The effect of seasonally and spatially varying chlorophyll on Bay of Bengal surface ocean properties and the South Asian Monsoon

Abstract. Chlorophyll absorbs solar radiation in the upper ocean, increasing mixed-layer radiative heating and sea surface temperatures (SST). The solar absorption caused by chlorophyll can be parameterised as an optical parameter, h2, the scale depth of absorption of blue light. Seasonally and spatially varying h2 in the Bay of Bengal was imposed in a coupled ocean-atmosphere model to investigate the effect of chlorophyll distributions on regional SST, atmospheric circulation and precipitation. There are both direct local upper-ocean effects, through changes in solar radiation absorption and indirect remote atmospheric responses. The depth of the mixed layer relative to the perturbed solar penetration depths modulates the response of SST to chlorophyll. The largest SST response to chlorophyll forcing occurs in coastal regions, where chlorophyll concentrations are high (> 1 mg m−3), and when climatological mixed layer depths shoal during the intermonsoon periods. Precipitation increases significantly by up to 3 mm day−1 across coastal Myanmar during the southwest monsoon onset and over northeast India and Bangladesh during the Autumn intermonsoon period, decreasing model biases.

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The effect of seasonally and spatially varying chlorophyll on Bay of Bengal surface ocean properties and the South Asian monsoon

Weather and Climate Dynamics ◽

10.5194/wcd-1-635-2020 ◽

2020 ◽

Vol 1 (2) ◽

pp. 635-655

Author(s):

Jack Giddings ◽

Adrian J. Matthews ◽

Nicholas P. Klingaman ◽

Karen J. Heywood ◽

Manoj Joshi ◽

...

Keyword(s):

Solar Radiation ◽

Mixed Layer ◽

Bay Of Bengal ◽

General Circulation ◽

Southwest Monsoon ◽

Radiative Heating ◽

Radiation Absorption ◽

Upper Ocean ◽

Monsoon Circulation ◽

Spatially Varying

Abstract. Chlorophyll absorbs solar radiation in the upper ocean, increasing the mixed layer radiative heating and sea surface temperatures (SST). Although the influence of chlorophyll distributions in the Arabian Sea on the southwest monsoon has been demonstrated, there is a current knowledge gap regarding how chlorophyll distributions in the Bay of Bengal influence the southwest monsoon. The solar absorption caused by chlorophyll can be parameterized as an optical parameter, h2, which expresses the scale depth of the absorption of blue light. Seasonally and spatially varying h2 fields in the Bay of Bengal were imposed in a 30-year simulation using an atmospheric general circulation model coupled to a mixed layer thermodynamic ocean model in order to investigate the effect of chlorophyll distributions on regional SST, the southwest monsoon circulation, and precipitation. There are both direct local upper-ocean effects, through changes in solar radiation absorption, and indirect remote atmospheric responses. The depth of the mixed layer relative to the perturbed solar penetration depths modulates the response of the SST to chlorophyll. The largest SST response of 0.5 ∘C to chlorophyll forcing occurs in coastal regions, where chlorophyll concentrations are high (> 1 mg m−3), and when climatological mixed layer depths shoal during the inter-monsoon periods. Precipitation increases significantly (by up to 3 mm d−1) across coastal Myanmar during the southwest monsoon onset and over northeast India and Bangladesh during the Autumn inter-monsoon period, decreasing model biases.

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Spatial and temporal variability of solar penetration depths in the Bay of Bengal and its impact on sea surface temperature (SST) during the summer monsoon

Ocean Science ◽

10.5194/os-17-871-2021 ◽

2021 ◽

Vol 17 (4) ◽

pp. 871-890

Author(s):

Jack Giddings ◽

Karen J. Heywood ◽

Adrian J. Matthews ◽

Manoj M. Joshi ◽

Benjamin G. M. Webber ◽

...

