Tropical-extratropical interaction associated with the Indian summer monsoon.

摘要

This study investigates the most recurrent coupled pattern of interannual variability between the Northern Hemisphere mid-latitude circulation and the tropical convection (between 15°S and 30°N) during the northern summer (June to September). The leading singular value decomposition (SVD) pattern reveals a significant, coupled interannual variation between a circumglobal teleconnection (CGT) pattern and strong tropical convection pattern associated with the La Nina phase of Equatorial Eastern Pacific sea surface temperature. The CGT, having a zonal wavenumber five structure, is primarily positioned within a waveguide associated with the westerly jetstream. The spatial phases of CGT tend to lock to preferred longitudes. The CGT is accompanied by significant rainfall and surface air temperature anomalies in the continental regions of West Europe, European Russia, India, East Asia and North America. This implies that the CGT may be a source of climate variability and predictability in the midlatitude regions.;This study also reveals a significant, coupled intraseasonal variation between a Rossby wavetrain across the Eurasian continent and the summer monsoon convection in northwestern India and Pakistan (referred to as NISM hereafter). The time-lagged SVD analysis shows that the mid-latitude wavetrain originates from the northeastern Atlantic and traverses Europe to central Asia. The wavetrain enhances the upper-level high pressure and reinforces the convection over the NISM region; meanwhile, it propagates further toward East Asia along the wave guide provided by the westerly jet. After an outbreak of NISM convection, the anomalous central Asian high retreats westward.;On the intraseasonal timescale, extreme active and break phases of the ISM often bring about devastating floods and severe droughts. The concurrent buildup of the anomalous high over Central Asia and the arrival of tropical convection over northern India increase the likelihood of occurrence of a heavy rainy period over the NISM region. Two predictors may be used to predict the extreme active/break phases of the northern ISM: normalized 200-hPa geopotential height over Central Asia and outgoing longwave radiation over southern India. Once the mean of the two predictors exceeds a threshold unit 1.0, an extreme phase is anticipated to occur over northern India after six to seven days.