Impact of a Narrow Coastal Bay of Bengal Sea Surface Temperature Front on an Indian Summer Monsoon Simulation

Dhrubajyoti Samanta,Saji N Hameed,Malay Ganai,Dachao Jin,Vishnu Thilakan,Suryachandra A Rao,Medha Deshpande

doi:10.1038/s41598-018-35735-3

Abstract

A dry bias in climatological Central Indian rainfall plagues Indian summer monsoon (ISM) simulations in multiple generations of climate models. Here, using observations and regional climate modeling, we focus on a warm coastal Bay of Bengal sea surface temperature (SST) front and its impact on Central Indian rainfall. The SST front, featuring sharp gradients as large as 0.5 °C/100 km, is colocated with a mixed layer depth (MLD) front, in a region where salinity variations are known to control MLD. Regional climate simulations coupling a regional atmospheric model with an ocean mixed layer model are performed. A simulation with observed MLD climatology reproduces SST, rainfall, and atmospheric circulation associated with ISM reasonably well; it also eliminates the dry bias over Central India significantly. Perturbing MLD structure in the simulations, we isolate the SST front’s impact on the simulated ISM climate state. This experiment offers insights into ISM climatological biases in the coupled NCEP Climate Forecast System version-2. We suggest that the warm SST front is essential to Central Indian rainfall as it helps to sustain deep and intense convection in its vicinity, which may be a source for the vortex cores seeding the monsoon low-pressure systems.

Highlights

Intense Indian summer monsoon (ISM) rainfall (Fig. 1a), CFSv2 underestimates rainfall intensity and variations in the synoptic and longer scales
The sea surface temperature (SST) simulated with the mixed-layer formulations (OML_OBS and OML_50M) have a warm bias, because the formulations do not include oceanic processes that remove the heat from the BoB36
Our findings provide fresh insights into the issue of ISM simulations, especially its climatological biases in CGCMs

Summary

Introduction

Intense ISM rainfall (Fig. 1a), CFSv2 underestimates rainfall intensity and variations in the synoptic and longer scales. A better representation of physical processes associated with coupled ocean dynamics, especially the BoB mixing remains a challenge for a realistic simulation for LPSs, Central Indian rainfall and mean ISM climate[37]. Due to the realistic representation of the mixed layer structure, the OML_OBS experiment reproduces the large-scale spatial structure of climatological SST in the BoB (Fig. 2d) reasonably well.

Results

Conclusion