132172311
Forced response and internal variability of summer climate over western North America
Over the past decade, anomalously hot summers and persistent droughts frequented over the western United States (wUS), the condition similar to the 1950s and 1960s. While atmospheric internal variability is important for mid-latitude interannual climate variability, it has been suggested that anthropogenic external forcing and multidecadal modes of variability in sea surface temperature, namely, the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO), also affect the occurrence of droughts and hot summers. In this study, 100-member ensemble simulations for 1951–2010 by an atmospheric general circulation model were used to explore relative contributions of anthropogenic warming, atmospheric internal variability, and atmospheric response to PDO and AMO to the decadal anomalies over the wUS. By comparing historical and sensitivity simulations driven by observed sea surface temperature, sea ice, historical forcing agents, and non-warming counterfactual climate forcing, we found that large portions of recent increases in mean temperature and frequency of hot summers (66 and 82 %) over the wUS can be attributed to the anthropogenic global warming. In contrast, multidecadal change in the wUS precipitation is explained by a combination of the negative PDO and the positive AMO after the 2000s. Diagnostics using a linear baroclinic model indicate that AMO- and PDO-related diabatic heating anomalies over the tropics contribute to the anomalous atmospheric circulation associated with the droughts and hot summers over wUS on multidecadal timescale. Those anomalies are not robust during the periods when PDO and AMO are in phase. The prolonged PDO–AMO antiphase period since the late twentieth century resulted in the substantial component of multidecadal anomalies in temperature and precipitation over the wUS.
129577984
Regional Precipitation Trends: Distinguishing Natural Variability from Anthropogenic Forcing
Abstract In this study, the nature and causes for observed regional precipitation trends during 1977–2006 are diagnosed. It is found that major features of regional trends in annual precipitation during 1977–2006 are consistent with an atmospheric response to observed sea surface temperature (SST) variability. This includes drying over the eastern Pacific Ocean that extends into western portions of the Americas related to a cooling of eastern Pacific SSTs, and broad increases in rainfall over the tropical Eastern Hemisphere, including a Sahelian rainfall recovery and increased wetness over the Indo–West Pacific related to North Atlantic and Indo–West Pacific ocean warming. It is further determined that these relationships between SST and rainfall change are generally not symptomatic of human-induced emissions of greenhouse gases (GHGs) and aerosols. The intensity of regional trends simulated in climate models using observed time variability in greenhouse gases, tropospheric sulfate aerosol, and solar and ...
15825012
Northern Hemisphere summer monsoon intensified by mega-El Niño/southern oscillation and Atlantic multidecadal oscillation
Prediction of monsoon changes in the coming decades is important for infrastructure planning and sustainable economic development. The decadal prediction involves both natural decadal variability and anthropogenic forcing. Hitherto, the causes of the decadal variability of Northern Hemisphere summer monsoon (NHSM) are largely unknown because the monsoons over Asia, West Africa, and North America have been studied primarily on a regional basis, which is unable to identify coherent decadal changes and the overriding controls on planetary scales. Here, we show that, during the recent global warming of about 0.4 °C since the late 1970s, a coherent decadal change of precipitation and circulation emerges in the entirety of the NHSM system. Surprisingly, the NHSM as well as the Hadley and Walker circulations have all shown substantial intensification, with a striking increase of NHSM rainfall by 9.5% per degree of global warming. This is unexpected from recent theoretical prediction and model projections of the 21st century. The intensification is primarily attributed to a mega-El Niño/Southern Oscillation (a leading mode of interannual-to-interdecadal variation of global sea surface temperature) and the Atlantic Multidecadal Oscillation, and further influenced by hemispherical asymmetric global warming. These factors driving the present changes of the NHSM system are instrumental for understanding and predicting future decadal changes and determining the proportions of climate change that are attributable to anthropogenic effects and long-term internal variability in the complex climate system.
129015715
Attributing the increase in Northern Hemisphere hot summers since the late 20th century
Anomalously high summertime temperatures have occurred with increasing frequency since the late 20th century. It is not clear why hot summers are becoming more frequent despite the recent slowdown in the rise in global surface air temperature. To examine factors affecting the historical variation in the frequency of hot summers over the Northern Hemisphere (NH), we conducted three sets of ensemble simulations with an atmospheric general circulation model. The model accurately reproduced interannual variation and long-term increase in the occurrence of hot summers. Decadal variabilities in the Pacific and Atlantic Oceans accounted for 43 ± 27% of the recent increase over the NH middle latitudes. In addition, direct influence of anthropogenic forcing also contributes to increasing the frequency since the late 20th century. The results suggest that the heat extremes can become more frequent in the coming decade even with the persistent slowdown in the global-mean surface warming.
542452
Combined effects of recent Pacific cooling and Indian Ocean warming on the Asian monsoon
Recent research indicates that the cooling trend in the tropical Pacific Ocean over the past 15 years underlies the contemporaneous hiatus in global mean temperature increase. During the hiatus, the tropical Pacific Ocean displays a La Niña-like cooling pattern while sea surface temperature (SST) in the Indian Ocean has continued to increase. This SST pattern differs from the well-known La Niña-induced basin-wide cooling across the Indian Ocean on the interannual timescale. Here, based on model experiments, we show that the SST pattern during the hiatus explains pronounced regional anomalies of rainfall in the Asian monsoon region and thermodynamic effects due to specific humidity change are secondary. Specifically, Indo-Pacific SST anomalies cause convection to intensify over the tropical western Pacific, which in turn suppresses rainfall in mid-latitude East Asia through atmospheric teleconnection. Overall, the tropical Pacific SST effect opposes and is greater than the Indian Ocean SST effect.
130091026
Long-term changes/trends in surface temperature and precipitation during the satellite era (1979–2012)
During the post-1979 period in which the satellite-based precipitation measurements with global coverage are available, global mean surface temperature rapidly increased up to late 1990s, followed by a period of temperature hiatus after about 1998/1999. Comparing observed surface temperature trends against the simulated ones by the CMIP5 historical experiments especially in the zonal mean context suggests that although the anthropogenic greenhouse-gases (GHG) forcing has played a major role, in addition to the anthropogenic aerosols and various natural forcings, the effects from decadal-to-interdecadal-scale internal modes specifically the Pacific Decadal Oscillation (PDO) are also very strong. Evident temperature changes associated with the PDO’s phase shift are seen in the Pacific basin, with decadal-scale cooling in the tropical central-eastern Pacific and most of the east basin and concurrent warming in the subtropics of both hemispheres, even though the PDO’s net effect on global mean temperature is relatively weak. The Atlantic Multidecadal Oscillation (AMO) also changed its phase in the mid-1990s, and hence its possible impact is estimated and assessed as well. However, comparisons with CMIP5 simulations suggest that the AMO may have not contributed as significantly as the PDO in terms of the changes/trends in global surface temperature, even though the data analysis technique used here suggests otherwise. Long-term precipitation changes or trends during the post-1979 period are further shown to have been modulated by the two major factors: anthropogenic GHG and PDO, in addition to the relatively weak effects from aerosols and natural forcings. The spatial patterns of observed precipitation trends in the Pacific, including reductions in the tropical central-eastern Pacific and increases in the tropical western Pacific and along the South Pacific Convergence Zone, manifest the PDO’s contributions. Removing the PDO effect from the total precipitation trends makes the spatial structures of precipitation trends more similar to those simulated by CMIP5 historical full forcing experiments particularly in the context of zonal-mean results. This also confirms that in spite of the PDO effect specifically on regional scales, the anthropogenic GHG signals are still discernible in observed precipitation during the time period. Following the increase of GHG, precipitation tends to increase roughly along the climatological ITCZ and decrease south of the equator and in the subtropics of both hemispheres.
