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.

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

2014 ◽  
Vol 11 (14) ◽  
pp. 3819-3843 ◽  
Author(s):  
J. Narvekar ◽  
S. Prasanna Kumar
Keyword(s):  
Mixed Layer ◽  
Bay Of Bengal ◽  
Mixed Layer Depth ◽  
Remote Sensing Data ◽  
Ekman Pumping ◽  
Layer Depth ◽  
Basin Scale ◽  
Deep Mixed Layer ◽  
Northern Bay Of Bengal

Abstract. The 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 oceanic primary productivity. We examined the basin-scale processes controlling the seasonal variability of mixed layer depth in the Bay of Bengal and its association with chlorophyll using a suite of in situ as well as remote sensing data. A coupling between mixed layer depth and chlorophyll was seen during spring intermonsoon and summer monsoon, but for different reasons. In spring intermonsoon the temperature-dominated stratification and associated shallow mixed layer makes the upper waters of the Bay of Bengal nutrient depleted and oligotrophic. In summer, although the salinity-dominated stratification in the northern Bay of Bengal shallows the mixed layer, the nutrient input from adjoining rivers enhance the surface chlorophyll. This enhancement is confined only to the surface layer and with increase in depth, the chlorophyll biomass decreases rapidly due to reduction in sunlight by suspended sediment. In the south, advection of high salinity waters from the Arabian Sea and westward propagating Rossby waves from the eastern Bay of Bengal led to the formation of deep mixed layer. In contrast, in the Indo–Sri Lanka region, the shallow mixed layer and nutrient enrichment driven by upwelling and Ekman pumping resulted in chlorophyll enhancement. The mismatch between the nitrate and chlorophyll indicated the inadequacy of present data to fully unravel its coupling to mixed layer processes.

scholarly journals Impact of ocean mixed‐layer depth initialization on the simulation of tropical cyclones over the Bay of Bengal using the WRF‐ARW model

2019 ◽  
Vol 27 (1) ◽  
Author(s):  
Viswanadhapalli Yesubabu ◽  
Vijaya Kumari Kattamanchi ◽  
Naresh Krishna Vissa ◽  
Hari Prasad Dasari ◽  
Vijaya Bhaskara Rao Sarangam
Keyword(s):  
Tropical Cyclones ◽  
Mixed Layer ◽  
Bay Of Bengal ◽  
Mixed Layer Depth ◽  
Ocean Mixed Layer ◽  
Layer Depth ◽  
Arw Model ◽  
Ocean Mixed Layer Depth

scholarly journals Ocean–Atmosphere Coupling in the Monsoon Intraseasonal Oscillation: A Simple Model Study

2008 ◽  
Vol 21 (20) ◽  
pp. 5254-5270 ◽  
Author(s):  
Gilles Bellon ◽  
Adam H. Sobel ◽  
Jerome Vialard
Keyword(s):  
Mixed Layer ◽  
Bay Of Bengal ◽  
Mixed Layer Depth ◽  
Intraseasonal Oscillation ◽  
Thermal Inertia ◽  
Coupled Model ◽  
Boreal Winter ◽  
Quasi Equilibrium ◽  
Layer Depth ◽  
Ocean Atmosphere

Abstract A simple coupled model is used in a zonally symmetric aquaplanet configuration to investigate the effect of ocean–atmosphere coupling on the Asian monsoon intraseasonal oscillation. The model consists of a linear atmospheric model of intermediate complexity based on quasi-equilibrium theory coupled to a simple, linear model of the upper ocean. This model has one unstable eigenmode with a period in the 30–60-day range and a structure similar to the observed northward-propagating intraseasonal oscillation in the Bay of Bengal/west Pacific sector. The ocean–atmosphere coupling is shown to have little impact on either the growth rate or latitudinal structure of the atmospheric oscillation, but it reduces the oscillation’s period by a quarter. At latitudes corresponding to the north of the Indian Ocean, the sea surface temperature (SST) anomalies lead the precipitation anomalies by a quarter of a period, similarly to what has been observed in the Bay of Bengal. The mixed layer depth is in phase opposition to the SST: a monsoon break corresponds to both a warming and a shoaling of the mixed layer. This behavior results from the similarity between the patterns of the predominant processes: wind-induced surface heat flux and wind stirring. The instability of the seasonal monsoon flow is sensitive to the seasonal mixed layer depth: the oscillation is damped when the oceanic mixed layer is thin (about 10 m deep or thinner), as in previous experiments with several models aimed at addressing the boreal winter Madden–Julian oscillation. This suggests that the weak thermal inertia of land might explain the minima of intraseasonal variance observed over the Asian continent.

