Consequences of inhibition of mixed‐layer deepening by the West India Coastal Current for winter phytoplankton bloom in the northeastern Arabian Sea

Vijith, V.; Vinayachandran, P. N.; Thushara, V.; Amol, P.; Shankar, D.; Anil, Arga Chandrashekar

doi:10.1002/2016jc012004

Cited by 39 publications

(24 citation statements)

References 56 publications

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“…The TOPAZ ecosystem model simulates the mean distribution of oceanic productivity well Pastor et al, 2013;Sedigh Marvasti et al, 2016), and the biophysical interactions associated with major climatic events including the Indian Ocean Dipole, El Niño-Southern Oscillation and Atlantic Multidecadal Oscillation (Park and Kug, 2014;Park et al, 2014;Gnanadesikan et al, 2014). The model provides a realistic representation of the monsoonal biophysical interactions in the Indian Ocean and has been used to explain the bloom dynamics of the northwestern BoB during the summer monsoon (Thushara and Vinayachandran, 2016) and the northeastern Arabian Sea during winter (Vijith et al, 2016).…”

Section: Model Simulationmentioning

confidence: 99%

Vertical distribution of chlorophyll in dynamically distinct regions of the southern Bay of Bengal

et al. 2019

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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.

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Section: Model Simulationmentioning

confidence: 99%

Vertical distribution of chlorophyll in dynamically distinct regions of the southern Bay of Bengal

et al. 2019

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“…The SW monsoon SST pattern in the modern Arabian Sea with its NW-SE-oriented gradient (Fig. 4a) is strongly modulated by upwelling off Oman and Somalia and the inflow of warm and low-saline surface water from the Bay of Bengal via the WICC (Vijith et al, 2016) (Fig. 2a).…”

Section: Temperature Differences Between Glacial and The Holocenementioning

confidence: 99%

Glacial–interglacial changes and Holocene variations in Arabian Sea denitrification

et al. 2018

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Abstract. At present, the Arabian Sea has a permanent oxygen minimum zone (OMZ) at water depths between about 100 and 1200 m. Active denitrification in the upper part of the OMZ is recorded by enhanced δ 15 N values in the sediments. Sediment cores show a δ 15 N increase during the middle and late Holocene, which is contrary to the trend in the other two regions of water column denitrification in the eastern tropical North and South Pacific. We calculated composite sea surface temperature (SST) and δ 15 N ratios in time slices of 1000 years of the last 25 kyr to better understand the reasons for the establishment of the Arabian Sea OMZ and its response to changes in the Asian monsoon system. Low δ 15 N values of 4-7 ‰ during the last glacial maximum (LGM) and stadials (Younger Dryas and Heinrich events) suggest that denitrification was inactive or weak during Pleistocene cold phases, while warm interstadials (ISs) had elevated δ 15 N. Fast changes in upwelling intensities and OMZ ventilation from the Antarctic were responsible for these strong millennial-scale variations during the glacial. During the entire Holocene δ 15 N values > 6 ‰ indicate a relatively stable OMZ with enhanced denitrification. The OMZ develops parallel to the strengthening of the SW monsoon and monsoonal upwelling after the LGM. Despite the relatively stable climatic conditions of the Holocene, the δ 15 N records show regionally different trends in the Arabian Sea. In the upwelling areas in the western part of the basin, δ 15 N values are lower during the mid-Holocene (4.2-8.2 ka BP) compared to the late Holocene (< 4.2 ka BP) due to stronger ventilation of the OMZ during the period of the most intense southwest monsoonal upwelling. In contrast, δ 15 N values in the northern and eastern Arabian Sea rose during the last 8 kyr. The displacement of the core of the OMZ from the region of maximum productivity in the western Arabian Sea to its present position in the northeast was established during the middle and late Holocene. This was probably caused by (i) reduced ventilation due to a longer residence time of OMZ waters and (ii) augmented by rising oxygen consumption due to enhanced northeast-monsoon-driven biological productivity. This concurs with the results of the Kiel Climate Model, which show an increase in OMZ volume during the last 9 kyr related to the increasing age of the OMZ water mass.

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“…The TOPAZ ecosystem model simulates well the mean distribution of oceanic productivity (Sarmiento et al, 2010;Pastor et al, 2013;Marvasti et al, 2016) Oscillation Gnanadesikan et al, 2014). The model provides a realistic representation of the monsoonal biophysical interactions in the Indian Ocean and has been used to explain the bloom dynamics of northwestern BoB during the summer monsoon (Thushara and Vinayachandran, 2016) and northeastern Arabian Sea during winter (Vijith et al, 2016).…”

Section: Model Simulationmentioning

confidence: 99%

Interactive comment on “Vertical distribution of chlorophyll in dynamically distinct regions of the southern Bay of Bengal” by Venugopal Thushara et al.

Vinayachandran¹

2018

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The Bay of Bengal (BoB) generally exhibits surface oligotrophy, due to nutrient limitation induced by strong salinity stratification. Nevertheless, there are hot spots of high chlorophyll in the BoB where the monsoonal forcings are strong enough to break the stratification; one such region being the southern BoB, east of Sri Lanka. A recent field program 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 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 on 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 thermo-1 cline 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. Stabilising 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 5 monsoon current exhibits 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 10 chlorophyll in the BoB, emphasizing the need for extensive in situ sampling and ecosystem model-based efforts for a better understanding of the bio-physical interactions and the potential climatic feedbacks.

show abstract

Consequences of inhibition of mixed‐layer deepening by the West India Coastal Current for winter phytoplankton bloom in the northeastern Arabian Sea

Cited by 39 publications

References 56 publications

Vertical distribution of chlorophyll in dynamically distinct regions of the southern Bay of Bengal

Vertical distribution of chlorophyll in dynamically distinct regions of the southern Bay of Bengal

Glacial–interglacial changes and Holocene variations in Arabian Sea denitrification

Interactive comment on “Vertical distribution of chlorophyll in dynamically distinct regions of the southern Bay of Bengal” by Venugopal Thushara et al.

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