123445359
Possible impacts of mega‐El Niño/Southern Oscillation and Atlantic Multidecadal Oscillation on Eurasian heatwave frequency variability
Identifying predictability sources of heatwave variations is a scientific challenge and of practical importance. This study investigates the summertime heatwave frequency (HWF) over Eurasia for 1950–2014. The Eurasian HWF is dominated by two distinct modes: the interdecadal (ID) mode, featured by an increasing pattern overall, centred around eastern Europe–central Asia and Mongolia–southwestern China; and the interannual (IA) mode, resembling a tripole anomaly pattern with three centres over western–northern Europe, northeastern Asia and East Asia. The ID mode is found to be influenced by mega-El Nino/Southern Oscillation (mega-ENSO) and the Atlantic Multidecadal Oscillation (AMO), and the latter has far more effect, whereas the IA mode is connected with mega-ENSO. 
 
 
 
Further analysis suggests that mega-ENSO variations can incite a Gill-type response spreading to Eurasia, while the AMO changes cause eastward-propagating Rossby wave trains toward Eurasia. These two teleconnection patterns together contribute to the large-scale circulation anomalies of the ID mode, and those related to the IA mode arise from the teleconnection pattern excited by mega-ENSO. A strong mega-ENSO triggers subsidence with high pressure anomalies, warms the surface and increases the HWF significantly over northeastern Asia particularly. Likewise, the warm AMO-induced circulation anomalies engender surface radiative heating and HWF growth in most of the Eurasian continent except some localized Siberian and Asian regions. The situation is opposite for a weak mega-ENSO and AMO. Those models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) which realistically capture the features of the ID mode can reproduce the AMO-like sea-surface temperature anomalies (SSTAs), while signals resembling mega-ENSO are found in those with favourable capability of simulating the IA mode. On the contrary, these relevant SSTAs linked to the respective modes vanish in the models with little skills. Thus, mega-ENSO and the AMO might provide two critical predictability sources for heat waves over Eurasia.
2198328
Indian Ocean warming modulates Pacific climate change
It has been widely believed that the tropical Pacific trade winds weakened in the last century and would further decrease under a warmer climate in the 21st century. Recent high-quality observations, however, suggest that the tropical Pacific winds have actually strengthened in the past two decades. Precise causes of the recent Pacific climate shift are uncertain. Here we explore how the enhanced tropical Indian Ocean warming in recent decades favors stronger trade winds in the western Pacific via the atmosphere and hence is likely to have contributed to the La Niña-like state (with enhanced east–west Walker circulation) through the Pacific ocean–atmosphere interactions. Further analysis, based on 163 climate model simulations with centennial historical and projected external radiative forcing, suggests that the Indian Ocean warming relative to the Pacific’s could play an important role in modulating the Pacific climate changes in the 20th and 21st centuries.
87155351
Atlantic-induced pan-tropical climate change over the past three decades
The dipole-like trend of tropical sea surface temperature is investigated and this study finds it to be initiated in the Atlantic Ocean. Atlantic warming drives wind and circulation changes and influences Pacific Ocean surface temperatures. During the past three decades, tropical sea surface temperature (SST) has shown dipole-like trends, with warming over the tropical Atlantic and Indo-western Pacific but cooling over the eastern Pacific. Competing hypotheses relate this cooling, identified as a driver of the global warming hiatus1,2, to the warming trends in either the Atlantic3,4 or Indian Ocean5. However, the mechanisms, the relative importance and the interactions between these teleconnections remain unclear. Using a state-of-the-art climate model, we show that the Atlantic plays a key role in initiating the tropical-wide teleconnection, and the Atlantic-induced anomalies contribute ∼55–75% of the tropical SST and circulation changes during the satellite era. The Atlantic warming drives easterly wind anomalies over the Indo-western Pacific as Kelvin waves and westerly anomalies over the eastern Pacific as Rossby waves. The wind changes induce an Indo-western Pacific warming through the wind–evaporation–SST effect6,7, and this warming intensifies the La Nina-type response in the tropical Pacific by enhancing the easterly trade winds and through the Bjerknes ocean dynamical processes8. The teleconnection develops into a tropical-wide SST dipole pattern. This mechanism, supported by observations and a hierarchy of climate models, reveals that the tropical ocean basins are more tightly connected than previously thought.
4466067
Recent global-warming hiatus tied to equatorial Pacific surface cooling
Despite the continued increase in atmospheric greenhouse gas concentrations, the annual-mean global temperature has not risen in the twenty-first century, challenging the prevailing view that anthropogenic forcing causes climate warming. Various mechanisms have been proposed for this hiatus in global warming, but their relative importance has not been quantified, hampering observational estimates of climate sensitivity. Here we show that accounting for recent cooling in the eastern equatorial Pacific reconciles climate simulations and observations. We present a novel method of uncovering mechanisms for global temperature change by prescribing, in addition to radiative forcing, the observed history of sea surface temperature over the central to eastern tropical Pacific in a climate model. Although the surface temperature prescription is limited to only 8.2% of the global surface, our model reproduces the annual-mean global temperature remarkably well with correlation coefficient r = 0.97 for 1970–2012 (which includes the current hiatus and a period of accelerated global warming). Moreover, our simulation captures major seasonal and regional characteristics of the hiatus, including the intensified Walker circulation, the winter cooling in northwestern North America and the prolonged drought in the southern USA. Our results show that the current hiatus is part of natural climate variability, tied specifically to a La-Niña-like decadal cooling. Although similar decadal hiatus events may occur in the future, the multi-decadal warming trend is very likely to continue with greenhouse gas increase.
30694639
Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus
The slowdown in global average surface warming has recently been linked to sea surface cooling in the eastern Pacific Ocean. This work shows that strengthening trade winds caused a reduction in the 2012 global average surface air temperature of 0.1–0.2 °C. This may account for much of the warming hiatus and is a result of increased subsurface ocean heat uptake.
85168034
Contribution of natural decadal variability to global warming acceleration and hiatus
The role of natural decadal variability in the global warming slowdown has been hinted at, but not quantified. This study looks at decadal average surface temperature anomalies for the 1980s, 1990s and 2000s. The results show that decadal variability is a large contributor to temperature trends, but its influence has decreased, from 47% in the 1980s to 27% in the 2000s, as anthropogenic warming has increased.
53682426
Seasonal aspects of the recent pause in surface warming
The slowdown in global warming has been identified predominately through changes in the Pacific Ocean. This study investigates the teleconnections and seasonal changes associated with the slowdown. The present forcing from the tropical Pacific is found to produce many of the changes in atmospheric circulation, for example, changes in the upper troposphere wave patterns increase the chances of cold European winters.
4470401
Tropical forcing of the recent rapid Arctic warming in northeastern Canada and Greenland
Rapid Arctic warming and sea-ice reduction in the Arctic Ocean are widely attributed to anthropogenic climate change. The Arctic warming exceeds the global average warming because of feedbacks that include sea-ice reduction and other dynamical and radiative feedbacks. We find that the most prominent annual mean surface and tropospheric warming in the Arctic since 1979 has occurred in northeastern Canada and Greenland. In this region, much of the year-to-year temperature variability is associated with the leading mode of large-scale circulation variability in the North Atlantic, namely, the North Atlantic Oscillation. Here we show that the recent warming in this region is strongly associated with a negative trend in the North Atlantic Oscillation, which is a response to anomalous Rossby wave-train activity originating in the tropical Pacific. Atmospheric model experiments forced by prescribed tropical sea surface temperatures simulate the observed circulation changes and associated tropospheric and surface warming over northeastern Canada and Greenland. Experiments from the Coupled Model Intercomparison Project Phase 5 (ref. 16) models with prescribed anthropogenic forcing show no similar circulation changes related to the North Atlantic Oscillation or associated tropospheric warming. This suggests that a substantial portion of recent warming in the northeastern Canada and Greenland sector of the Arctic arises from unforced natural variability.
