Differences in the Timing of Peak Precipitation and Peak Storm Track Activity over Central Southwest Asia

Understanding the nature of precipitation over central southwest Asia (CSWA), a data-sparse, semi-arid region, is important given its relation to agricultural productivity and the likelihood of hazards such as flooding. The present study considers how daily precipitation and local vertical motion – represented by warm conveyor belts (WCBs) – evolve from November to April over CSWA. We first compare several precipitation datasets, indicating that the seasonality of precipitation is consistent, while accumulation totals differ. A common feature across datasets is that the majority of precipitation occurs on the few days when area-averaged accumulation exceeds 4 mm, which are most frequent in February and March, and is offset from the timing of peak upper-level storm track activity in January.

The circulation pattern associated with heavy (> 4 mm day^-1) precipitation days evolves within the wet season from a southwest-northeast tilted couplet of circulation anomalies in January and February to a neutrally-tilted monopole pattern in April. El Niño conditions are associated with more heavy precipitation days than La Niña conditions, with both enhanced WCB frequency and moisture transport observed during the former. An exception to this is found in January when precipitation, WCB frequency and moisture do not increase, despite a similar increase in surface cyclones to other months, suggesting precipitation changes cannot always be inferred from cyclone frequency changes.