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

2018 ◽  
Author(s):  
Venugopal Thushara ◽  
Puthenveettil Narayana Menon Vinayachandran ◽  
Adrian J. Matthews ◽  
Benjamin G. M. Webber ◽  
Bastien Y. Queste
Keyword(s):  
Sri Lanka ◽  
Vertical Distribution ◽  
Mixed Layer ◽  
Bay Of Bengal ◽  
Dominant Role ◽  
Chlorophyll Concentration ◽  
Ecosystem Model ◽  
Boreal Summer ◽  
South West Monsoon ◽  
The Vertical Distribution

Abstract. The Bay of Bengal (BoB) generally exhibits surface oligotrophy, due to nutrient limitation induced by strong salinity stratification. Nevertheless, there are hot spots of biological activity 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 during a suppressed phase of the Boreal Summer Intraseasonal Oscillation (BSISO), along a longitudinal transect at 8° N, extending from 85.3–89° E, covering the dynamically active regions of the Sri Lanka Dome (SLD) and the South- west Monsoon Current (SMC). Mixing and upwelling induced by the monsoonal wind forcing enhanced chlorophyll concentrations (0.3–0.7 mg m−3) in the surface layers. Observations reveal the presence of prominent deep chlorophyll maxima (DCM; 0.3–1.2 mg m−3) at intermediate depths (20–50 m), generally below the mixed layer and above the thermocline, signifying the contribution of subsurface productivity on the biological carbon cycling in the BoB. The shape of chlorophyll profiles varied in different dynamical regimes indicating that the mechanisms determining the vertical distribution of chlorophyll are intricate; upwelling favoured sharp and intense DCM, whereas mixing resulted in diffuse and weaker DCM. Within the SLD, open ocean Ekman pumping and the doming of thermocline favoured a substantial increase in chlorophyll concentration. Farther east, the thermocline was deeper and moderate surface blooms were triggered by intermittent mixing events. Stabilising surface freshening events and barrier layer formation were often observed to inhibit the surface blooms. The pathway of SMC intrusion was marked by a distinct band of chlorophyll, indicating the advective effect of biologically rich Arabian Sea waters. The region of 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, which reduces the self-shading effect of phytoplankton and shallow mixed layers, which prevent the vertical redistribution of subsurface phytoplankton. 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, which is not captured in satellite mea- surements, emphasizing the need for extensive in situ sampling and ecosystem model-based efforts for a better understanding of the monsoonal bio-physical interactions and the potential climatic feedbacks.

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

2019 ◽  
Vol 16 (7) ◽  
pp. 1447-1468 ◽  
Author(s):  
Venugopal Thushara ◽  
Puthenveettil Narayana Menon Vinayachandran ◽  
Adrian J. Matthews ◽  
Benjamin G. M. Webber ◽  
Bastien Y. Queste
Keyword(s):  
Sri Lanka ◽  
Vertical Distribution ◽  
Bay Of Bengal ◽  
Dominant Role ◽  
Active Regions ◽  
Ecosystem Model ◽  
Ocean Dynamics ◽  
Nutrient Distribution ◽  
Enhanced Surface ◽  
The Vertical Distribution

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.

scholarly journals Seasonal cycle of desert aerosols in western Africa: analysis of the coastal transition with passive and active sensors

2017 ◽  
Vol 17 (13) ◽  
pp. 8395-8410 ◽  
Author(s):  
Habib Senghor ◽  
Éric Machu ◽  
Frédéric Hourdin ◽  
Amadou Thierno Gaye
Keyword(s):  
Vertical Distribution ◽  
Satellite Observations ◽  
Boreal Summer ◽  
Wide Field ◽  
Aerosol Layer ◽  
Good Representation ◽  
Mineral Aerosols ◽  
Western Africa ◽  
The Impact ◽  
The Vertical Distribution