129489696
Atmosphere and Ocean Origins of North American Droughts
AbstractThe atmospheric and oceanic causes of North American droughts are examined using observations and ensemble climate simulations. The models indicate that oceanic forcing of annual mean precipitation variability accounts for up to 40% of total variance in northeastern Mexico, the southern Great Plains, and the Gulf Coast states but less than 10% in central and eastern Canada. Observations and models indicate robust tropical Pacific and tropical North Atlantic forcing of annual mean precipitation and soil moisture with the most heavily influenced areas being in southwestern North America and the southern Great Plains. In these regions, individual wet and dry years, droughts, and decadal variations are well reproduced in atmosphere models forced by observed SSTs. Oceanic forcing was important in causing multiyear droughts in the 1950s and at the turn of the twenty-first century, although a similar ocean configuration in the 1970s was not associated with drought owing to an overwhelming influence of in...
28741045
A Link between the Hiatus in Global Warming and North American Drought
AbstractPortions of western North America have experienced prolonged drought over the last decade. This drought has occurred at the same time as the global warming hiatus—a decadal period with little increase in global mean surface temperature. Climate models and observational analyses are used to clarify the dual role of recent tropical Pacific changes in driving both the global warming hiatus and North American drought. When observed tropical Pacific wind stress anomalies are inserted into coupled models, the simulations produce persistent negative sea surface temperature anomalies in the eastern tropical Pacific, a hiatus in global warming, and drought over North America driven by SST-induced atmospheric circulation anomalies. In the simulations herein the tropical wind anomalies account for 92% of the simulated North American drought during the recent decade, with 8% from anthropogenic radiative forcing changes. This suggests that anthropogenic radiative forcing is not the dominant driver of the curre...
39411790
Hot days induced by precipitation deficits at the global scale
Global warming increases the occurrence probability of hot extremes, and improving the predictability of such events is thus becoming of critical importance. Hot extremes have been shown to be induced by surface moisture deficits in some regions. In this study, we assess whether such a relationship holds at the global scale. We find that wide areas of the world display a strong relationship between the number of hot days in the regions’ hottest month and preceding precipitation deficits. The occurrence probability of an above-average number of hot days is over 70% after precipitation deficits in most parts of South America as well as the Iberian Peninsula and Eastern Australia, and over 60% in most of North America and Eastern Europe, while it is below 30–40% after wet conditions in these regions. Using quantile regression analyses, we show that the impact of precipitation deficits on the number of hot days is asymmetric, i.e. extreme high numbers of hot days are most strongly influenced. This relationship also applies to the 2011 extreme event in Texas. These findings suggest that effects of soil moisture-temperature coupling are geographically more widespread than commonly assumed.
128616068
The Precipitation Response over the Continental United States to Cold Tropical Pacific Sea Surface Temperatures
AbstractThe dominant pattern of SST variability in the Pacific during its cold phase produces pronounced precipitation deficits over the continental United States throughout the annual cycle. This study investigates the observed physical and dynamical processes through which the cold Pacific pattern affects U.S. precipitation, particularly the causes for the peak dry impacts in fall, as well as the nature of the differences between the summer and fall responses.Results show that the peak precipitation deficit over the United States during fall is primarily due to reduced atmospheric moisture transport from the Gulf of Mexico into the central and eastern United States and secondarily a reduction in local evaporation from land–atmosphere feedback. The former is associated with a strong and systematic low-level northeasterly flow anomaly over the southeastern United States that counteracts the northwest branch of the climatological North Atlantic subtropical high. The above northeasterly anomaly is maintaine...
27418729
A Pacific Interdecadal Climate Oscillation with Impacts on Salmon Production
Evidence gleaned from the instrumental record of climate data identifies a robust, recurring pattern of ocean–atmosphere climate variability centered over the midlatitude North Pacific basin. Over the past century, the amplitude of this climate pattern has varied irregularly at interannual-to-interdecadal timescales. There is evidence of reversals in the prevailing polarity of the oscillation occurring around 1925, 1947, and 1977; the last two reversals correspond to dramatic shifts in salmon production regimes in the North Pacific Ocean. This climate pattern also affects coastal sea and continental surface air temperatures, as well as streamflow in major west coast river systems, from Alaska to California.
6868642
Pacific Interdecadal Climate Variability: Linkages between the Tropics and the North Pacific during Boreal Winter since 1900
This study examines the tropical linkages to interdecadal climate fluctuations over the North Pacific during boreal winter through a comprehensive and physically based analysis of a wide variety of observational datasets spanning the twentieth century. Simple difference maps between epochs of high sea level pressure over the North Pacific (1900‐24 and 1947‐76) and epochs of low pressure (1925‐46 and 1977‐97) are presented for numerous climate variables throughout the tropical Indo-Pacific region, including rainfall, cloudiness, sea surface temperature (SST), and sea level pressure. The results support the notion that the Tropics play a key role in North Pacific interdecadal climate variability. In particular, SST anomalies in the tropical Indian Ocean and southeast Pacific Ocean, rainfall and cloudiness anomalies in the vicinity of the South Pacific convergence zone, stratus clouds in the eastern tropical Pacific, and sea level pressure differences between the tropical southeast Pacific and Indian Oceans all exhibit prominent interdecadal fluctuations that are coherent with those in sea level pressure over the North Pacific. The spatial patterns of the interdecadal tropical climate anomalies are compared with those associated with ENSO, a predominantly interannual phenomenon; in general, the two are similar with some differences in relative spatial emphasis. Finally, a published 194-yr coral record in the western tropical Indian Ocean is shown to compare favorably with the twentieth-century instrumental records, indicating the potential for extending knowledge of tropical interdecadal climate variability to earlier time periods.
4824093
The Pacific Decadal Oscillation, Revisited
AbstractThe Pacific decadal oscillation (PDO), the dominant year-round pattern of monthly North Pacific sea surface temperature (SST) variability, is an important target of ongoing research within the meteorological and climate dynamics communities and is central to the work of many geologists, ecologists, natural resource managers, and social scientists. Research over the last 15 years has led to an emerging consensus: the PDO is not a single phenomenon, but is instead the result of a combination of different physical processes, including both remote tropical forcing and local North Pacific atmosphere–ocean interactions, which operate on different time scales to drive similar PDO-like SST anomaly patterns. How these processes combine to generate the observed PDO evolution, including apparent regime shifts, is shown using simple autoregressive models of increasing spatial complexity. Simulations of recent climate in coupled GCMs are able to capture many aspects of the PDO, but do so based on a balance of ...
140183219
Megadroughts in the Indian Monsoon Region and Southwest North America and a Mechanism for Associated Multidecadal Pacific Sea Surface Temperature Anomalies
Abstract A 1360-yr control run from a global coupled climate model (the Parallel Climate Model) is analyzed. It simulates “megadroughts” in the southwestern United States and Indian monsoon regions. The megadroughts represent extreme events of naturally occurring multidecadal precipitation variations linked to the dominant pattern of multidecadal SST variability in the Indian and Pacific Oceans. Gaining insight into the occurrence of megadroughts thus requires an understanding of the mechanism that is producing this multidecadal SST variability. Analysis of the model variability shows that the mechanism involves atmosphere–ocean and tropical–midlatitude interactions, with a crucial element being wind-forced ocean Rossby waves near 20°N and 25°S in the Pacific whose transit times set the decadal time scale. At the western boundary, the Rossby waves reflect into the equatorial Pacific to affect thermocline depth. The resulting feedbacks, involving surface temperature, winds, and the strength of the subtropi...
