A Physical Model for Extreme Drought over Southwest Asia

Abstract:

The socioeconomic difficulties of Southwest Asia, defined as the area bound by the domain 25°N-40°N and 40°E-70°E, which includes the countries of Iran, Afghanistan and Pakistan, are exacerbated by extreme precipitation deficits during the November-April rainy season. The precipitation deficits during many Southwest Asia droughts have been examined in terms of the forcing by climate variability originating over the Pacific Ocean as a result of the El Niño-Southern Oscillation (ENSO), Pacific Decadal Variability (PDV) and the long-term warming of Pacific (LT) sea surface temperatures (SST). Here, we 1) examine how the most extreme November-April Southwest Asia droughts relate to global SSTs and the associated large-scale atmospheric circulation anomalies, 2) analyze the specific atmospheric forcing mechanisms responsible for changes in regional Southwest Asian precipitation and 3) examine the causal mechanisms responsible for the increased frequency of Southwest Asia drought in recent decades.

The driest November-April seasons during 1948-2012 over Southwest Asia are forced by subsidence and reductions of moisture fluxes as a result of the interaction of the mean flow with anomalous zonally-symmetric high pressure throughout the Northern Hemisphere. The anomalous zonally-symmetric high pressure throughout the Northern Hemisphere occurs simultaneously with cool central and eastern Pacific SST anomalies associated with La Niña and the negative phase of PDV and a warm west Pacific Ocean caused in part by the long-term warming of the west Pacific Ocean. The long-term warming of the Pacific Ocean has driven the regional precipitation declines in recent decades, with the strongest signal occurring over areas bordering the Arabian Sea.