The effect of seasonally and spatially varying chlorophyll on Bay of Bengal surface ocean properties and the South Asian monsoon

Jack Giddings,Karen J Heywood,Adrian J Matthews,Benjamin G M Webber,Manoj Joshi,Nicholas P Klingaman

doi:10.5194/wcd-1-635-2020

Abstract

Abstract. Chlorophyll absorbs solar radiation in the upper ocean, increasing the mixed layer radiative heating and sea surface temperatures (SST). Although the influence of chlorophyll distributions in the Arabian Sea on the southwest monsoon has been demonstrated, there is a current knowledge gap regarding how chlorophyll distributions in the Bay of Bengal influence the southwest monsoon. The solar absorption caused by chlorophyll can be parameterized as an optical parameter, h2, which expresses the scale depth of the absorption of blue light. Seasonally and spatially varying h2 fields in the Bay of Bengal were imposed in a 30-year simulation using an atmospheric general circulation model coupled to a mixed layer thermodynamic ocean model in order to investigate the effect of chlorophyll distributions on regional SST, the southwest monsoon circulation, and precipitation. There are both direct local upper-ocean effects, through changes in solar radiation absorption, and indirect remote atmospheric responses. The depth of the mixed layer relative to the perturbed solar penetration depths modulates the response of the SST to chlorophyll. The largest SST response of 0.5 ∘C to chlorophyll forcing occurs in coastal regions, where chlorophyll concentrations are high (> 1 mg m−3), and when climatological mixed layer depths shoal during the inter-monsoon periods. Precipitation increases significantly (by up to 3 mm d−1) across coastal Myanmar during the southwest monsoon onset and over northeast India and Bangladesh during the Autumn inter-monsoon period, decreasing model biases.

Highlights

The strong coupling of the Indian Ocean to the atmosphere is a major factor in South Asian monsoon seasonal variability (Ju and Slingo, 1995)
We have identified that the influence of biological warming on the South Asian monsoon strongly depends upon the seasonality of the chlorophyll concentration and the depth of the mixed layer, which is further dependent on the timing of the monsoon itself
The effect of chlorophyll on the sea surface temperatures (SST) is amplified during the inter-monsoon periods when shallow mixed layer depth (MLD) are comparable to the perturbed solar penetration depths

Summary

Introduction

The strong coupling of the Indian Ocean to the atmosphere is a major factor in South Asian monsoon seasonal variability (Ju and Slingo, 1995). Strong southwesterly winds transport heat and moisture from the Indian Ocean surface to sustain deep convection over the Indian subcontinent. The South Asian summer monsoon provides up to 90 % of the annual rainfall for the Indian subcontinent (Vecchi and Harrison, 2002), so it is important to accurately predict the seasonal variability of monsoon rainfall given its economic importance to agriculture and other water-intensive industries. The South Asian monsoon is initiated when lowertropospheric winds, transporting heat and moisture, begin to flow northward from the Equator to the Asian continent in response to increasing summer insolation and increasing land–sea thermal and pressure gradients (grey arrows in Fig. 1; Webster et al, 1998). From June to September (June–July–August–September, JJAS), Published by Copernicus Publications on behalf of the European Geosciences Union

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