129130089
Simulated U.S. Drought Response to Interannual and Decadal Pacific SST Variability
AbstractIdealized atmospheric general circulation model (AGCM) experiments by the U.S. Climate Variability and Predictability Program (CLIVAR) Drought Working Group were used in order to study the influence of natural modes of sea surface temperature (SST) variability in the Pacific on drought in the contiguous United States. The current study expands on previous results by examining the atmospheric response of three AGCMs to three different patterns of the idealized Pacific SST anomalies that operate on different time scales: low-frequency (decadal), high-frequency (interannual), and a pan-Pacific pattern that retains characteristics of interannual and decadal variability. While forcing patterns are generally similar in appearance, results indicate that differences in the relative amplitude of the equatorial and extratropical components of the SST forcing are sufficient to give rise to differing teleconnections, leading to regional differences in the amplitude and significance of the precipitation respon...
129505926
Mechanisms of Tropical Atlantic SST Influence on North American Precipitation Variability
Abstract The dynamical mechanisms associated with the impact of year-to-year variability in tropical North Atlantic (TNA) sea surface temperatures (SSTs) on North American precipitation, during the cold and warm halves of the hydrological year (October–September) are examined. Observations indicate that during both seasons warmer-than-normal TNA SSTs are associated with a reduction of precipitation over North America, mainly west of ∼90°W, and that the effect can be up to 30% of the year-to-year seasonal precipitation RMS variability. This finding confirms earlier studies with observations and models. During the cold season (October–March) the North American precipitation variability associated with TNA fluctuations is considerably weaker than its association with ENSO. During the warm season (April–September), however, the Atlantic influence, per one standard deviation of SST anomalies, is larger than that of ENSO. The observed association between TNA SST anomalies and global and North American precipita...
53637430
Influence of Atlantic sea surface temperatures on persistent drought in North America
This study investigates the relationship between North Atlantic sea surface temperatures (SST) and persistent drought in North America using modern observations, proxy paleo-data, and simulations from multiple climate models. The observational results show that persistent droughts in the Great Plains and the southwest North America are closely related to multidecadal variations of North Atlantic SST (Atlantic Multidecadal Oscillations, AMO). During the AMO warm (cold) phases, most of North America is dry (wet). This relationship is persistent since at least 1567 AD, as based on proxy SST for the North Atlantic and the reconstructed drought index for North America. On centennial timescales, proxy SST records from the North Atlantic and proxy drought records for North America suggest that major periods of AMO-like warm (cold) SST anomalies during the last 7.0 ka correspond to dry (wet) conditions in the Great Plains. The influence of North Atlantic SST on North American droughts is examined using simulations made by five global climate models. When forced by warm North Atlantic SST anomalies, all models captured significant drying over North America, despite some regional differences. Specifically, dry summers in the Great Plains and the southwest North America are simulated by all models. The precipitation response to a cold North Atlantic is much weaker and contains greater disagreement among the models. Overall, the ensemble of the five models could well reproduce the statistical relationship between the dry/wet fluctuations in the North America and North Atlantic SST anomalies. Our results suggest that North Atlantic SSTs are likely a major driver of decadal and centennial timescale circulation, including droughts, in North America. Possible mechanisms that connect North Atlantic SST with North American drought, as well as interactions between North Atlantic and tropical Pacific SST and their relative roles on drought are also discussed.
140710921
Attribution of observed historical near‒surface temperature variations to anthropogenic and natural causes using CMIP5 simulations
[1] We have carried out an investigation into the causes of changes in near‒surface temperatures from 1860 to 2010. We analyze the HadCRUT4 observational data set which has the most comprehensive set of adjustments available to date for systematic biases in sea surface temperatures and the CMIP5 ensemble of coupled models which represents the most sophisticated multi‒model climate modeling exercise yet carried out. Simulations that incorporate both anthropogenic and natural factors span changes in observed temperatures between 1860 and 2010, while simulations of natural factors do not warm as much as observed. As a result of sampling a much wider range of structural modeling uncertainty, we find a wider spread of historic temperature changes in CMIP5 than was simulated by the previous multi‒model ensemble, CMIP3. However, calculations of attributable temperature trends based on optimal detection support previous conclusions that human‒induced greenhouse gases dominate observed global warming since the mid‒20th century. With a much wider exploration of model uncertainty than previously carried out, we find that individually the models give a wide range of possible counteracting cooling from the direct and indirect effects of aerosols and other non‒greenhouse gas anthropogenic forcings. Analyzing the multi‒model mean over 1951–2010 (focusing on the most robust result), we estimate a range of possible contributions to the observed warming of approximately 0.6 K from greenhouse gases of between 0.6 and 1.2 K, balanced by a counteracting cooling from other anthropogenic forcings of between 0 and −0.5 K.
327749
Anthropogenic warming has increased drought risk in California
Significance California ranks first in the United States in population, economic activity, and agricultural value. The state is currently experiencing a record-setting drought, which has led to acute water shortages, groundwater overdraft, critically low streamflow, and enhanced wildfire risk. Our analyses show that California has historically been more likely to experience drought if precipitation deficits co-occur with warm conditions and that such confluences have increased in recent decades, leading to increases in the fraction of low-precipitation years that yield drought. In addition, we find that human emissions have increased the probability that low-precipitation years are also warm, suggesting that anthropogenic warming is increasing the probability of the co-occurring warm–dry conditions that have created the current California drought. California is currently in the midst of a record-setting drought. The drought began in 2012 and now includes the lowest calendar-year and 12-mo precipitation, the highest annual temperature, and the most extreme drought indicators on record. The extremely warm and dry conditions have led to acute water shortages, groundwater overdraft, critically low streamflow, and enhanced wildfire risk. Analyzing historical climate observations from California, we find that precipitation deficits in California were more than twice as likely to yield drought years if they occurred when conditions were warm. We find that although there has not been a substantial change in the probability of either negative or moderately negative precipitation anomalies in recent decades, the occurrence of drought years has been greater in the past two decades than in the preceding century. In addition, the probability that precipitation deficits co-occur with warm conditions and the probability that precipitation deficits produce drought have both increased. Climate model experiments with and without anthropogenic forcings reveal that human activities have increased the probability that dry precipitation years are also warm. Further, a large ensemble of climate model realizations reveals that additional global warming over the next few decades is very likely to create ∼100% probability that any annual-scale dry period is also extremely warm. We therefore conclude that anthropogenic warming is increasing the probability of co-occurring warm–dry conditions like those that have created the acute human and ecosystem impacts associated with the “exceptional” 2012–2014 drought in California.
7883170
Towards predictive understanding of regional climate change
This Review considers recent advances in our understanding of regional climate change, critically discusses outstanding issues, and recommends targets for future research. Regional information on climate change is urgently needed but often deemed unreliable. To achieve credible regional climate projections, it is essential to understand underlying physical processes, reduce model biases and evaluate their impact on projections, and adequately account for internal variability. In the tropics, where atmospheric internal variability is small compared with the forced change, advancing our understanding of the coupling between long-term changes in upper-ocean temperature and the atmospheric circulation will help most to narrow the uncertainty. In the extratropics, relatively large internal variability introduces substantial uncertainty, while exacerbating risks associated with extreme events. Large ensemble simulations are essential to estimate the probabilistic distribution of climate change on regional scales. Regional models inherit atmospheric circulation uncertainty from global models and do not automatically solve the problem of regional climate change. We conclude that the current priority is to understand and reduce uncertainties on scales greater than 100 km to aid assessments at finer scales.
