16 September 2017

Decadal Variability of the Indian and Pacific Walker Cells Since the 1960s: Do They Co-vary on Decadal Timescale?


Previous studies have investigated the centennial and multi-decadal trends of the Pacific and Indian Ocean Walker Cells (WCs) during the past century, but have obtained no consensus due to data uncertainties and weak signals of the long-term trends. The warm pool convection exhibits apparent decadal variability, generally co-varies with the Indian and Pacific WCs during winter (Nov-Apr) with enhanced convection corresponding to intensified WCs, and the Indian-Pacific WCs co-vary. 


This paper focuses on decadal variability (periods of one to few decades), by first documenting the variability of the WCs and warm pool convection, and their co-variability since the 1960s using in situ and satellite observations and reanalysis products. It then explores the causes for the variability and co-variability using a Bayesian Dynamic Linear Model (DLM), which can extract non-stationary effects of climate modes. 


During summer (May-Oct), the warm pool convection still highly co-varies with the Pacific WC but does not co-vary with the Indian WC, and the Indian-Pacific WCs are uncorrelated. The wintertime coherent variability results from the vital influence of ENSO decadal variation, which reduces warm pool convection and weakens the WCs during El Niño-like conditions. During summer, while ENSO decadal variability still dominates the Pacific WC, decadal variations of ENSO, IOD, Indian summer monsoon convection and tropical Indian Ocean SST have comparable effects on the Indian WC overall, with monsoon convection having the largest effect since the 1990s. The complex causes for the Indian WC during summer result in its poor co- variability with the Pacific WC and warm pool convection.

Aixue Hu
National Center for Atmospheric Research (NCAR)