Abstract. The impact of desert aerosols on climate, atmospheric processes, and the environment is still debated in the scientific community. The extent of their influence remains to be determined and particularly requires a better understanding of the variability of their distribution. In this work, we studied the variability of these aerosols in western Africa using different types of satellite observations. SeaWiFS (Sea-Viewing Wide Field-of-View Sensor) and OMI (Ozone Monitoring Instrument) data have been used to characterize the spatial distribution of mineral aerosols from their optical and physical properties over the period 2005–2010. In particular, we focused on the variability of the transition between continental western African and the eastern Atlantic Ocean. Data provided by the lidar scrolling CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) onboard the satellite CALIPSO (Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations) for the period 2007–2013 were then used to assess the seasonal variability of the vertical distribution of desert aerosols. We first obtained a good representation of aerosol optical depth (AOD) and single-scattering albedo (SSA) from the satellites SeaWiFS and OMI, respectively, in comparison with AERONET estimates, both above the continent and the ocean. Dust occurrence frequency is higher in spring and boreal summer. In spring, the highest occurrences are located between the surface and 3 km above sea level, while in summer the highest occurrences are between 2 and 5 km altitude. The vertical distribution given by CALIOP also highlights an abrupt change at the coast from spring to fall with a layer of desert aerosols confined in an atmospheric layer uplifted from the surface of the ocean. This uplift of the aerosol layer above the ocean contrasts with the winter season during which mineral aerosols are confined in the atmospheric boundary layer. Radiosondes at Dakar Weather Station (17.5° W, 14.74° N) provide basic thermodynamic variables which partially give a causal relationship between the layering of the atmospheric circulation over western Africa and their aerosol contents throughout the year. A SSA increase is observed in winter and spring at the transition between the continent and the ocean. The analysis of mean NCEP (National Centers for Environmental Prediction) winds at 925 hPa between 2000 and 2012 suggest a significant contribution of coastal sand sources from Mauritania in winter which would increase SSA over the ocean.

scholarly journals Mixed layer variability and chlorophyll <i>a</i> biomass in the Bay of Bengal

2013 ◽  
Vol 10 (10) ◽  
pp. 16405-16452 ◽  
Author(s):  
J. Narvekar ◽  
S. Prasanna Kumar
Keyword(s):  
Mixed Layer ◽  
Bay Of Bengal ◽  
Mixed Layer Depth ◽  
Chlorophyll Concentration ◽  
Short Wave ◽  
Wave Radiation ◽  
Layer Depth ◽  
Short Wave Radiation ◽  
Deep Mixed Layer ◽  
Monsoon Winds

Abstract. Mixed layer is the most variable and dynamically active part of the marine environment that couples the underlying ocean to the atmosphere and plays an important role in determining the chlorophyll concentration. In this paper we examined the seasonal variability of the mixed layer depth in the Bay of Bengal, the factors responsible for it and the coupling of mixed layer processes to the chlorophyll biomass using a suite of in situ as well as remote sensing data. The basin-wide mixed layer depth was the shallowest during spring intermonsoon, which was associated with strong themohaline stratification of the upper water column. The prevailing winds which were the weakest of all the seasons were unable to break the stratification leading to the observed shallow mixed layer. Consistent with the warm oligotrophic upper ocean, the surface chlorophyll concentrations were the least and the vertical profile of chlorophyll was characterized by a subsurface chlorophyll maximum. Similarly, during summer though the monsoon winds were the strongest they were unable to break the upper ocean haline-stratification in the northern Bay brought about by a combination of excess precipitation over evaporation and fresh water influx from rivers adjoining the Bay of Bengal. Consistent with this though the nitrate concentrations were high in the northern part of the Bay, the chlorophyll concentrations were low indicating the light limitation. In contrast, in the south, advection of high salinity waters from the Arabian Sea coupled with the westward propagating Rossby waves of annual periodicity were able to decrease stability of the upper water column and the prevailing monsoon winds were able to initiate deep mixing leading to the observed deep mixed layer. The high chlorophyll concentration observed in the south resulted from the positive wind stress curl which pumped nutrient rich subsurface waters to the euphotic zone. The southward extension of the shallow mixed layer in fall intermonsoon resulted from the advection of low salinity waters from the northern Bay combined with the secondary heating by the incoming short wave radiation. The satellite-derived chlorophyll pigment concentration during fall intermonsoon was similar to that of summer but with reduced values. The basin-wide deep mixed layer during winter resulted from a combination of reduced short wave radiation, increase in salinity and comparatively stronger winds. The mismatch between the low nitrate and comparatively higher chlorophyll biomass during winter indicated the efficacy of the limited nitrate data to adequately resolve the coupling between the mixed layer processes and the chlorophyll biomass.

