Abstract
Abstract. Chlorophyll has long been known to influence air–sea gas exchange and CO2 drawdown. But chlorophyll also influences regional climate through its effect on solar radiation absorption and thus sea surface temperature (SST). In the Bay of Bengal, the effect of chlorophyll on SST has been demonstrated to have a significant impact on the Indian summer (southwest) monsoon. However, little is known about the drivers and impacts of chlorophyll variability in the Bay of Bengal during the southwest monsoon. Here we use observations of downwelling irradiance measured by an ocean glider and three profiling floats to determine the spatial and temporal variability of solar absorption across the southern Bay of Bengal during the 2016 summer monsoon. A two-band exponential solar absorption scheme is fitted to vertical profiles of photosynthetically active radiation to determine the effective scale depth of blue light. Scale depths of blue light are found to vary from 12 m during the highest (0.3–0.5 mg m−3) mixed-layer chlorophyll concentrations to over 25 m when the mixed-layer chlorophyll concentrations are below 0.1 mg m−3. The Southwest Monsoon Current and coastal regions of the Bay of Bengal are observed to have higher mixed-layer chlorophyll concentrations and shallower solar penetration depths than other regions of the southern Bay of Bengal. Substantial sub-daily variability in solar radiation absorption is observed, which highlights the importance of near-surface ocean processes in modulating mixed-layer chlorophyll. Simulations using a one-dimensional K-profile parameterization ocean mixed-layer model with observed surface forcing from July 2016 show that a 0.3 mg m−3 increase in chlorophyll concentration increases sea surface temperature by 0.35 ∘C in 1 month, with SST differences growing rapidly during calm and sunny conditions. This has the potential to influence monsoon rainfall around the Bay of Bengal and its intraseasonal variability.
Highlights
Absorption of incoming solar radiation at the ocean surface modulates the upper-ocean heat content, which controls the exchange of heat and moisture to the lower troposphere (Zaneveld et al, 1981; Lewis et al, 1990)
The Sri Lanka Dome (SLD) is a prominent feature in the southwest Bay of Bengal (BoB) during the summer monsoon and is typically associated with high surface chlorophyll concentrations (Thushara et al, 2019)
Chlorophyll concentrations peak on 1 July with values of 0.8 mg m−3 at a depth of 18 m, indicating high surface chlorophyll concentrations (Fig. 3d)
Summary
Absorption of incoming solar radiation at the ocean surface modulates the upper-ocean heat content, which controls the exchange of heat and moisture to the lower troposphere (Zaneveld et al, 1981; Lewis et al, 1990). In the Arabian Sea, the inclusion of seasonally varying chlorophyll due to phytoplankton blooms in a coupled climate model led to a 50 % reduction in mixed-layer depth (MLD) biases, an increase in local SST, and a subsequent increase in rainfall of up to 2 mm d−1 over western India during the southwest monsoon onset (Turner et al, 2012). Coupling a biogeochemistry model to a coupled ocean–atmosphere GCM to derive chlorophyll-dependent attenuation rates of solar radiation led to an SST increase of 1 ◦C in the Arabian Sea during autumn and an increase in summer monsoon rainfall of 3 mm d−1 along the west coast of India (Wetzel et al, 2006). Little is known about the influence of surface chlorophyll on the temporal and spatial variability of solar penetration depths across the BoB and how surface chlorophyll directly impacts SST during the summer southwest monsoon
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