130628672
Analysis of Low-Frequency Precipitation Variability in CMIP5 Historical Simulations for Southwestern North America
AbstractDrier future conditions are projected for the arid southwest of North America, increasing the chances of the region experiencing severe and prolonged drought. To examine the mechanisms of decadal variability, 47 global climate model historical simulations performed for phase 5 of the Coupled Model Intercomparison Project (CMIP5) were assessed. On average, the CMIP5 models have higher climatological precipitation over the past century in southwestern North America than current instrumental or reanalysis products. The timing of the winter peak in climatological precipitation over California and Nevada is accurately represented. Models with resolutions coarser than 2° show a larger spread in the location and strength of the North American monsoon ridge and subsequent summer precipitation, in comparison with the higher-resolution models. Less than 20% of decadal variability in wintertime precipitation over California is associated with North Pacific sea surface temperature anomalies, a larger proporti...
16201075
The Version 2 Global Precipitation Climatology Project (GPCP) Monthly Precipitation Analysis (1979-Present)
Abstract The Global Precipitation Climatology Project (GPCP) Version-2 Monthly Precipitation Analysis is described. This globally complete, monthly analysis of surface precipitation at 2.5° latitude × 2.5° longitude resolution is available from January 1979 to the present. It is a merged analysis that incorporates precipitation estimates from low-orbit satellite microwave data, geosynchronous-orbit satellite infrared data, and surface rain gauge observations. The merging approach utilizes the higher accuracy of the low-orbit microwave observations to calibrate, or adjust, the more frequent geosynchronous infrared observations. The dataset is extended back into the premicrowave era (before mid-1987) by using infrared-only observations calibrated to the microwave-based analysis of the later years. The combined satellite-based product is adjusted by the rain gauge analysis. The dataset archive also contains the individual input fields, a combined satellite estimate, and error estimates for each field. This m...
126055011
Over 5,000 years of ensemble future climate simulations by 60-km global and 20-km regional atmospheric models
AbstractAn unprecedentedly large ensemble of climate simulations with a 60-km atmospheric general circulation model and dynamical downscaling with a 20-km regional climate model has been performed to obtain probabilistic future projections of low-frequency local-scale events. The climate of the latter half of the twentieth century, the climate 4 K warmer than the preindustrial climate, and the climate of the latter half of the twentieth century without historical trends associated with the anthropogenic effect are each simulated for more than 5,000 years. From large ensemble simulations, probabilistic future changes in extreme events are available directly without using any statistical models. The atmospheric models are highly skillful in representing localized extreme events, such as heavy precipitation and tropical cyclones. Moreover, mean climate changes in the models are consistent with those in phase 5 of the Coupled Model Intercomparison Project (CMIP5) ensembles. Therefore, the results enable the a...
128714423
Centennial-Scale Sea Surface Temperature Analysis and Its Uncertainty
AbstractA new sea surface temperature (SST) analysis on a centennial time scale is presented. In this analysis, a daily SST field is constructed as a sum of a trend, interannual variations, and daily changes, using in situ SST and sea ice concentration observations. All SST values are accompanied with theory-based analysis errors as a measure of reliability. An improved equation is introduced to represent the ice–SST relationship, which is used to produce SST data from observed sea ice concentrations. Prior to the analysis, biases of individual SST measurement types are estimated for a homogenized long-term time series of global mean SST. Because metadata necessary for the bias correction are unavailable for many historical observational reports, the biases are determined so as to ensure consistency among existing SST and nighttime air temperature observations. The global mean SSTs with bias-corrected observations are in agreement with those of a previously published study, which adopted a different appro...
96177362
Development of an MRI Chemical Transport Model for the Study of Stratospheric Chemistry
A three-dimensional chemical transport model(CTM) was developed at the Meteorological Research Institute by coupling a chemical module with the MJ98 general circulation model (dynamical module) for the study of stratospheric chemistry.This model, MJ98-CTM, ran for approximately 15 years and the simulated chemical species were investigated, focusing on the time-mean fields. The chemical module was based on the family method and contains major stratospheric species, i.e., 34 long-lived species including 7 families and 15 short-lived species with 79 gas phase reactions and 34 photodissociations. Two types (I and II) of polar stratospheric clouds (PSCs) and sulfate aerosols were included with six heterogeneous reactions on PSCs and three heterogeneous reactions on sulfate aerosols. MJ98-CTMs of T21L45 and T42L45 versions were integrated using climatological monthly mean values of sea-surface temperature and ozone for the dynamical module, and fixed values at the surface for surface-origin species in the chemical module. Radiatively active gases such as ozone, methane, and nitrous oxide were not treated interactively between the two modules. Horizontal resolutions were 5.6°(∼600 km) for the T21 model and 2.8°(∼300 km) for the T42 model. The vertical resolution L45 had a vertical spacing of about 2 km in the stratosphere with a top at 0.01 hPa (80km).   Qualitatively, MJ98-CTM suitably reproduced the temporal and spatial features of observed ozone and other chemical species in the middle atmosphere. However, the lower stratosphere held crucial errors for the distributions of chemical species, particularly ozone, yielding positive errors for column ozone. A major cause of these errors can be ascribed to the errors associated with transport: one based on the wind field bias produced by MJ98, and the other from the coarse vertical resolution.
140742722
Estimates of Tropical Diabatic Heating Profiles: Commonalities and Uncertainties
This study aims to evaluate the consistency and discrepancies in estimates of diabatic heating profiles associated with precipitation based on satellite observations and microphysics and those derived from the thermodynamics of the large-scale environment. It presents a survey of diabatic heating profile estimates from four Tropical Rainfall Measuring Mission (TRMM) products, four global reanalyses, and in situ sounding measurements from eight field campaigns at various tropical locations. Common in most of the estimates are the following: (i) bottom-heavy profiles, ubiquitous over the oceans, are associated with relatively low rain rates, while top-heavy profiles are generally associated with high rain rates; (ii) temporal variability of latent heating profiles is dominated by two modes, a deep mode with a peak in the upper troposphere and a shallow mode with a low-level peak; and (iii) the structure of the deep modes is almost the same in different estimates and different regions in the tropics. The primary uncertainty is in the amount of shallow heating over the tropical oceans, which differs substantially among the estimates.
129235009
Change in the Odds of Warm Years and Seasons Due to Anthropogenic Influence on the Climate
AbstractThe new Hadley Centre system for attribution of weather and climate extremes provides assessments of how human influence on the climate may lead to a change in the frequency of such events. Two different types of ensembles of simulations are generated with an atmospheric model to represent the actual climate and what the climate would have been in the absence of human influence. Estimates of the event frequency with and without the anthropogenic effect are then obtained. Three experiments conducted so far with the new system are analyzed in this study to examine how anthropogenic forcings change the odds of warm years, summers, or winters in a number of regions where the model reliably reproduces the frequency of warm events. In all cases warm events become more likely because of human influence, but estimates of the likelihood may vary considerably from year to year depending on the ocean temperature. While simulations of the actual climate use prescribed observational data of sea surface tempera...