Keyword(s):

Surface Temperature ◽

Sea Surface Temperature ◽

Mixed Layer ◽

Bay Of Bengal ◽

Southwest Monsoon ◽

Sea Surface ◽

Radiation Absorption ◽

Spatial And Temporal Variability ◽

Solar Absorption ◽

Chlorophyll Concentrations

Abstract. Chlorophyll has long been known to influence air–sea gas exchange and CO2 drawdown. But chlorophyll also influences regional climate through its effect on solar radiation absorption and thus sea surface temperature (SST). In the Bay of Bengal, the effect of chlorophyll on SST has been demonstrated to have a significant impact on the Indian summer (southwest) monsoon. However, little is known about the drivers and impacts of chlorophyll variability in the Bay of Bengal during the southwest monsoon. Here we use observations of downwelling irradiance measured by an ocean glider and three profiling floats to determine the spatial and temporal variability of solar absorption across the southern Bay of Bengal during the 2016 summer monsoon. A two-band exponential solar absorption scheme is fitted to vertical profiles of photosynthetically active radiation to determine the effective scale depth of blue light. Scale depths of blue light are found to vary from 12 m during the highest (0.3–0.5 mg m−3) mixed-layer chlorophyll concentrations to over 25 m when the mixed-layer chlorophyll concentrations are below 0.1 mg m−3. The Southwest Monsoon Current and coastal regions of the Bay of Bengal are observed to have higher mixed-layer chlorophyll concentrations and shallower solar penetration depths than other regions of the southern Bay of Bengal. Substantial sub-daily variability in solar radiation absorption is observed, which highlights the importance of near-surface ocean processes in modulating mixed-layer chlorophyll. Simulations using a one-dimensional K-profile parameterization ocean mixed-layer model with observed surface forcing from July 2016 show that a 0.3 mg m−3 increase in chlorophyll concentration increases sea surface temperature by 0.35 ∘C in 1 month, with SST differences growing rapidly during calm and sunny conditions. This has the potential to influence monsoon rainfall around the Bay of Bengal and its intraseasonal variability.

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How Does Solar Attenuation Depth Affect the Ocean Mixed Layer? Water Turbidity and Atmospheric Forcing Impacts on the Simulation of Seasonal Mixed Layer Variability in the Turbid Black Sea*

Journal of Climate ◽

10.1175/jcli-3159.1 ◽

2005 ◽

Vol 18 (3) ◽

pp. 389-409 ◽

Cited By ~ 19

Author(s):

A. Birol Kara ◽

Alan J. Wallcraft ◽

Harley E. Hurlburt

Keyword(s):

Solar Radiation ◽

Black Sea ◽

Mixed Layer ◽

Sea Surface ◽

Shortwave Radiation ◽

Altimeter Data ◽

The Black Sea ◽

Upper Ocean ◽

Clear Water ◽

Water Turbidity

Abstract A fine-resolution (≈3.2 km) Hybrid Coordinate Ocean Model (HYCOM) is used to investigate the impact of solar radiation attenuation with depth on the predictions of monthly mean sea surface height (SSH), mixed layer depth (MLD), buoyancy and heat fluxes, and near-sea surface circulation as well. The model uses spatially and temporally varying attenuation of photosynthetically available radiation (kPAR) climatologies as processed from the remotely sensed Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) to take water turbidity into account in the Black Sea. An examination of the kPAR climatology reveals a strong seasonal cycle in the water turbidity, with a basin-averaged annual climatological mean value of 0.19 m−1 over the Black Sea. Climatologically forced HYCOM simulations demonstrate that shortwave radiation below the mixed layer can be quite different based on the water turbidity, thereby affecting prediction of upper-ocean quantities in the Black Sea. The clear water constant solar attenuation depth assumption results in relatively deeper MLD (e.g., ≈+15 m in winter) in comparison to standard simulations due to the unrealistically large amount of shortwave radiation below the mixed layer, up to 100 W m−2 during April to October. Such unrealistic sub–mixed layer heating causes weaker stratification at the base of the mixed layer. The buoyancy gain associated with high solar heating in summer effectively stabilizes the upper ocean producing shallow mixed layers and elevated SSH over the most of the Black Sea. In particular, the increased stability resulting from the water turbidity reduces vertical mixing in the upper ocean and causes changes in surface-layer currents, especially in the easternmost part of the Black Sea. Monthly mean SSH anomalies from the climatologically forced HYCOM simulations were evaluated against a monthly mean SSH anomaly climatology, which was constructed using satellite altimeter data from TOPEX/ Poseidon (T/P), Geosat Follow-On (GFO), and the Earth Remote Sensing Satellite-2 (ERS-2) over 1993–2002. Model–data comparisons show that the basin-averaged root-mean-square (rms) difference is ≈4 cm between the satellite-based SSH climatology and that obtained from HYCOM simulations using spatial and temporal kPAR fields. In contrast, when all solar radiation is absorbed at the sea surface or clear water constant solar attenuation depth values of 16.7 m are used in the model simulations, the basin-averaged SSH rms difference with respect to the climatology is ≈6 cm (≈50% more). This demonstrates positive impact from using monthly varying solar attenuation depths in simulating upper-ocean quantities in the Black Sea. The monthly mean kPAR and SSH anomaly climatologies presented in this paper can also be used for other Black Sea studies.

