Spatial and temporal variability of solar penetration depths in the Bay of Bengal and its impact on sea surface temperature (SST) during the summer monsoon
2021
Abstract. Chlorophyll has long been known to influence air–sea gas exchange
and CO 2 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.
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