13568152
Attributing Historical Changes in Probabilities of Record-Breaking Daily Temperature and Precipitation Extreme Events
Author(s): Shiogama, H; Imada, Y; Mori, M; Mizuta, R; Stone, D; Yoshida, K; Arakawa, O; Ikeda, M; Takahashi, C; Arai, M; Ishii, M; Watanabe, M; Kimoto, M | Abstract: © 2016, the Meteorological Society of Japan. We describe two unprecedented large (100-member), longterm (61-year) ensembles based on MRI-AGCM3.2, which were driven by historical and non-warming climate forcing. These ensembles comprise the "Database for Policy Decision making for Future climate change (d4PDF)". We compare these ensembles to large ensembles based on another climate model, as well as to observed data, to investigate the influence of anthropogenic activities on historical changes in the numbers of record-breaking events, including: the annual coldest daily minimum temperature (TNn), the annual warmest daily maximum temperature (TXx) and the annual most intense daily precipitation event (Rx1day). These two climate model ensembles indicate that human activity has already had statistically significant impacts on the number of record-breaking extreme events worldwide mainly in the Northern Hemisphere land. Specifically, human activities have altered the likelihood that a wider area globally would suffer record-breaking TNn, TXx and Rx1day events than that observed over the 2001- 2010 period by a factor of at least 0.6, 5.4 and 1.3, respectively. However, we also find that the estimated spatial patterns and amplitudes of anthropogenic impacts on the probabilities of record-breaking events are sensitive to the climate model and/or natural-world boundary conditions used in the attribution studies.
1731300
Perception of climate change
“Climate dice,” describing the chance of unusually warm or cool seasons, have become more and more “loaded” in the past 30 y, coincident with rapid global warming. The distribution of seasonal mean temperature anomalies has shifted toward higher temperatures and the range of anomalies has increased. An important change is the emergence of a category of summertime extremely hot outliers, more than three standard deviations (3σ) warmer than the climatology of the 1951–1980 base period. This hot extreme, which covered much less than 1% of Earth’s surface during the base period, now typically covers about 10% of the land area. It follows that we can state, with a high degree of confidence, that extreme anomalies such as those in Texas and Oklahoma in 2011 and Moscow in 2010 were a consequence of global warming because their likelihood in the absence of global warming was exceedingly small. We discuss practical implications of this substantial, growing, climate change.
59584217
Avoiding Inhomogeneity in Percentile-Based Indices of Temperature Extremes
Using a Monte Carlo simulation, it is demonstrated that percentile-based temperature indices computed for climate change detection and monitoring may contain artificial discontinuities at the beginning and end of the period that is used for calculating the percentiles (base period). This would make these exceedance frequency time series unsuitable for monitoring and detecting climate change. The problem occurs because the threshold calculated in the base period is affected by sampling error. On average, this error leads to overestimated exceedance rates outside the base period. A bootstrap resampling procedure is proposed to estimate exceedance frequencies during the base period. The procedure effectively removes the inhomogeneity.
59430669
Quantifying changes in climate variability and extremes: Pitfalls and their overcoming
Hot temperature extremes have increased substantially in frequency and magnitude over past decades. A widely used approach to quantify this phenomenon is standardizing temperature data relative to the local mean and variability of a reference period. Here we demonstrate that this conventional procedure leads to exaggerated estimates of increasing temperature variability and extremes. For example, the occurrence of ‘2-sigma extremes’ would be overestimated by 48.2% compared to a given reference period of 30 years with time-invariant simulated Gaussian data. This corresponds to an increase from a 2.0% to 2.9% probability of such events. We derive an analytical correction revealing that these artifacts prevail in recent studies. Our analyses lead to a revision of earlier reports [e.g. Huntingford et al., 2013]: For instance we show that there is no evidence for a recent increase in normalized temperature variability. In conclusion, we provide an analytical pathway to describe changes in variability and extremes in climate observations and model simulations.
54925002
Mechanisms Contributing to the Warming Hole and the Consequent U.S. East–West Differential of Heat Extremes
AbstractA linear trend calculated for observed annual mean surface air temperatures over the United States for the second-half of the twentieth century shows a slight cooling over the southeastern part of the country, the so-called warming hole, while temperatures over the rest of the country rose significantly. This east–west gradient of average temperature change has contributed to the observed pattern of changes of record temperatures as given by the ratio of daily record high temperatures to record low temperatures with a comparable east–west gradient. Ensemble averages of twentieth-century climate simulations in the Community Climate System Model, version 3 (CCSM3), show a slight west–east warming gradient but no warming hole. A warming hole appears in only several ensemble members in the Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel dataset and in one ensemble member of simulated twentieth-century climate in CCSM3. In this model the warming hole is produced mostly from internal de...
128709832
North American Climate in CMIP5 Experiments. Part II: Evaluation of Historical Simulations of Intraseasonal to Decadal Variability
AbstractThis is the second part of a three-part paper on North American climate in phase 5 of the Coupled Model Intercomparison Project (CMIP5) that evaluates the twentieth-century simulations of intraseasonal to multidecadal variability and teleconnections with North American climate. Overall, the multimodel ensemble does reasonably well at reproducing observed variability in several aspects, but it does less well at capturing observed teleconnections, with implications for future projections examined in part three of this paper. In terms of intraseasonal variability, almost half of the models examined can reproduce observed variability in the eastern Pacific and most models capture the midsummer drought over Central America. The multimodel mean replicates the density of traveling tropical synoptic-scale disturbances but with large spread among the models. On the other hand, the coarse resolution of the models means that tropical cyclone frequencies are underpredicted in the Atlantic and eastern North Pa...
131625345
Running dry: The U.S. Southwest's drift into a drier climate state
Changes in precipitation have far-reaching consequences on human society and ecosystems as has been demonstrated by recent severe droughts in California and the Oklahoma region. Droughts are beside tropical cyclones the most costly weather and climate related extreme events in the U.S. We apply a weather type (WT) analysis to reanalysis data from 1979–2014 that characterize typical weather conditions over the contiguous United States. This enables us to assign precipitation trends within 1980–2010 to changes in WT frequencies and changes in precipitation intensities. We show that in the North Atlantic and Midwest region precipitation intensity changes are the major driver of increasing precipitation trends. In the U.S. Southwest, however, WT frequency changes lead to a significant precipitation decrease of up to −25% related to an increase in anticyclonic conditions in the North East Pacific. This trend is partly counteracted by increasing precipitation intensities.
128625867
Influence of ENSO and the Atlantic Multidecadal Oscillation on Drought over the United States
Abstract Composites based on observations and model outputs from the Climate Variability and Predictability (CLIVAR) drought experiments were used to examine the impact of El Nino–Southern Oscillation (ENSO) and the Atlantic multidecadal oscillation (AMO) on drought over the United States. Because drought implies persistent dryness, the 6-month standardized precipitation index, standardized runoff index, and soil moisture anomalies are used to represent drought. The experiments were performed by forcing an AGCM with prescribed sea surface temperature anomalies (SSTAs) superimposed on the monthly mean SST climatology. Four model outputs from the NCEP Global Forecast System (GFS), NASA’s Seasonal-to-Interannual Prediction Project, version 1 (NSIPP1), GFDL’s global atmospheric model, version 2.1 (AM2.1), and the Lamont-Doherty Earth Observatory (LDEO)/NCAR Community Climate System Model, version 3 (CCM3) were analyzed in this study. Each run lasts from 36 to 51 yr. The impact of ENSO on drought over the Unit...