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Adrift Upon a Salinity-Stratified Sea: A View of Upper-Ocean Processes in the Bay of Bengal During the Southwest Monsoon

Oceanography ◽

10.5670/oceanog.2016.46 ◽

2016 ◽

Vol 29 (2) ◽

pp. 134-145 ◽

Cited By ~ 22

Author(s):

Andrew Lucas ◽

Jonathan Nash ◽

Robert Pinkel ◽

Jennifer MacKinnon ◽

Amit Tandon ◽

...

Keyword(s):

Bay Of Bengal ◽

Southwest Monsoon ◽

Upper Ocean ◽

Ocean Processes

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Mixing, heat fluxes and heat content evolution of the Arctic Ocean mixed layer

Ocean Science Discussions ◽

10.5194/osd-8-247-2011 ◽

2011 ◽

Vol 8 (1) ◽

pp. 247-289 ◽

Cited By ~ 2

Author(s):

A. Sirevaag ◽

S. de la Rosa ◽

I. Fer ◽

M. Nicolaus ◽

M. Tjernström ◽

...

Keyword(s):

Heat Flux ◽

Solar Radiation ◽

Arctic Ocean ◽

Mixed Layer ◽

Heat Content ◽

The Arctic ◽

Upper Ocean ◽

Ocean Mixed Layer ◽

The Arctic Ocean ◽

Melting Season

Abstract. A comprehensive measurement program was conducted during 16 days of a 3 week long ice pack drift, from 15 August to 1 September 2008 in the Central Amundsen Basin, Arctic Ocean. The data, sampled as part of the Arctic Summer Cloud Ocean Study (ASCOS), included upper ocean stratification, mixing and heat transfer as well as transmittance of solar radiation through the ice. The observations give insight into the evolution of the upper layers of the Arctic Ocean in the transition period from melting to freezing. The ocean mixed layer was found to be heated from above and, for summer conditions, the net heat flux through the ice accounted for 22% of the observed change in mixed layer heat content. Heat was mixed downward within the mixed layer and a small, downward heat flux across the pycnocline accounted for the accumulated heat in the upper cold halocline during the melting season. On average, the ocean mixed layer was cooled by an ocean heat flux at the ice/ocean interface (1.2 W m−2) and heated by solar radiation through the ice (−2.6 W m−2). An abrupt change in surface conditions halfway into the drift due to freezing and snowfall showed distinct signatures in the data set and allowed for inferences and comparisons to be made for cases of contrasting forcing conditions. Transmittance of solar radiation was reduced by 59% in the latter period. From hydrographic observations obtained earlier in the melting season, in the same region, we infer a total fresh water equivalent of 3.3 m accumulated in the upper ocean, which together with the observed saltier winter mixed layer indicates a transition towards a more seasonal ice cover in the Arctic.

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Mixing, heat fluxes and heat content evolution of the Arctic Ocean mixed layer

Ocean Science ◽

10.5194/os-7-335-2011 ◽

2011 ◽

Vol 7 (3) ◽

pp. 335-349 ◽

Cited By ~ 31

Author(s):

A. Sirevaag ◽

S. de la Rosa ◽

I. Fer ◽

M. Nicolaus ◽

M. Tjernström ◽

...

