scholarly journals Delineation of marine ecosystem zones in the northern Arabian Sea during winter

2018 ◽  
Vol 15 (5) ◽  
pp. 1395-1414 ◽  
Author(s):  
Saleem Shalin ◽  
Annette Samuelsen ◽  
Anton Korosov ◽  
Nandini Menon ◽  
Björn C. Backeberg ◽  
...  
Keyword(s):  
Mixed Layer ◽  
Temporal Variability ◽  
Arabian Sea ◽  
Mixed Layer Depth ◽  
Marine Ecosystem ◽  
Chlorophyll Concentration ◽  
Spatial And Temporal Variability ◽  
Chl A ◽  
Northern Arabian Sea ◽  
Cross Correlation Analysis

Abstract. The spatial and temporal variability of marine autotrophic abundance, expressed as chlorophyll concentration, is monitored from space and used to delineate the surface signature of marine ecosystem zones with distinct optical characteristics. An objective zoning method is presented and applied to satellite-derived Chlorophyll a (Chl a) data from the northern Arabian Sea (50–75∘ E and 15–30∘ N) during the winter months (November–March). Principal component analysis (PCA) and cluster analysis (CA) were used to statistically delineate the Chl a into zones with similar surface distribution patterns and temporal variability. The PCA identifies principal components of variability and the CA splits these into zones based on similar characteristics. Based on the temporal variability of the Chl a pattern within the study area, the statistical clustering revealed six distinct ecological zones. The obtained zones are related to the Longhurst provinces to evaluate how these compared to established ecological provinces. The Chl a variability within each zone was then compared with the variability of oceanic and atmospheric properties viz. mixed-layer depth (MLD), wind speed, sea-surface temperature (SST), photosynthetically active radiation (PAR), nitrate and dust optical thickness (DOT) as an indication of atmospheric input of iron to the ocean. The analysis showed that in all zones, peak values of Chl a coincided with low SST and deep MLD. The rate of decrease in SST and the deepening of MLD are observed to trigger the algae bloom events in the first four zones. Lagged cross-correlation analysis shows that peak Chl a follows peak MLD and SST minima. The MLD time lag is shorter than the SST lag by 8 days, indicating that the cool surface conditions might have enhanced mixing, leading to increased primary production in the study area. An analysis of monthly climatological nitrate values showed increased concentrations associated with the deepening of the mixed layer. The input of iron seems to be important in both the open-ocean and coastal areas of the northern and north-western parts of the northern Arabian Sea, where the seasonal variability of the Chl a pattern closely follows the variability of iron deposition.

scholarly journals Delineation of marine ecosystem zones in the northern Arabian Sea using an objective method

2017 ◽  
Author(s):  
Saleem Shalin ◽  
Annette Samuelsen ◽  
Anton Korosov ◽  
Nandini Menon ◽  
Björn C. Backeberg ◽  
...  
Keyword(s):  
Mixed Layer ◽  
Temporal Variability ◽  
Arabian Sea ◽  
Mixed Layer Depth ◽  
Marine Ecosystem ◽  
Chlorophyll Concentration ◽  
Spatial And Temporal Variability ◽  
Chl A ◽  
Northern Arabian Sea ◽  
Cross Correlation Analysis

Abstract. The spatial and temporal variability of marine autotrophic abundance, expressed as chlorophyll concentration, is monitored from space and used to delineate the surface signature of marine ecosystem zones with distinct optical characteristics. An objective zoning method is presented and applied to satellite-derived Chlorophyll a (Chl-a) data from the northern Arabian Sea (50°–75° E and 15°–30° N) during the winter months (November–March). Principal Component Analysis (PCA) and Cluster Analysis (CA) were used to statistically delineate the Chl-a into zones with similar surface distribution patterns and temporal variability. The PCA identifies principal components of variability and the CA splits these into zones based on similar characteristics. Based on the temporal variability of Chl-a pattern within the study area, the statistical clustering revealed six distinct ecological zones. The obtained zones are related to the Longhurst provinces to evaluate how these compared to established ecological provinces. The Chl-a variability within each zone was then compared with the variability of oceanic and atmospheric properties viz. mixed-layer depth (MLD), wind speed, sea-surface temperature (SST), Photosynthetically Active Radiation (PAR), nitrate and Dust Optical Thickness (DOT) as an indication of atmospheric input of iron to the ocean. The analysis showed that in all zones, peak values of Chl-a coincided with low SST and deep MLD. Rate of decrease in SST and deepening of MLD are observed to trigger the intensity of the algae bloom events in the first four zones. Lagged cross-correlation analysis shows that peak Chl-a follows peak MLD and SST minima. The MLD time-lag is shorter than the SST lag by eight days, indicating that the cool surface conditions might have enhanced mixing, leading to increased primary production in the study area. An analysis of monthly climatological nitrate values showed increased concentrations associated with the deepening of the mixed-layer. The input of iron seems to be important in both the open ocean and coastal areas of the northern and north-western part of the Northern Arabian Sea, where the seasonal variability of the Chl-a pattern closely follows the variability of iron deposition.