9015242
Variations in North American Summer Precipitation Driven by the Atlantic Multidecadal Oscillation
Understanding the development and variation of the atmospheric circulation regimes driven by the Atlantic multidecadaloscillation(AMO)isessentialbecausethesecirculationsinteractwithotherforcingsondecadaland interannual time scales. Collectively, they determine the summer (June, July, and August) precipitation variations for North America. In this study, a general circulation model (GCM) is used to obtain such understanding, with a focus on physical processes connecting the AMO and the summertime precipitation regime change in NorthAmerica.Twoexperimentalrunsareconductedwithseasurfacetemperature(SST)anomaliesimposedin the North Atlantic Ocean that represent the warm and cold phases of the AMO. Climatological SSTs are used elsewhere in the oceans. Model results yield summertime precipitation anomalies in North America closely matching the observed anomaly patterns in North America, suggesting that the AMO provides a fundamental control on summertime precipitation in North America at decadal time scales. The impacts of the AMO are achieved by a chain of events arising from different circulation anomalies during warm and cold phases of the AMO. During the warm phase, the North Atlantic subtropical high pressure system (NASH) weakens, and the North American continent is much less influenced by it. A massive body of warm air develops over the heated land in North America from June‐August, associated with high temperature and low pressure anomalies in the lowertroposphereandhighpressureanomaliesintheuppertroposphere.Incontrast,duringthecoldphaseofthe AMO, the North American continent, particularly to the west, is much more influenced by an enhanced NASH. Cooler temperatures and high pressure anomalies prevail in the lower troposphere, and a frontal zone forms in the upper troposphere. These different circulation anomalies further induce a three-cell circulation anomaly pattern over North America in the warm and cold phases of the AMO. In particular, during the cold phase, the three-cell circulation anomaly pattern features a broad region of anomalous low-level southerly flow from the Gulf of Mexico into the U.S. Great Plains. Superimposed with an upper-troposphere front, more frequent summertime storms develop and excess precipitation occurs over most of North America. A nearly reversed condition occurs during the warm phase of the AMO, yielding drier conditions in North America. This new understandingprovides a foundation for further studyand better prediction of the variations of North American summer precipitation, especially when modulated by other multidecadal variations—for example, the Pacific decadal oscillation and interannual variations associated with the ENSO and the Arctic Oscillation.
129156144
AMO- and ENSO-Driven Summertime Circulation and Precipitation Variations in North America
AbstractInterannual and multidecadal time-scale anomalies in sea surface temperatures (SST) of the North Atlantic and North Pacific Oceans could result in persistent atmospheric circulation and regional precipitation anomalies for years to decades. Understanding the processes that connect such SST forcings with circulation and precipitation anomalies is thus important for understanding climate variations and for improving predictions at interannual–decadal time scales. This study focuses on the interrelationship between the Atlantic multidecadal oscillation (AMO) and El Nino–Southern Oscillation (ENSO) and their resulting interannual to multidecadal time-scale variations in summertime precipitation in North America. Major results show that the ENSO forcing can strongly modify the atmospheric circulation variations driven by the AMO. Moreover, these modifications differ considerably between the subtropics and the mid- and high-latitude regions. In the subtropics, ENSO-driven variations in precipitation are...
16588748
Impact of Atlantic multidecadal oscillations on India/Sahel rainfall and Atlantic hurricanes
[3] Linkages between the AMO, Sahel summer rainfall and Atlantic Hurricane activity during the 20th century were mainly based on statistical analyses of observed data. A previous modeling study [Vitart and Anderson, 2001] simulated the impact of interdecadal variability in Atlantic SST on Atlantic Hurricane activity. In this study, with both statistical analyses of observed data and carefully designed experiments using the GFDL CM2.1 climate model, we show that the AMO plays a major role in forcing the 20th century multidecadal variations of India and Sahel summer rainfall, and of tropical Atlantic atmospheric circulation that is of crucial relevance for Atlantic Hurricane activity. This leads to an in-phase relationship among low frequency India and Sahel summer rainfall, and Atlantic hurricane activity. This observed variability falls within the range spanned by the individual ensemble members of our modeling experiment. In this study, we adopt the above commonly accepted definition for both the observed and modeled AMO index, as that used in many previous studies [Enfield et al., 2001; Sutton and Hodson, 2005; Knight et al., 2005]. The simple detrending applied in this definition of the AMO may not cleanly separate the THC-induced AMO signal from the anthropogenic climate change and the exact mechanism causing such defined AMO is still uncertain. However, our focus here is not on the definition or mechanism of the AMO, but on the response of the global climate system to the Atlantic multidecadal fluctuations. Here our modeling results are from just one climate model, and need to be further tested with other models. The idealized linear detrending applied to the observed variability discussed in this paper does not necessarily remove all the influence of changing external forcings. Fluctuations of the external forcings about a linear trend may have contributed to some of the observed multidecadal variability. 2. Analyses of the Impact of AMO With Observed Data
56030998
Sahel rainfall and decadal to multi-decadal sea surface temperature variability
Decadal Sahelian rainfall variability was mainly driven by sea surface temperatures (SSTs) during the twentieth century. At the same time SSTs showed a marked long-term global warming (GW) trend. Superimposed on this long-term trend decadal and multi-decadal variability patterns are observed like the Atlantic Multidecadal Oscillation (AMO) and the inter-decadal Pacific Oscillation (IPO). Using an atmospheric general circulation model we investigate the relative contribution of each component to the Sahelian precipitation variability. To take into account the uncertainty related to the use of different SST data sets, we perform the experiments using HadISST1 and ERSSTv3 reconstructed sets. The simulations show that all three SST signals have a significant impact over West Africa: the positive phases of the GW and the IPO lead to drought over the Sahel, while a positive AMO enhances Sahel rainfall. The tropical SST warming is the main cause for the GW impact on Sahel rainfall. Regarding the AMO, the pattern of anomalous precipitation is established by the SSTs in the Atlantic and Mediterranean basins. In turn, the tropical SST anomalies control the impact of the IPO component on West Africa. Our results suggest that the low-frequency evolution of Sahel rainfall can be interpreted as the competition of three factors: the effect of the GW, the AMO and the IPO. Following this interpretation, our results show that 50% of the SST-driven Sahel drought in the 1980s is explained by the change to a negative phase of the AMO, and that the GW contribution was 10%. In addition, the partial recovery of Sahel rainfall in recent years was mainly driven by the AMO.
130574275
Southward shift of the Northern tropical belt from 1945 to 1980
The width of the tropical belt affects the subtropical dry zones and has expanded since 1980. Analyses of observations and climate–chemistry model simulations suggest that the northern tropical edge retracted between 1945 and 1980.
84055293
Communication of the role of natural variability in future North American climate
As climate models improve, decision-makers' expectations for accurate climate predictions are growing. Natural climate variability, however, limits climate predictability and hampers the ability to guide adaptation in many regions such as North America. Scientists, policymakers and the public need to improve communication and avoid raising expectations for accurate regional predictions everywhere.
128547414
Variability of Tropical Cyclone Track Density in the North Atlantic: Observations and High-Resolution Simulations*
AbstractInterannual–decadal variability of tropical cyclone (TC) track density over the North Atlantic (NA) between 1979 and 2008 is studied using observations and simulations with a 25-km-resolution version of the High Resolution Atmospheric Model (HiRAM) forced by observed sea surface temperatures (SSTs). The variability on decadal and interannual time scales is examined separately. On both time scales, a basinwide mode dominates, with the time series being related to variations in seasonal TC counts. On decadal time scales, this mode relates to SST contrasts between the tropical NA and the tropical northeast Pacific as well as the tropical South Atlantic, whereas on interannual time scales it is controlled by SSTs over the central–eastern equatorial Pacific and those over the tropical NA. The temporal evolution of the spatial distribution of track density is further investigated by normalizing the track density with seasonal TC counts. On decadal time scales, two modes emerge: one is an oscillation bet...
