Low-Frequency Interactions between the Summer Monsoon and the Northern Hemisphere Westerlies

Abstract

Inter-correlations between the active/break cycles of Indian summer monsoon and the circulation change in the northern middle and high latitude westerlies are investigated particularly relevant to the low frequency (30-50 day period) mode.
Empirical orthogonal functions and composite analysis revealed the standing-type east-west oscillations of the geopotential height field between central and far-east Asia with the node over Tibet.
Lag-correlations between the monsoon trough and the 500mb heights in the northern hemisphere suggest that this east-west oscillation is part of the response of the mid-latitude westerlies to thenorthward-moving monsoon heat source. The response in the higher latitudes seems to reach its maximum when the heat source approaches to the southern periphery of the westerlies; i.e., near to the break phase of monsoon.
A plausible mechanism of this interaction between the monsoon and the westerly flow in the higher latitudes are also briefly discussed.