Variability of the Cold Season Climate in Central Asia. Part I: Weather Types and Their Tropical and Extratropical Drivers

To understand the atmospheric mechanisms resulting in a pronounced cold season climate variability in central Asia, an objective weather-type classification is conducted, utilizing a k-means-based clustering approach applied to 500-hPa geopotential height (GPH) fields. Eight weather types (WT) are identified and analyzed with regard to characteristic pressure patterns and moisture fluxes over Eurasia and specific near-surface climate conditions over central Asia. To identify remote drivers of the central Asian climate, WT frequencies are analyzed for their relationships with tropical and extratropical teleconnection modes. The results indicate an influence of Northern Hemispheric planetary wave tracks on westerly moisture fluxes with positive anomalies of precipitation associated with the formation of a Rossby trough over central Asia. Particularly the propagation of the east Atlantic-western Russia and the Scandinavian patterns is shown to modulate regional climate conditions. Variations of ENSO are shown to affect the frequency of particular WTs because of the formation of an anticyclonic anomaly over the Indian Ocean and an increase of tropical fluxes of moisture and heat into central Asia during El Nino events. Further a WT internal influence of ENSO is distinctly defined, with enhanced moisture supply during the ENSO warm phase. The analysis of climatic trends shows that 50% of observed temperature changes can be assigned to variations of the WT composition, indicating that most likely changing regional circulation characteristics account for the enhanced warming rates in central Asia. Trends of precipitation sums are likewise shown to be associated with changing WT frequencies, although the WT-precipitation relationships include large uncertainties.