Abstract

Abstract. The Bay of Bengal (BoB) generally exhibits surface oligotrophy due to nutrient limitation induced by strong salinity stratification. Nevertheless, there are hotspots of high chlorophyll in the BoB where the monsoonal forcings are strong enough to break the stratification; one such region is the southern BoB, east of Sri Lanka. A recent field programme conducted during the summer monsoon of 2016, as a part of the Bay of Bengal Boundary Layer Experiment (BoBBLE), provides a unique high-resolution dataset of the vertical distribution of chlorophyll in the southern BoB using ocean gliders along with shipboard conductivity–temperature–depth (CTD) measurements. Observations were carried out for a duration of 12–20 days, covering the dynamically active regions of the Sri Lanka Dome (SLD) and the Southwest Monsoon Current (SMC). Mixing and upwelling induced by the monsoonal wind forcing enhanced surface chlorophyll concentrations (0.3–0.7 mg m−3). Prominent deep chlorophyll maxima (DCM; 0.3–1.2 mg m−3) existed at intermediate depths (20–50 m), signifying the contribution of subsurface productivity to the biological carbon cycling in the BoB. The shape of chlorophyll profiles varied in different dynamical regimes; upwelling was associated with sharp and intense DCM, whereas mixing resulted in a diffuse and weaker DCM. Within the SLD, open-ocean Ekman suction favoured a substantial increase in chlorophyll. Farther east, where the thermocline was deeper, enhanced surface chlorophyll was associated with intermittent mixing events. Remote forcing by the westward propagating Rossby waves influenced the upper-ocean dynamics and chlorophyll distribution in the southern BoB. Stabilizing surface freshening events and barrier-layer formation often inhibited the generation of surface chlorophyll. The pathway of the SMC intrusion was marked by a distinct band of chlorophyll, indicating the advective effect of biologically rich Arabian Sea waters. The region of the monsoon current exhibited the strongest DCM as well as the highest column-integrated chlorophyll. Observations suggest that the persistence of DCM in the southern BoB is promoted by surface oligotrophy and shallow mixed layers. Results from a coupled physical–ecosystem model substantiate the dominant role of mixed layer processes associated with the monsoon in controlling the nutrient distribution and biological productivity in the southern BoB. The present study provides new insights into the vertical distribution of chlorophyll in the BoB, emphasizing the need for extensive in situ sampling and ecosystem model-based efforts for a better understanding of the biophysical interactions and the potential climatic feedbacks.

Highlights

  • The Bay of Bengal (BoB) is fascinating, with its unique upper-ocean features strongly linked to the Indian Summer Monsoon (ISM) variability (Gadgil et al, 1984; Vecchi and Harrison, 2002; Shankar et al, 2007)

  • The Bay of Bengal Boundary Layer Experiment (BoBBLE) field programme coincided with a suppressed phase of the Boreal Summer Intraseasonal Oscillation (BSISO), when the convective activity was weak over the southern BoB

  • In the presence of heavy cloud cover associated with the monsoon, light availability is generally believed to limit the growth of phytoplankton in the Bay of Bengal

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Summary

Introduction

The Bay of Bengal (BoB) is fascinating, with its unique upper-ocean features strongly linked to the Indian Summer Monsoon (ISM) variability (Gadgil et al, 1984; Vecchi and Harrison, 2002; Shankar et al, 2007). Though the basin-averaged productivity is weak in the BoB, satellite and in situ observations reveal the presence of intense regional chlorophyll blooms (Vinayachandran and Mathew, 2003; Kumar et al, 2004, 2007). Past observational studies (Vinayachandran and Mathew, 2003; Vinayachandran et al, 2004; Kumar et al, 2004, 2007; Jyothibabu et al, 2015) have contributed to our understanding of the biological productivity in the BoB, suggesting that the dynamics controlling the chlorophyll distribution are complex, determined by the competing effects of winds (local as well as remote) and freshwater flux on the mixed layer processes. A summary and conclusions are given in the last section

Observations and modelling
In situ measurements of chlorophyll
Coupled physical–ecosystem model
Results and discussion
Hydrography
Surface bloom events
Deep chlorophyll maxima
Role of light limitation
Model simulation
Simulated chlorophyll distribution
Mixed layer nutrient budget
Summary and conclusions
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