Effects of climate modes on decadal variability of upwelling in the tropical Indian Ocean

In the tropical Pacific and Atlantic Oceans, easterly trade winds prevail and upwelling occurs in the eastern equatorial basin. In contrast, the Indian Ocean (IO) is subject to intense seasonally reversing monsoon wind forcing and the annual mean prevailing winds in the equatorial basin are westerlies. Corresponding to the wind stress forcing, there are two Upwelling regions in the Indian Ocean, namely the mean upwelling zone of the Seychelles-Chagos thermocline ridge (SCTR) and the seasonal upwelling area of the Eastern tropical Indian Ocean (EIO). The upwelling in the Indian Ocean is mainly caused by two climate modes, namely, ENSO and IOD. Previous studies have been trying to understand the impact from these two climate modes but the contribution from each mode has never been quantified. Through analyzing observational datasets from 1993-2016 using the conventional Static Linear regression Model (SLM) and Bayesian Dynamical Linear Model (DLM), and performing model experiments using a linear ocean model, we showed that on interannual scale, ENSO is more important than the IOD over the SCTR region, however they play comparable roles in the EIO. On decadal time scale, What the relative importance of IPO and IOD is in driving the Western Indian Ocean and Eastern Indian Ocean upwelling as well as the related dynamics are still not clear. Furthermore how important the monsoon is and why there is a distinct reversal of basin-wide sea level trend patterns from 1993-2000 to 2000-2006 as observed in the Indian ocean is still not fully explained. In this work, through analyzing observational datasets from 1958-2019 using SLM and Bayesian DLM, and combing with the linear ocean model, we are trying to answer these questions. Preliminary results have been obtained but the quantified estimation and the related dynamics are currently still under investigation, and an up-to date analysis will be presented.