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.

scholarly journals Temporal variability of chlorophyll distribution in the Gulf of Mexico: bio-optical data from profiling floats

2017 ◽  
Vol 14 (24) ◽  
pp. 5647-5662 ◽  
Author(s):  
Orens Pasqueron de Fommervault ◽  
Paula Perez-Brunius ◽  
Pierre Damien ◽  
Victor F. Camacho-Ibar ◽  
Julio Sheinbaum
Keyword(s):  
Gulf Of Mexico ◽  
Mixed Layer ◽  
Temporal Variability ◽  
Mixed Layer Depth ◽  
Chlorophyll Concentration ◽  
Optical Data ◽  
Layer Depth ◽  
Basin Scale ◽  
In Situ Experiments ◽  
Surface Increase

Abstract. Chlorophyll concentration is a key oceanic biogeochemical variable. In the Gulf of Mexico (GOM), its distribution, which is mainly obtained from satellite surface observations and scarce in situ experiments, is still poorly understood. In 2011–2012, eight profiling floats equipped with biogeochemical sensors were deployed for the first time in the GOM and generated an unprecedented dataset that significantly increased the number of chlorophyll vertical distribution measurements in the region. The analysis of these data, once calibrated, permits us to reconsider the spatial and temporal variability of the chlorophyll concentration in the water column. At a seasonal scale, results confirm the surface signal seen by satellites, presenting maximum concentrations in winter and low values in summer. It is shown that the deepening of the mixed layer is the primary factor triggering the chlorophyll surface increase in winter. In the GOM, a possible interpretation is that this surface increase corresponds to a biomass increase. However, the present dataset suggests that the basin-scale climatological surface increase in chlorophyll content results from a vertical redistribution of subsurface chlorophyll and/or photoacclimation processes, rather than a net increase of biomass. One plausible explanation for this is the decoupling between the mixed-layer depth and the deep nutrient reservoir since mixed-layer depth only reaches the nitracline in sporadic events in the observations. Float measurements also provide evidence that the depth and the magnitude of the deep chlorophyll maximum is strongly controlled by the mesoscale variability, with higher chlorophyll biomass generally observed in cyclones rather than anticyclones.

scholarly journals Temporal variability of chlorophyll distribution in the Gulf of Mexico: bio-optical data from profiling floats

2017 ◽  
Author(s):  
Orens Pasqueron de Fommervault ◽  
Paula Perez-Brunius ◽  
Pierre Damien ◽  
Julio Sheinbaum
Keyword(s):  
Gulf Of Mexico ◽  
Mixed Layer ◽  
Temporal Variability ◽  
Mixed Layer Depth ◽  
Chlorophyll Concentration ◽  
Optical Data ◽  
Spatial And Temporal Variability ◽  
Layer Depth ◽  
In Situ Experiments ◽  
Surface Increase

Abstract. Chlorophyll concentration is a key oceanic biogeochemical variable. In the Gulf of Mexico (GOM), its distribution, which is mainly obtained from satellite surface observations and scarce in situ experiments, is still poorly understood. In 2011–2012, eight profiling floats equipped with biogeochemical sensors were deployed for the first time in the GOM and generated an unprecedented dataset that significantly increased the number of chlorophyll vertical distribution measurements in the region. The analysis of these data, once calibrated, permits us to reconsider the spatial and temporal variability of the chlorophyll concentration in the water column. At a seasonal scale, results confirm the surface signal seen by satellites, presenting maximum concentrations in winter and low values in summer. It is shown that the deepening of the mixed layer depth is the primary factor triggering the chlorophyll surface increase in winter. In the GOM, current belief is that this surface increase corresponds to a biomass increase. However, the present dataset reveals a vertically integrated content of chlorophyll which remains constant throughout the year, suggesting that the surface increase results from a vertical redistribution of subsurface chlorophyll or photoacclimation processes, rather than a net increase of primary productivity. One plausible explanation for this is the decoupling between the mixed layer depth and the deep nutrient reservoir since mixed layer depth only reaches the nitracline in sporadic events in the observations. Float measurements also provide evidence that the depth and the magnitude of the deep chlorophyll maximum is strongly controlled by the mesoscale variability, with higher chlorophyll biomass generally observed in cyclones rather than anticyclones.

scholarly journals Observations of wind and waves in the central Bay of Bengal during BOBMEX-99 and their effect on mixed layer depth variability due to forced mixing

2003 ◽  
Vol 112 (2) ◽  
pp. 255-266
Author(s):  
J. Swain ◽  
R. K. Shukla ◽  
A. Raghunadha Rao ◽  
J. K. Panigrahi ◽  
N. R. Venkitachalam
Keyword(s):  
Mixed Layer ◽  
Bay Of Bengal ◽  
Mixed Layer Depth ◽  
Layer Depth ◽  
Forced Mixing
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