Strikingly contrasting Indian monsoon progressions during 2013 and 2014: role of Western Tibetan Plateau and the South China Sea

This study presents some new perspectives on the progression of the Indian summer monsoon (June through September) during two contrasting summer seasons of 2013 and 2014. Monsoon 2013 witnessed above normal rainfall (105% of long period June-September average rainfall); on the other hand, 2014 experienced a severe drought (87% average rainfall). Furthermore, the south to north progression of monsoon 2013 was very rapid, but very slow during monsoon 2014. The disparity of monsoon progressions during the contrasting monsoons is demonstrated through the interaction between the vertical thermal contrast (VTC) of the upper troposphere over the western Tibetan Plateau (TP) and the South China Sea (SCS): a new dynamic perspective. This new perspective suggests interactions of the large-scale circulation anomalies driven by the thermal wind relation, which causes contrasts in the progression of the Indian monsoon. While during 2013 the SCS provided substantial moisture flux towards the Indian subcontinent, the year 2014 witnessed comparatively very few moisture flux incursions towards the Indian subcontinent. This highlights the role of the SCS in driving the differences during the contrasting monsoons. The role of VTC gradient (i.e., the difference in the VTC between the SCS and the western TP) is further demonstrated using a newly developed methodology, which exhibits causal relations of the VTC gradient between the two aforementioned regions with longwave flux (LWF) at the top of the atmosphere.