SEOE Seminar Series: Dr. Chellappan Gnanseelan, Indian Institute of Tropical Meterology PuneThursday, September 7, 2017 -
Title: "Indian Ocean warming: the causes, feedbacks and impact on regional climate"
The increased rate of Tropical Indian Ocean (TIO) surface warming has gained a lot of attention in the recent years mainly due to its regional climatic impacts. The processes associated with this increased surface warming is highly complex and none of the mechanisms in the past studies could comprehend the important features associated with this warming such as the negative trends in surface net heat fluxes and the decreasing temperature trends at thermocline level. The changing surface wind patterns, the subsequent changes in the ocean dynamics, the changes in the large scale surface ocean circulation pattern etc. modulate the surface warming pattern over TIO. The changes in the large scale mass and salt exchange between different basins are also found to be playing an important role in locating the warming pattern to the central Indian Ocean. Changes in the meridional overturning circulation and the oceanic pathways such as Indonesian Through Flow contribute considerably on the Indian Ocean warming. Ocean reanalysis datasets and a suit of Ocean General Circulation Model (OGCM) experiments are used to address this challenging problem. Both reanalysis and OGCM reveal strengthening large scale surface circulation pattern in the recent years. The most striking feature is the intensification of cyclonic gyre circulation around the thermocline ridge region in the southwestern TIO. The study reveals a positive feedback mechanism between changing atmospheric westerlies and the ocean circulation changes supporting the strengthening of SST trends in the equatorial Indian Ocean. The OGCM experiments showed that the wind induced circulation changes redistribute the excess heat received in the western TIO to central and east thereby enhancing warming in the central equatorial Indian Ocean. The increased surface warming in central TIO increases the latent heat loss, and keeps the net heat flux trends negative. On the other hand warming induces more convection over the central TIO and subsidence over the Indian land mass affecting the rainfall trends. Model sensitivity experiments reveal that the subsurface cooling at thermocline level in TIO is contributed mostly by variability in Pacific via Indonesian Through Flow whereas the surface warming trend is mainly induced by the changes in the local forcing. The long term changes in Indian Ocean Rossby waves are also primarily induced by Pacific forcing. The thermocline shoaling in the west is therefore amplified by the remote influence of Pacific via wave transmission through Indonesian archipelago.