Oceanic Forcing of Seasonal to Decadal Western Asia Precipitation During Winter

The long-term variability of November-April western Asia precipitation, defined hereafter as the region bound by 25°N-40°N and 40°E-70°E, and its relationship with Pacific sea surface temperatures (SSTs) is examined using a large ensemble of atmospheric model simulations forced by 1901-2012 time-varying boundary conditions. Previous studies have identified the links between western Asia precipitation during November-April and SSTs associated with the El Nino-Southern Oscillation (ENSO) and warming of Pacific SSTs. However, due to data limitations, these studies have focused primarily on the mid-20thcentury and later. Here, a 43-member ensemble of atmospheric model simulations is used to investigate the causality, sensitivity and changing temporal relationships of western Asia precipitation to SST patterns associated with ENSO and warming of Pacific SST. The atmospheric models adequately simulate the November-April western Asia precipitation during 1960-2005 in terms of precipitation means, the leading patterns of precipitation variability, the global SST and atmospheric circulation patterns associated with the leading patterns of precipitation variability and temporal precipitation variability. Using the atmospheric models, we find that a long-term drying of western Asia during ENSO neutral conditions is associated with Pacific warming, with a nearly 10% reduction in the regional precipitation during 1938-2012. The historical influence of ENSO on western Asia precipitation varied in strength: strong prior to the 1950s, weak between 1950-1980, and strongest after the 1980s. These variations were not antisymmetric between ENSO phases. El Niño was persistently related with anomalously wet conditions throughout 1901-2012, whereas La Niña was not closely linked to precipitation anomalies prior to the 1970s, but has been associated with exceptionally dry conditions since. We hypothesize that the immersion of La Nina within a warmer global ocean is responsible for exceptional November-April western Asia dry conditions since the 1970s, and we anticipate for these dry conditions during La Nina to continue as the global oceans warm further.