Quantifying the effect of chlorophyll on upper ocean processes and regional climate.

Through the absorption of solar radiation, chlorophyll influences mixed-layer radiative heating and sea surface temperatures and hence influences regional climate. Although the effect of chlorophyll on the El Niño-Southern Oscillation and the South Asian summer monsoon has been examined by previous studies, little is known about the effect of chlorophyll in the Bay of Bengal on the southwest monsoon. The absorption of solar radiation by chlorophyll is represented by varying the scale depth of blue light, h2, in ocean general circulation models. Here, a two-band solar absorption scheme is fitted to in-water photosynthetically active radiation profiles, measured from ocean gliders and profiling floats, to determine h2 across the southern Bay of Bengal during the 2016 southwest monsoon. Values of h2 are low (⇠14 m) in the Southwest Monsoon Current, Sri Lanka Dome and Bay of Bengal coastal regions when chlorophyll concentrations are high (0.3-0.5 mg m−3). A one-month surface-forced idealised simulation, using a one-dimensional K-profile parameterisation ocean mixed layer model, shows that a 0.3 mg m−3 increase in chlorophyll concentration increases sea surface temperature by 0.37#C in one month. Imposing seasonally and spatially varying h2 in the Bay of Bengal in a coupled ocean-atmosphere model shows that the response of SST to chlorophyll is modulated by the depth of the mixed layer relative to the perturbed h2. The largest SST response to chlorophyll-induced warming occurs in the coastal regions where chlorophyll concentrations are high (>1 mg m−3), and when mixed layer depths shoal during the intermonsoon periods. Precipitation rates increase significantly by up to 3 mm day−1 across coastal Myanmar during the southwest monsoon onset and up to 3 mm day−1 over northeastern India and Bangladesh during the autumn intermonsoon period. The increase in precipitation rates improve model biases. Thus, imposing seasonally varying chlorophyll into models can improve intermonsoon rainfall amount and distribution. This thesis further investigates the effect of chlorophyll on radiant heating rates in the Sub-Antarctic Zone of the Southern Ocean during the austral spring and summer season of 2012/13. The chlorophyll concentration variability depends on the shoaling of the mixed layer depth, which is dependent on the strength of the wind-induced turbulent mixing. Although chlorophyll concentrations are high (>1 mg m−3) during summer, they are too small to modulate the mixed layer temperature