Preprints
https://doi.org/10.5194/os-2020-125
https://doi.org/10.5194/os-2020-125

  08 Jan 2021

08 Jan 2021

Review status: this preprint is currently under review for the journal OS.

Spatial and temporal variability of solar penetration depths in the Bay of Bengal and its impact on SST during the summer monsoon

Jack Giddings1, Karen J. Heywood1, Adrian J. Matthews2, Manoj M. Joshi1, Benjamin G. M. Webber1, Alejandra Sanchez-Franks3, Brian A. King3, and Puthenveettil N. Vinayachandran4 Jack Giddings et al.
  • 1Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK
  • 2Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences and School of Mathematics, University of East Anglia, Norwich, NR4 7TJ, UK
  • 3National Oceanography Centre, Southampton, SO14 3ZH, UK
  • 4Center for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India

Abstract. Chlorophyll 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 radiadion absorption is observed, which highlights the importance of near-surface ocean processes in modulating mixed layer chlorophyll. Simulations using a one-dimensional K-profile parameterisation 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 one 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.

Jack Giddings et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on os-2020-125', Isabelle Giddy, 22 Feb 2021
  • RC2: 'Comment on os-2020-125', Anonymous Referee #2, 07 Mar 2021

Jack Giddings et al.

Jack Giddings et al.

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Short summary
Little is known about the impact of chlorophyll on SST in the Bay of Bengal (BoB). Solar irradiance measured by an ocean glider and three Argo floats are used to determine the effect of chlorophyll on BoB SST during the 2016 summer monsoon. The Southwest Monsoon Current has high chlorophyll concentrations (~ 0.5 mg m−3) and shallow solar penetration depths (~ 14 m). Ocean mixed layer model simulations show that SST increases by 0.35 °C month−1 with the potential to influence monsoon rainfall.