| Summary: | <p>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, <span class="inline-formula"><i>h</i><sub>2</sub></span>, which expresses the scale depth of the absorption of blue light.
Seasonally and spatially varying <span class="inline-formula"><i>h</i><sub>2</sub></span> 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 <span class="inline-formula"><sup>∘</sup></span>C to chlorophyll
forcing occurs in coastal regions, where chlorophyll concentrations are high
(<span class="inline-formula"><i>></i> 1</span> mg m<span class="inline-formula"><sup>−3</sup></span>), and when climatological mixed layer depths
shoal during the inter-monsoon periods. Precipitation increases significantly
(by up to 3 mm d<span class="inline-formula"><sup>−1</sup></span>) across coastal Myanmar during the southwest monsoon
onset and over northeast India and Bangladesh during the Autumn inter-monsoon
period, decreasing model biases.</p>
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