Pluvial climatic episodes in North America and North Africa: types and correlation with global climate
Abstract Differences in Late Quaternary pluvial chronologies among the world's deserts can be related to fundamental differences in the climatology of those pluvial episodes. Paleoenvironmental records and climate model simulations for two Northern Hemisphere desert regions are used to illustrate two types of pluvial climatic regime. In the higher-latitude deserts of the American Southwest, pluvial conditions during the last glacial maximum were caused by the southward displacement of the westerly jet stream, in turn caused by the blocking effect of Laurentide Ice Sheet and a steepened arctic-equator pressure gradient. Winter seasonality of precipitation and substantially lower mean annual temperatures are reflected in the paleoenvironmental record, and simulated by models of 18,000 yr B.P. climate. A second pluvial climate type which strongly influences the lower-latitude deserts of North Africa is characterized by enhanced summer monsoons, and occurs during interglacials and, perhaps, interstadials. The primary forcing mechanism of the last pluvial episode in the Sahara and Libyan Deserts appears to have been increased solar radiation (Milankovitch forcing), causing thermally enhanced ocean-land pressure gradients and increased flow of maritime tropical air into the interior of the continent. Poleward displacement of the summer monsoon is consistent with floristic evidence for summer seasonality of precipitation, and replicated by model simulations of 9000 and 6000 yr B.P. climate. Chronologies from some deserts, such as the southern portion of the American Southwest, suggest the consecutive occurrence of both types of pluvial climatic regime. Pluvial conditions during the last glacial maximum were brought about by the equatorward displacement of the prevailing westerlies. A subsequent, more subdued pluvial period occurred later, between ∼ 12,500 and 8000 yr B.P., caused by the poleward displacement of summer monsoons. Characterization of the climatology of pluvial episodes generates explanations for their temporal distribution and, therefore, this approach may be useful in assessing the Quaternary chronology of pluvial episodes in other desert regions.