Impact of biennial SST oscillation on the Southeast Asian summer monsoon

Abstract:

How the biennial oscillation of global SST, one of the main components of ENSO, affects the Southeast Asia summer monsoon is analyzed. The biennial mode is extracted from the 142-year (1871-2012) Extended Reconstruction SST version 3 data using cyclostationary EOF (CSEOF) analysis. Based on regression analysis in CSEOF space, evolutions of key atmospheric variables are obtained to be consistent with the long-term variation of the biennial mode. Atmospheric variables are derived from the twentieth century (20C) reanalysis version 2 data. The biennial oscillation, primarily in the tropical Pacific, influences the monsoons in the Indo-Pacific region. Summer monsoonal change can be explained in terms of the change in monsoon precipitation accompanied with low-level moisture convergence and large-scale atmospheric circulation. In the equatorial region, SST anomaly directly triggers the vertical motion and horizontal wind such that zonal circulation across the Pacific and Indian Oceans is set up. In the subtropical Asian region, both cyclonic or anticyclonic circulation over the northwestern Pacific and the meridional circulation over the Indo-Pacific region induced by the equatorial SST change affects the Southeast Asian monsoon, and henceforth the monsoon precipitation. When positive SST anomaly develops in the eastern tropical Pacific, precipitation decreases over the tropical Indian Ocean and the Maritime Continent (10°S-5°N, 40°-150°E) and increases over Southeast Asia (5°N-20°N, 90°-150°E). With negative SST anomaly in the eastern tropical Pacific, the situation reverses. Based on the spatio-temporal evolution patterns for key physical variables and corresponding long-term variability, physical link through atmosphere-ocean interactions is explored between the biennial mode of SST and the Southeast Asian summer monsoon.