Extreme droughts and corresponding Summer Monsoon: A Case Study of 2009 Indian Drought

Drought is a sustained result of continuous surface and atmospheric heating and moisture deficit. In general, drought assessment is made based on various indices. However, the drought dynamics and associated surface energetics about the corresponding Indian Summer Monsoon (ISM, i.e., June, July, August and September-JJAS) still needs to be better understood. Prolonged surface heating causes reduction of soil moisture, surface/subsurface runoff and atmospheric moisture. Excess surface heating results from positive surface energy budget, which is computed using the term, surface net solar radiation (SNSR), surface net thermal radiation (SNTR), surface sensible heat flux (SSHF), and surface latent heat flux (SLHF). It may also be the possible reason for enhanced convection. Convection also depends on the moisture holding capacity of the atmosphere, which increases with increasing air temperature. These processes lead to moisture content deficit and rainfall suppression due to moisture divergence from the convective sources. India Meteorological Department (IMD) (station and 0.25° gridded) and European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim reanalysis (0.25° resolution) datasets are used to study the extreme drought events. Drought indices such as Standardized Precipitation Index (SPI), Palmer Drought Severity Index (PDSI), Standardized Precipitation Evapotranspiration Index (SPEI), Soil Moisture Index (SMI) and Sensible Heat Index (SHI) are used. Results indicate that anomalous lowering of available surface soil moisture and increase of surface sensible heat flux is a possible cause for enhancing extreme drought during the 2009 ISM. The associated Hadley circulation shows anomalous weakening, which led to reduced northward moisture transport from the southern oceans, further acerbating moisture deficit. Most parts of India suffered from anomalous decrement in specific humidity in the lower to upper troposphere and related precipitation scarcity during the drought period. The atmosphere’s increased moisture holding capacity sets a weak monsoon due to moisture divergence from ocean/local convection. However, likely, the excessive surface warming (due to SNSR/SNTR trapped into the surface) led to the extreme drought during 2009 ISM.