16023249
Forced and Internal Variability of Tropical Cyclone Track Density in the Western North Pacific
AbstractForced interannual-to-decadal variability of annual tropical cyclone (TC) track density in the western North Pacific between 1979 and 2008 is studied using TC tracks from observations and simulations by a 25-km-resolution version of the GFDL High-Resolution Atmospheric Model (HiRAM) that is forced by observed sea surface temperatures (SSTs). Two modes dominate the decadal variability: a nearly basinwide mode, and a dipole mode between the subtropics and lower latitudes. The former mode links to variations in TC number and is forced by SST variations over the off-equatorial tropical central North Pacific, whereas the latter might be associated with the Atlantic multidecadal oscillation. The interannual variability is also controlled by two modes: a basinwide mode driven by SST anomalies of opposite signs located in the tropical central Pacific and eastern Indian Ocean, and a southeast–northwest dipole mode connected to the conventional eastern Pacific ENSO. The seasonal evolution of the ENSO effect...
128405504
Investigating soil moisture-climate interactions in a changing climate: A review
Abstract Soil moisture is a key variable of the climate system. It constrains plant transpiration and photosynthesis in several regions of the world, with consequent impacts on the water, energy and biogeochemical cycles. Moreover it is a storage component for precipitation and radiation anomalies, inducing persistence in the climate system. Finally, it is involved in a number of feedbacks at the local, regional and global scales, and plays a major role in climate-change projections. In this review, we provide a synthesis of past research on the role of soil moisture for the climate system, based both on modelling and observational studies. We focus on soil moisture–temperature and soil moisture–precipitation feedbacks, and their possible modifications with climate change. We also highlight further impacts of soil moisture on climate, and the state of research regarding the validation of the relevant processes. There are promises for major advances in this research field in coming years thanks to the development of new validation datasets and multi-model initiatives. However, the availability of ground observations continues to be critical in limiting progress and should therefore strongly be fostered at the international level. Exchanges across disciplines will also be essential for bridging current knowledge gaps in this field. This is of key importance given the manifold impacts of soil moisture on climate, and their relevance for climate-change projections. A better understanding and quantification of the relevant processes would significantly help to reduce uncertainties in future-climate scenarios, in particular with regard to changes in climate variability and extreme events, as well as ecosystem and agricultural impacts.
53418312
Decadal predictability of soil water, vegetation, and wildfire frequency over North America
The potential decadal predictability of land hydrological and biogeochemical variables in North America is examined using a 900-year-long pre-industrial control simulation, conducted with the NCAR Community Earth System Model (CESM) version 1.0.3. The leading modes of simulated North American precipitation and soil water storage are characterized essentially by qualitatively similar meridional seesaw patterns associated with the activity of the westerly jet. Whereas the corresponding precipitation variability can be described as a white noise stochastic process, power spectra of vertically integrated soil water exhibit significant redness on timescales of years to decades, since the predictability of soil water storage arises mostly from the integration of precipitation variability. As a result, damped persistence hindcasts following a 1st order Markov process are skillful with lead times of up to several years. This potential multi-year skill estimate is consistent with ensemble hindcasts conducted with the CESM for various initial conditions. Our control simulation further suggests that decadal variations in soil water storage also affect vegetation and wildfire occurrences. The long-term potential predictability of soil water variations in combination with the slow regrowth of vegetation after major disruptions leads to enhanced predictability on decadal timescales for vegetation, terrestrial carbon stock, and fire frequency, in particular in the Southern United States (US)/Mexico region. By contrast, the prediction skill of fire frequency in the Northern US is limited to 1 year. Our results demonstrate that skillful decadal predictions of soil water storage, carbon stock, and fire frequency are feasible with proper initialization of soil conditions. Although the potential predictability in our idealized modeling framework would overestimate the real predictability of the coupled climate-land-vegetation system, the decadal climate prediction may become beneficial for water resource management, forestry, and agriculture.
128382310
Assessing the relative influence of surface soil moisture and ENSO SST on precipitation predictability over the contiguous United States
This study assesses the relative influence of soil moisture memory and tropical sea surface temperature (SST) in seasonal rainfall over the contiguous United States. Using observed precipitation, the NINO3.4 index, and soil moisture and evapotranspiration simulated by a land surface model for 61 years, analysis was performed using partial correlations to evaluate to what extent land surface and SST anomaly of El Nino–Southern Oscillation (ENSO) can affect seasonal precipitation over different regions and seasons. Results show that antecedent soil moisture is as important as concurrent ENSO condition in controlling rainfall anomalies over the U.S., but they generally dominate in different seasons with SST providing more predictability during winter while soil moisture, through its linkages to evapotranspiration and snow water, has larger influence in spring and early summer. The proposed methodology is applicable to climate model outputs to evaluate the intensity of land-atmosphere coupling and its relative importance.
130432216
Robust Arctic sea-ice influence on the frequent Eurasian cold winters in past decades
Severe winters have occurred frequently in mid-latitude Eurasia during the past decade. Simulations with a 100-member ensemble of an atmospheric model detect an influence of declining Arctic sea-ice cover.
128833855
Change in the precipitation intensity of the East Asian summer monsoon projected by CMIP3 models
Future change in precipitation intensity of East Asian summer monsoon is investigated using the present-day climate simulations (1991–2000) and the future climate simulations (2091–2100, A1B emission scenario) by the Couple Model Intercomparison Project 3 (CMIP3) models. Target period is the month from June to July which is the main part of the rainy season over Japan and Korea. In the present-day climate simulations, we have quantitatively evaluated model’s reproducibility of precipitation climatology and Simple Daily precipitation Intensity Index (SDII), using skill S proposed by Taylor (J Geophys Res 106:7183–7192, 2001). Models with higher reproducibility of precipitation climatology tend to show higher reproducibility of precipitation intensity. In the future climate simulations, simple Multi-Model Ensemble (MME) average using all models show the increase of precipitation intensity over almost all regions of East Asia. Introduction of S as weighting factor does not make large difference in the geographical distribution of precipitation intensity change. In case of MME using five best models, the geographical pattern of precipitation intensity change is qualitatively similar to that using all models, but the local magnitude of changes are much affected by the best model. In order to interpret future change in precipitation climatology and SDII, we have calculated change in vertically integrated horizontal transport of moisture. Changes in precipitation climatology and SDII can be interpreted as the moisture convergence change associated with change in horizontal transport of moisture. Large moisture convergence was found due to either intensification or weakening of subtropical high depending on models.
123634783
Annual maximum 5-day rainfall total and maximum number of consecutive dry days over Central America and the Caribbean in the late twenty-first century projected by an atmospheric general circulation model with three different horizontal resolutions
We simulated changes in annual maximum 5-day rainfall (RX5D) and annual maximum number of consecutive dry days (CDD) in Central America, Mexico, and the Caribbean with three different horizontal resolution atmospheric global general circulation models (AGCMs) and quantified the uncertainty of the projections. The RX5Ds and CDDs were projected to increase in most areas in response to global warming. However, consistent changes were confined to small areas: for RX5D, both coastal zones of northern Mexico and the Yucatan Peninsula; for CDD, the Pacific coastal zone of Mexico, the Yucatan Peninsula, and Guatemala. All three AGCMs projected that RX5Ds and CDDs averaged over only the land area and over the entire area (land and ocean) would increase. The dependence of RX5D probability density functions on the horizontal resolutions was complex. Precipitation unrelated to tropical cyclones was primarily responsible for the projected increases in the frequency of RX5Ds greater than 300 mm.