Cold Season Southwest Asia Precipitation Sensitivity to El Niño–Southern Oscillation Events

The sensitivity of Southwest Asia (25˚N-40˚N, 40˚E-70˚E) precipitation during the November-April rainy season to four types of El Niño-Southern Oscillation (ENSO) events, eastern Pacific (EP) and central Pacific (CP) El Niño and La Niña, is assessed using an ensemble of atmospheric model simulations forced by 1979-2015 boundary conditions. Sensitivity is assessed in terms of: (1) the spread of precipitation across the ensemble members around the ensemble mean, (2) the probability of precipitation falling into the upper and lower terciles of the historical distribution and (3) how the tropical atmosphere communicates information between Southwest Asia and ENSO. Such an assessment of Southwest Asia precipitation predictability during ENSO advises the possible failure or extreme excess of precipitation that can severely impact the water resources and the rain-fed farming upon which the people of Afghanistan, Pakistan and Turkmenistan rely.

During CP La Niña, the magnitude of the below average mean precipitation exceeds the magnitude of the precipitation spread, thereby conditioning the probability of above tercile Southwest Asia precipitation to greater than 70%. By contrast, EP La Niña does not alter the odds of Southwest Asia precipitation terciles, as the magnitude of the near-zero mean precipitation is overwhelmed by the magnitude of the precipitation spread. EP and CP El Niño similarly result in above average mean precipitation whose magnitude approaches the magnitude of the precipitation spread, thereby conditioning the probability of upper tercile Southwest Asia precipitation to around 50% region-wide. However, the notable effect of the precipitation spread during El Niño allows for a 20-30% probability that the regional precipitation falls into the lower tercile. Results suggest that the tropical eastern Indian Ocean atmosphere serves as the medium by which ENSO forcing is communicated to Southwest Asia. ENSO types modify the probability of tropical eastern Indian Ocean precipitation and thereby Southwest Asia precipitation probabilities.