Vertical distribution and diel migration of Crangon septemspinosa Say, 1818 (Decapoda, Caridea) on Georges Bank, northwest Atlantic

Crustaceana ◽  
2014 ◽  
Vol 87 (13) ◽  
pp. 1486-1499 ◽  
Author(s):  
Harmon Brown ◽  
Harmon Brown ◽  
Stephen M. Bollens ◽  
Harmon Brown ◽  
Stephen M. Bollens ◽  
...  
Keyword(s):  
Vertical Distribution ◽  
Water Column ◽  
Chlorophyll Concentration ◽  
Georges Bank ◽  
Lunar Periodicity ◽  
Northwest Atlantic ◽  
Diel Migration ◽  
Crangon Septemspinosa ◽  
The Vertical Distribution ◽  
Lower Water Column

We sampled for vertical distribution and possible diel vertical migration (DVM) of Crangon septemspinosa Say, 1818 on and around Georges Bank, Northwest Atlantic, between 1995 and 1999. Both juveniles and adults were found to undergo DVM, being distributed within the lower water column (and perhaps on or in the bottom) during the day, and distributed throughout the water column at night, with higher abundances seen in all depth strata at night. Differences in vertical distribution were also found based on location and chlorophyll concentration for juveniles, but no effects were seen of season, salinity, temperature, lunar periodicity, year, or copepod prey for either juveniles or adults. Variation in vertical distribution and DVM were only moderately well explained (50% of the total variance) by the above factors, suggesting that some other factor(s) not measured by us (e.g., predation) were potentially also controlling the vertical distribution and diel migration of C. septemspinosa on Georges Bank.

scholarly journals Seasonal and inter-annual variability of the sea surface temperature and mixed layer depth in the southern Bay of Bengal

2013 ◽  
Vol 4 (1) ◽  
pp. 70
Author(s):  
R. Ranith ◽  
L. Senthilnathan ◽  
M. Machendiranathan ◽  
T. Thangaradjou ◽  
A. Saravanakumar
Keyword(s):  
Wind Speed ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Mixed Layer ◽  
Bay Of Bengal ◽  
Mixed Layer Depth ◽  
Sea Surface ◽  
Climate Change Scenarios ◽  
Layer Depth ◽  
South West Monsoon

Argo float data supplemented with satellite measurements was used to study the seasonal and inter-annual variation in wind speed, sea surface temperature (SST) and mixed layer depth (MLD) of the southern Bay of Bengal from 2003 through 2010. Due to persistence of wind, clear sky and high insolation an increase in SST by about 2°C is evident during summer months (March-May) and is followed by shallowed MLD with a minimum depth of 9.3 m during summer 2004. MLD reached the maximum depth during monsoon season (November-December) and often extends to post monsoon (February) owing to strong monsoon wind, cloudy sky and SST plummeted by 3°C. During the inter-monsoon period (August-October) the MLD shallowed and maintained a depth of 20–30 m all through the study period. High wind accompanied with moderate temperature (SST) due to the south west monsoon leads to decreased MLD with an average depth of 44 m in July. Analysis of wind speed, SST and MLD suggested that out of various meteorological parameters wind speed and induced mixing are highly influential in MLD formation. Reduced occurrence and amplitude of MLD deepening noticed in recent years can be attributed to the evident climate change scenarios. Large scale upper ocean variability observed from the present study has innumerable antagonistic consequences on the marine ecosystem which is evident from various events of seagrass burns and coral bleaching which have occurred in the last decade.

Effect of physical factors on the vertical distribution of Phytoplankton in Eutrophic Coastal waters

1989 ◽  
Vol 40 (5) ◽  
pp. 559 ◽  
Author(s):  
AJ Gabric ◽  
J Parslow
Keyword(s):  
Vertical Distribution ◽  
Mixed Layer ◽  
Algal Cell ◽  
Vertical Mixing ◽  
Finite Depth ◽  
Physical Factors ◽  
Turbid Waters ◽  
Sea Bed ◽  
Analytical Tools ◽  
The Vertical Distribution

A new analytical model for the vertical distribution of phytoplankton in shallow, turbid waters is presented. The effects of vertical mixing, light-attenuated growth and algal-cell sinking rates are included. The two cases of a finite-depth water column and a mixed layer are investigated. The results for negatively buoyant cells suggest that concentration maxima can occur above the sea bed for either strong or weak mixing in a mixed layer but are always at the bed in finite-depth waters. Large-celled species with high sinking rates are least sustainable in stable water columns, especially a weakly mixed, mixed layer. Conversely, cell sinking rates have little effect on net growth rate when mixing is strong in finite-depth waters. The effect of turbidity on net growth is accentuated under weakly mixed conditions. Comparison with field data shows quite good agreement with the model predictions and underlines the fact that simple analytical tools are often quite adequate when dealing with a sparse experimental data base.

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