Oceanic Origins of Historical Southwest Asia Precipitation During the Boreal Cold Season

While a strong influence on cold season southwest Asia precipitation by Pacific sea surface temperatures (SSTs) has been previously established, the scarcity of southwest Asia precipitation observations prior to 1960 renders the region's long-term precipitation history largely unknown. Here a large ensemble of atmospheric model simulations forced by observed time-varying boundary conditions for 1901-2012 is used to examine the long-term sensitivity of November-April southwest Asia precipitation to Pacific SSTs. It is first established that the models are able to reproduce the key features of regional variability during the best-observed 1960-2005 period and then the pre-1960 variability is investigated using the model simulations. During the 1960-2005 period, both the mean precipitation and the two leading modes of precipitation variability during November-April are reasonably simulated by the atmospheric models, which include the previously identified relationships with El Nino-Southern Oscillation (ENSO) and the multidecadal warming of Indo-Pacific SSTs. Over the full 1901-2012 period, there are notable variations in precipitation and in the strength of the SST influence. A long-term drying of the region is associated with the Indo-Pacific warming, with a nearly 10% reduction in western most southwest Asia precipitation during 1938-2012. The influence of ENSO on southwest Asia precipitation varied in strength throughout the period: strong prior to the 1950s, weak between 1950 and 1980, and strongest after the 1980s. These variations were not antisymmetric between ENSO phases. El Nino was persistently related with anomalously wet conditions throughout 1901-2012, whereas La Nina was not closely linked to precipitation anomalies prior to the 1970s but has been associated with exceptionally dry conditions thereafter.