What controls the initiation of algal bloom during the winter monsoon in the Arabian Sea??

2021 ◽  
Author(s):  
Lakshmi Shenoy
Keyword(s):  
Water Column ◽  
Mixed Layer ◽  
Arabian Sea ◽  
Mixed Layer Depth ◽  
Chlorophyll Concentration ◽  
Winter Monsoon ◽  
Physical Interaction ◽  
Stoichiometric Ratio ◽  
Noctiluca Scintillans ◽  
Northern Arabian Sea

<p>A winter monsoon cruise was undertaken in the northern Arabian Sea to understand the bio-physical interaction responsible for the occurrence of phytoplankton bloom in the region. The observation shows strong convective mixing with a dense and deeper mixed layer (MLD: 100-140 m) and well-oxygenated upper water column (>95% saturation). The chlorophyll concentration was low (0.1 -0.3 µg/l) despite having ample nitrate (~2.5 µM) in the surface layer. The region, however, was deprived of micro phytoplankton, especially diatomic species and Noctiluca Scintillans, and was dominated by the picophytoplankton (77%-85%). The mean Si/N ratio in the upper 100 m was 0.72 indicating “Silicate stressed” condition for the proliferation of diatoms. Even a deeper mixed layer could not penetrate into the silicicline (~150m) which was deeper than the nitracline (~110m). In addition, the euphotic depth (~49m) was much shallower than the mixed layer depth suggesting the Sverdrup critical depth limitation in the northern Arabian Sea. We further show that the bloom initiated only when the mixed layer shoals towards the euphotic zone. Our observations suggest that two primary factors, the stoichiometric ratio of nutrients, especially Si/N ratio, in the mixed layer and re-stratification of the upper water column, govern the phytoplankton blooming in the northern Arabian Sea during the later winter monsoon.</p>

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.

Winter Convective Mixing in the northern Arabian Sea during Contrasting Monsoons

2021 ◽  
Keyword(s):  
Mixed Layer ◽  
Arabian Sea ◽  
Mixed Layer Depth ◽  
Specific Humidity ◽  
Atmospheric Forcing ◽  
Convective Mixing ◽  
Northern Arabian Sea ◽  
Winter Mixed Layer ◽  
D Model ◽  
Buoyancy Loss

Abstract Along-track Argo observations in the northern Arabian Sea during 2017 – 19 showed by far the most contrasting winter convective mixing; 2017 – 18 was characterized by less intense convective mixing resulting in a mixed layer depth of 110 m, while 2018 – 19 experienced strong and prolonged convective mixing with the mixed layer deepening to 150 m. The response of the mixed layer to contrasting atmospheric forcing and the associated formation of Arabian Sea High Salinity Water (ASHSW) in the northeastern Arabian Sea are studied using a combination of Argo float observations, gridded observations, a data assimilative general circulation model and a series of 1-D model simulations. The 1-D model experiments show that the response of winter mixed layer to atmospheric forcing is not only influenced by winter surface buoyancy loss, but also by a preconditioned response to freshwater fluxes and associated buoyancy gain by the ocean during the summer that is preceding the following winter. A shallower and short-lived winter mixed layer occurred during 2017 – 18 following the exceptionally high precipitation over evaporation during the summer monsoon in 2017. The precipitation induced salinity stratification (a salinity anomaly of -0.7 psu) during summer inhibited convective mixing in the following winter resulting in a shallow winter mixed layer (103 m). Combined with weak buoyancy loss due to weaker surface heat loss in the northeastern Arabian Sea, this caused an early termination of the convective mixing (February 26, 2018). In contrast, the winter convective mixing during 2018 – 19 was deeper (143 m) and long-lived. The 2018 summer, by comparison, was characterized by normal or below normal precipitation which generated a weakly stratified ocean pre-conditioned to winter mixing. This combined with colder and drier air from the land mass to the north with low specific humidity lead to strong buoyancy loss, and resulted in prolonged winter convective mixing through March 25, 2019.

scholarly journals Physical control of interannual variations of the winter chlorophyll bloom in the northern Arabian Sea

2017 ◽  
Vol 14 (15) ◽  
pp. 3615-3632 ◽  
Author(s):  
Madhavan Girijakumari Keerthi ◽  
Matthieu Lengaigne ◽  
Marina Levy ◽  
Jerome Vialard ◽  
Vallivattathillam Parvathi ◽  
...  
Keyword(s):  
Mixed Layer ◽  
Arabian Sea ◽  
Model Simulation ◽  
Mixed Layer Depth ◽  
Remotely Sensed ◽  
Physical Parameters ◽  
Thermocline Depth ◽  
Interannual Variations ◽  
Layer Depth ◽  
Northern Arabian Sea

