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 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.

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 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 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 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.

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.

scholarly journals Subduction over the Southern Indian Ocean in a High-Resolution Atmosphere–Ocean Coupled Model

2011 ◽  
Vol 24 (15) ◽  
pp. 3830-3849 ◽  
Author(s):  
Mei-Man Lee ◽  
A. J. George Nurser ◽  
I. Stevens ◽  
Jean-Baptiste Sallée
Keyword(s):  
Indian Ocean ◽  
Mixed Layer ◽  
Current System ◽  
Mixed Layer Depth ◽  
Coupled Model ◽  
Horizontal Resolution ◽  
Layer Depth ◽  
Mode Water ◽  
Coarse Resolution ◽  
Winter Mixed Layer

Abstract This study examines the subduction of the Subantarctic Mode Water in the Indian Ocean in an ocean–atmosphere coupled model in which the ocean component is eddy permitting. The purpose is to assess how sensitive the simulated mode water is to the horizontal resolution in the ocean by comparing with a coarse-resolution ocean coupled model. Subduction of water mass is principally set by the depth of the winter mixed layer. It is found that the path of the Agulhas Current system in the model with an eddy-permitting ocean is different from that with a coarse-resolution ocean. This results in a greater surface heat loss over the Agulhas Return Current and a deeper winter mixed layer downstream in the eddy-permitting ocean coupled model. The winter mixed layer depth in the eddy-permitting ocean compares well to the observations, whereas the winter mixed layer depth in the coarse-resolution ocean coupled model is too shallow and has the wrong spatial structure. To quantify the impacts of different winter mixed depths on the subduction, a way to diagnose local subduction is proposed that includes eddy subduction. It shows that the subduction in the eddy-permitting model is closer to the observations in terms of the magnitudes and the locations. Eddies in the eddy-permitting ocean are found to 1) increase stratification and thus oppose the densification by northward Ekman flow and 2) increase subduction locally. These effects of eddies are not well reproduced by the eddy parameterization in the coarse-resolution ocean coupled model.

scholarly journals Study of the mixed layer depth variations within the north Indian Ocean using a 1-D model

2004 ◽  
Vol 109 (C8) ◽  
pp. n/a-n/a ◽  
Author(s):  
K. N. Babu ◽  
Rashmi Sharma ◽  
Neeraj Agarwal ◽  
Vijay K. Agarwal ◽  
R. A. Weller
Keyword(s):  
Indian Ocean ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
North Indian Ocean ◽  
Layer Depth ◽  
The North ◽  
D Model

scholarly journals Phytoplankton bloom phenomena in the North Atlantic Ocean and Arabian Sea

2015 ◽  
Vol 72 (6) ◽  
pp. 2021-2028 ◽  
Author(s):  
John F. Marra ◽  
Tommy D. Dickey ◽  
Albert J. Plueddemann ◽  
Robert A. Weller ◽  
Christopher S. Kinkade ◽  
...  
Keyword(s):  
Water Column ◽  
Mixed Layer ◽  
Arabian Sea ◽  
Phytoplankton Bloom ◽  
Mixed Layer Depth ◽  
Ocean Dynamics ◽  
Layer Depth ◽  
Diurnal Variability ◽  
Water Column Stratification ◽  
Iceland Basin

Abstract We review bio-optical and physical data from three mooring experiments, the Marine Light–Mixed Layers programme in spring 1989 and 1991 in the Iceland Basin (59°N/21°W), and the Forced Upper Ocean Dynamics Experiment in the central Arabian Sea from October 1994 to 1995 (15.5°N/61.5°E). In the Iceland Basin, from mid-April to mid-June in 1989, chlorophyll-a concentrations are sensitive to small changes in stratification, with intermittent increases early in the record. The spring increase occurs after 20 May, coincident with persistent water column stratification. In 1991, the bloom occurs 2 weeks earlier than in 1989, with a background of strong short-term and diurnal variability in mixed layer depth and minimal horizontal advection. In the Arabian Sea, the mixing response to the northeast and southwest monsoons, plus the response to mesoscale eddies, produces four blooms over the annual cycle. The mixed layer depth in the Arabian Sea never exceeds the euphotic zone, allowing interactions between phytoplankton and grazer populations to become important. For all three mooring experiments, change in water column stratification is key in producing phytoplankton blooms.

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