Regionalization of a Global Climate Change Scenario for the Bay of Bengal under Special Consideration of Rossby Waves and Coastal Kelvin Waves

Abstract

A projected forecast of hydrographic physical parameters and their variabilities in the Bay of Bengal (BoB) is performed for the 21st century by employing a dynamic downscaling approach using the Hamburg Shelf Ocean Model (HAMSOM). The investigation is based on the high emissions pathway, RCP8.5 scenario. In order to evaluate the impact of climate change caused by anthropogenic factors, a supplementary experiment under a pre-industrial control scenario is also performed. The HAMSOM model is driven by outputs from the MPI-ESM, the Earth System Model developed at the Max-Planck-Institute for Meteorology. The open boundary forces, in terms of lateral open boundary forcing, atmospheric forcing, and river discharge, are bias corrected on the monthly climatological scale relative to the World Ocean Atlas (WOA18) climatology, ERA5 of ECMWF, and the global hydrological model WaterGap, respectively. The overall simulation period is from 1951 to 2099, applying the RCP8.5 scenario from 2006 onwards. The analysis focuses on three aspects of the hydrodynamic conditions in the BoB, i.e., the mechanism of remote forcing modulating interior properties, the oceanic climate change under the RCP8.5 scenario, and the anthropogenic contribution to projected changes. Based on the HAMSOM results under the historical scenario from 1951 to 2005 and other third-party data sets, it is demonstrated that coastal Kelvin waves and associated westward moving Rossby waves play a crucial role in the teleconnection of variations between the equatorial Indian Ocean and the interior BoB. According to the HAMSOM forecast under the RCP8.5 scenario, climate changes of hydrographic parameters generally exhibit significant spatial differences. Compared to the current climate state of 1980-2009, the sea surface temperature in the BoB will rise by 2.55 to 2.91◦C on the climatological scale, while the subsurface temperature at a depth of 100m will rise by 0.39 to 1.52◦C. The seasonality of temperature will be enhanced both at the surface and subsurface. In contrast, the seasonality of salinity will be enhanced at the surface, but weakened in the subsurface. As a consequence of climate change in temperature and salinity, the stratification strength is enhancing and the pycnocline depth is getting shallower in most areas of the BoB. For the upper ocean circulation, one notable change is the weakening of East Indian Coastal Current in winter, leading in turn also to a weakening of the seasonal variability. Interestingly, the variabilities of internal coastal Kelvin waves (CKWs) along the coasts show remarkable changes under the RCP8.5 scenario. The occurrence of significant CKWs will gen- erally become more frequent. Correlations between the CKWs and other variables indicate that the intraseasonal variability from the equator is losing influence on the internal CKWs along the eastern boundary, while the local wind is gaining influence. In addition, the HAMSOM results under the pre-industrial control scenario prove that these predicted changes are mainly attributed to the anthropogenic activities.