Abstract. The northern Arabian Sea hosts a winter chlorophyll bloom, triggered by convective overturning in response to cold and dry northeasterly monsoon winds. Previous studies of interannual variations of this bloom only relied on a couple of years of data and reached no consensus on the associated processes. The current study aims at identifying these processes using both  ∼  10 years of observations (including remotely sensed chlorophyll data and physical parameters derived from Argo data) and a 20-year-long coupled biophysical ocean model simulation. Despite discrepancies in the estimated bloom amplitude, the six different remotely sensed chlorophyll products analysed in this study display a good phase agreement at seasonal and interannual timescales. The model and observations both indicate that the interannual winter bloom fluctuations are strongly tied to interannual mixed layer depth anomalies ( ∼  0.6 to 0.7 correlation), which are themselves controlled by the net heat flux at the air–sea interface. Our modelling results suggest that the mixed layer depth control of the bloom amplitude ensues from the modulation of nutrient entrainment into the euphotic layer. In contrast, the model and observations both display insignificant correlations between the bloom amplitude and thermocline depth, which precludes a control of the bloom amplitude by daily dilution down to the thermocline depth, as suggested in a previous study.

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 Physical control of the interannual variations of the winter chlorophyll bloom in the northern Arabian Sea

2016 ◽  
Author(s):  
M. G. Keerthi ◽  
M. Lengaigne ◽  
M. Levy ◽  
J. Vialard ◽  
V. Parvathi ◽  
...  
Keyword(s):  
Mixed Layer ◽  
Arabian Sea ◽  
Model Simulation ◽  
Mixed Layer Depth ◽  
Remotely Sensed ◽  
Physical Parameters ◽  
Thermocline Depth ◽  
Interannual Variations ◽  
Layer Depth ◽  
Northern Arabian Sea

Abstract. The northern Arabian Sea hosts a winter chlorophyll bloom, triggered by convective overturning in response to cold and dry northeasterly monsoon winds. There is currently no consensus about the processes responsible for the interannual variations of the magnitude of this bloom. The current study aims at identifying these processes using both observations (including remotely sensed chlorophyll data and physical parameters derived from Argo data) and a coupled biophysical ocean model simulation. Six different remotely sensed chlorophyll products are compared and they show a good phase agreement at seasonal and interannual timescales, but significant discrepancies in bloom amplitude. Both model and observations indicate that the interannual fluctuations of the winter bloom amplitude are strongly tied to mixed layer depth interannual anomalies (correlation ~ 0.6 to 0.7), which are themselves controlled by the net heat flux at the air-sea interface. Our results suggest that mixed layer depth control of the bloom amplitude ensues from the modulation of nutrient entrainment into the mixed layer. In contrast, our results show insignificant correlations between the bloom amplitude and thermocline depth, which precludes a control of the bloom amplitude by daily dilution down to the thermocline depth, as suggested in a previous study.

scholarly journals Oceanic Mixing over the Northern Arabian Sea in a Warming Scenario: Tug of War between Wind and Buoyancy Forces

2020 ◽  
Vol 50 (4) ◽  
pp. 945-964 ◽  
Author(s):  
V. Praveen ◽  
V. Valsala ◽  
R. S. Ajayamohan ◽  
Sridhar Balasubramanian
Keyword(s):  
Climate Change ◽  
Mixed Layer ◽  
Arabian Sea ◽  
Mixed Layer Depth ◽  
Future Climate ◽  
First Century ◽  
Layer Depth ◽  
Surface Ocean ◽  
Warming Scenario ◽  
Northern Arabian Sea

AbstractA consensus of the twenty-first-century climate change in the ocean is surface warming, stratification due to extreme freshening and subsequent weakening of mixing, overturning circulation, and biological production. Counterintuitively, certain parts of the tropical ocean may develop a resistance to changes in mixing, where the climate change impacts of atmosphere and ocean are complementary to each other. Under the poleward shift of monsoon low-level jet (LLJ) in the twenty-first century, a part of the northern Arabian Sea has a tendency to maintain the mixed layer depth intact. The process is studied using a set of high-resolution regional ocean model downscaling experiments for the present and future climate. It is found that the wind intensification caused by the shift in LLJ tends to counteract the stratification gained by surface ocean warming and maintains the mixing process in a warming scenario. The mixing energetics shed light on the way in which this is achieved. Intensified winds promote shear production and surface ocean warming demotes buoyancy production of turbulent kinetic energy (TKE), with a net effect of an increase in TKE. However, TKE appears to be dissipating quickly because of the presence of a larger number of small-scale eddies. This causes the mixing length and mixed layer depth to remain intact. Therefore, the interpretations of impacts of future climate change in ocean mixing should be viewed with caution, at least regionally, by focusing on the detailed changes of the governing mechanisms.

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