Seasonal Effects of Indian Ocean Freshwater Forcing in a Regional Coupled Model*

Abstract

Effects of freshwater forcing from river discharge into the Indian Ocean on oceanic vertical structure and the Indian monsoons are investigated using a fully coupled, high-resolution, regional climate model. The effect of river discharge is included in the model by restoring sea surface salinity (SSS) toward observations. The simulations with and without this effect in the coupled model reveal a highly seasonal influence of salinity and the barrier layer (BL) on oceanic vertical density stratification, which is in turn linked to changes in sea surface temperature (SST), surface winds, and precipitation.During both boreal summer and winter, SSS relaxation leads to a more realistic spatial distribution of salinity and the BL in the model. In summer, the BL in the Bay of Bengal enhances the upper-ocean stratification and increases the SST near the river mouths where the freshwater forcing is largest. However, the warming is limited to the coastal ocean and the amplitude is not large enough to give a significant impact on monsoon rainfall.The strengthened BL during boreal winter leads to a shallower mixed layer. Atmospheric heat flux forcing acting on a thin mixed layer results in an extensive reduction of SST over the northern Indian Ocean. Relatively suppressed mixing below the mixed layer warms the subsurface layer, leading to a temperature inversion. The cooling of the sea surface induces a large-scale adjustment in the winter atmosphere with amplified northeasterly winds. This impedes atmospheric convection north of the equator while facilitating it in the austral summer intertropical convergence zone, resulting in a hemispheric-asymmetric response pattern. Overall, the results suggest that freshwater forcing from the river discharges plays an important role during the boreal winter by affecting SST and the coupled ocean-atmosphere interaction, with potential impacts on the broadscale regional climate.

Representation of Bay of Bengal Upper-Ocean Salinity in General Circulation Models

“…Similarly, CFSv2 displays a positive bias in freshwater flux during fall, which is responsible for the positive surface salinity bias (Figure 2k). Consistent with other coupled model studies, precipitation in the atmospheric component of CFSv2 contains strong biases, with overestimation of evaporation resulting in poor representation of surface salinity in the BoB (e.g., Seo et al, 2009). MOM5 displays a weak bias in E−P, which is reflected in a weak positive bias in surface salinity ( Figures 4c and 2e).…”

“…However, factors responsible for such salinity biases in the BoB (both in the surface and in the subsurface) are unclear and need examination. Biases in subsurface salinity and temperature in GCMs can alter the ocean circulation, sea level, vertical mixing, and the coupling between ocean and atmosphere (e.g., Seo et al, 2009;Brown et al, 2013 Ravichandran et al, 2013; a data assimilation product; see Table 1 for details). This article describes details of different models and data sets used in the study, provides insights into salinity biases in the BoB vertical structure and associated processes, and discusses thoughts for future work.…”

Mixed layer variability and chlorophyll <i>a</i> biomass in the Bay of Bengal

“…Gopalakrishna et al, 1988;Prasad, 2004;Narvekar and Prasanna Kumar, 2006). A major conclusion of these studies was that wind-driven mixing and net heat flux largely controlled the seasonal cycle of mixed layer depth in the open Bay of Bengal (see Kirthi et al, 2012, and the references therein), while fresh water flux driven stratification was important in the northern Bay of Bengal during summer monsoon (Han et al, 2001;Shenoi et al, 2002;Anitha et al, 2008;Seo et al, 2009). The present study explored the role of Rossby wave propagation and advection of high salinity Arabian Sea waters in regulating the basin-wide mixed layer depth, in addition to the fluxes of heat and fresh water.…”

Roles of land surface albedo and horizontal resolution on the Indian summer monsoon biases in a coupled ocean–atmosphere tropical-channel model

“…This allows to distinguish the effects of local versus remote 84 forcings on the ISM (Seo et al 2009, Samala et al 2013, to test the sensitivity of the 85 simulated ISM to different physical parameterizations (Mukhopadhyay et al 2010, 86 Srinivas et al 2013, Samson et al 2014 or to prescribe the orography in a more 87 realistic way (Ma et al 2014). But despite those specificities, significant biases still 88 exist in terms of precipitation and surface temperature (Lucas-Picher et al 2011), 89 which suggest that high resolution is not the unique missing ingredient in order to 90 improve ISM rainfall in current CGCMs and RCMs.…”