Modulation of the interannual variation of the India-Burma Trough on the winter moisture supply over Southwest China

Anomalous moisture circulation associated with the interannual variation of the India-Burma Trough was investigated to examine how the trough affects the moisture supply to Southwest China in winter. When the India-Burma Trough is strong, abnormal moisture is transported to Southwest China mainly via the southern boundary in the 750-650 hPa layer instead of via the western boundary, in contrast to the climatological situation. The abnormal moisture convergence is constrained in a thin layer along the windward side of the Tibetan Plateau. The moisture circulation response to the variation of the India-Burma Trough shows great baroclinicity over the Bay of Bengal (BOB). Northwesterly transport behind and southwesterly transport ahead of the trough appear only between 850 and 650 hPa, with anticyclonic moisture circulation underneath and cyclonic moisture circulation on top. Beyond the BOB, the moisture circulation response can be traced upstream with abnormal anticyclonic/cyclonic moisture circulations over the Mediterranean Sea and Arabian Sea/Red Sea above 850 hPa, and downstream with anticyclonic moisture circulation over the Philippine Sea near the surface. The activity of the India-Burma Trough is not a local phenomenon, but a part of the South Asian jet wave trains, and is modulated by the ENSO via the Philippine Sea anticyclone. Further investigation of the separate role of wind and moisture disturbances shows that the abnormal moisture flux is dominated by the wind disturbance, as is the abnormal moisture divergence in the lower level; while that in the upper level is dominated by both the wind and moisture disturbances.