Keyword(s):

Heat Flux ◽

Solar Radiation ◽

Arctic Ocean ◽

Mixed Layer ◽

Heat Content ◽

The Arctic ◽

Upper Ocean ◽

Ocean Mixed Layer ◽

The Arctic Ocean ◽

Melting Season

Abstract. A comprehensive measurement program was conducted during 16 days of a 3 week long ice pack drift, from 15 August to 1 September 2008 in the central Amundsen Basin, Arctic Ocean. The data, sampled as part of the Arctic Summer Cloud Ocean Study (ASCOS), included upper ocean stratification, mixing and heat transfer as well as transmittance solar radiation through the ice. The observations give insight into the evolution of the upper layers of the Arctic Ocean in the transition period from melting to freezing. The ocean mixed layer was found to be heated from above and, for summer conditions, the net heat flux through the ice accounted for 22 % of the observed change in mixed layer heat content. Heat was mixed downward within the mixed layer and a small, downward heat flux across the base of the mixed layer accounted for the accumulated heat in the upper cold halocline during the melting season. On average, the ocean mixed layer was cooled by an ocean heat flux at the ice/ocean interface (1.2 W m−2) and heated by solar radiation through the ice (−2.6 W m−2). An abrupt change in surface conditions halfway into the drift due to freezing and snowfall showed distinct signatures in the data set and allowed for inferences and comparisons to be made for cases of contrasting forcing conditions. Transmittance of solar radiation was reduced by 59 % in the latter period. From hydrographic observations obtained earlier in the melting season, in the same region, we infer a total fresh water equivalent of 3.3 m accumulated in the upper ocean, which together with the observed saltier winter mixed layer indicates a transition towards a more seasonal ice cover in the Arctic.

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Spatial and temporal variability of solar penetration depths in the Bay of Bengal and its impact on SST during the summer monsoon

10.5194/os-2020-125 ◽

2021 ◽

Author(s):

Jack Giddings ◽

Karen J. Heywood ◽

Adrian J. Matthews ◽

Manoj M. Joshi ◽

Benjamin G. M. Webber ◽

...

Keyword(s):

Mixed Layer ◽

Bay Of Bengal ◽

Temporal Variability ◽

Southwest Monsoon ◽

Sea Surface ◽

Spatial And Temporal Variability ◽

Solar Absorption ◽

Penetration Depths ◽

Solar Penetration ◽

Chlorophyll Concentrations

Abstract. Chlorophyll influences regional climate through its effect on solar radiation absorption and thus sea surface temperature (SST). In the Bay of Bengal, the effect of chlorophyll on SST has been demonstrated to have a significant impact on the Indian summer (southwest) monsoon. However, little is known about the drivers and impacts of chlorophyll variability in the Bay of Bengal during the southwest monsoon. Here we use observations of downwelling irradiance measured by an ocean glider and three profiling floats to determine the spatial and temporal variability of solar absorption across the southern Bay of Bengal during the 2016 summer monsoon. A two-band exponential solar absorption scheme is fitted to vertical profiles of photosynthetically active radiation to determine the effective scale depth of blue light. Scale depths of blue light are found to vary from 12 m during the highest (0.3–0.5 mg m−3) mixed layer chlorophyll concentrations, to over 25 m when the mixed layer chlorophyll concentrations are below 0.1 mg m−3. The Southwest Monsoon Current and coastal regions of the Bay of Bengal are observed to have higher mixed layer chlorophyll concentrations and shallower solar penetration depths than other regions of the southern Bay of Bengal. Substantial sub-daily variability in solar radiadion absorption is observed, which highlights the importance of near-surface ocean processes in modulating mixed layer chlorophyll. Simulations using a one-dimensional K-profile parameterisation ocean mixed layer model with observed surface forcing from July 2016 show that a 0.3 mg m−3 increase in chlorophyll concentration increases sea surface temperature by 0.35 °C in one month with SST differences growing rapidly during calm and sunny conditions. This has the potential to influence monsoon rainfall around the Bay of Bengal and its intraseasonal variability.

Download Full-text