Indo-Pak heavy rainfall events: Role of Indo-Pacific SST and large-scale dynamics

Special Symposium on the South Asia Monsoon, Annual AMS meeting at Pheonix, Arizona, USA on 8th January 2015. Topic: Monsoon Dynamics

Indo-Pak heavy rainfall events: Role of Indo-Pacific SST and large-scale dynamics.

Milind Mujumdar1, Priya P. 1, Sabin T.P. 1, Pascal Terray1,2,3 and Krishnan R. 1 1Centre for Climate Change Research, Indian Institute of Tropical Meteorology, Pune 411008, India 2Sorbonne Universités (UPMC, Univ Paris 06)-CNRS-IRD-MNHN, LOCEAN Laboratoire, 4 place Jussieu, Paris, France 3Indo-French cell for Water Sciences, IISc-IITM-NIO–IRD Joint International Laboratoire, IITM, Pune

Abstract The evolution of recent heavy rainfall over northwest Pakistan and adjacent Indian region during 2010 is diagnosed through a set of simulation experiments with a very high-resolution atmospheric model. The analysis of these heavy rainfall events in observations depicts the westward shift of the large-scale circulation over Indo-Pacific sector and the northwest Pacific sub-tropical high as well as an anomalous northward moisture transport from western Indian Ocean (IO) during the strong 2010 La Niña episode. The realistic simulation of 2010 Indo-Pak heavy rainfall events using real-time SST boundary conditions motivated us to explore the specific influence of IO and Pacific SST anomalies through additional simulation experiments using ENSO-related and ENSO-unrelated SST forcings. As expected, the simulation using ENSO-related SST anomalies during 2010 induces a westward shift of the large-scale monsoon circulation and significantly weakens the convection over Bay of Bengal (BoB). However, the Indo-Pak heavy rainfall anomalies are poorly simulated. On the other hand, the ENSO-unrelated SST forcing, with warm anomalies over the southeast IO and north Arabian Sea (AS), significantly strengthens the cyclonic convergence (divergence) over southeastern IO (BoB) and promotes enhanced convection over northern AS. This in turn, intensifies the northward moisture transport from AS into the subtropical Indo-Pak region and leads to positive rainfall anomalies as observed in 2010. Thus, ENSO-unrelated SST anomalies play a key role in inducing the northward moisture transport from AS into sub-tropical Indo-Pak region in the background of the modulated large-scale summer monsoon circulation by ENSO during 2010.