Articles written in Journal of Earth System Science

    • A revisit to the regulation of oxygen minimum zone in the Bay of Bengal


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      Occurrence of intense oxygen minimum zone (OMZ) is known in the Bay of Bengal (BoB), but it has been recently reported to have become more acute and is at its tipping point. Here, we show that the intensification of OMZ to acute condition is a random and short-term rather than perennial phenomenon based on re-evaluation of old and recent information in the BoB. Short-term modifications in dissolved oxygen (DO) in the OMZ are caused by balance among physical forcings: salinity stratification, occurrence of cyclonic (CE), and anticyclonic eddies (ACE). Our analysis reveals that ‘acute OMZ’ is only a transient phenomenon in the Bay since the dynamic periodic physical forcings, particularly ACEs, do not allow it to become a dead zone.

    • Phosphate removal through adsorption on suspended matter in the Bay of Bengal: Possible implications to primary production


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      Low primary production in the Bay of Bengal (BoB) could not be explained as this region receives nutrients from atmospheric deposition, rivers, eddies, and mixing due to depression/cyclones. In addition to nutrients, BoB also receives significant amount of suspended particulate matter (SPM) from the major rivers and their concentrations are in an order of magnitude higher than elsewhere in the open ocean region. Here we show that the removal of phosphorus (P) through adsorption on SPM may be a potential mechanism to decrease in primary production in the northern BoB. Significant fraction of P removal (5–50% of total dissolved inorganic phosphate) in association with SPM was observed in the BoB. The magnitude of removal of P through SPM is linearly related with dissolved inorganic phosphate (DIP), particulate organic carbon (POC) in the water column suggesting that P is removed in association with organic matter. The fraction of P removed from that of DIP showed inverse relation with salinity, and linear relation with SPM concentration suggesting that SPM brought by river discharge removed P from the water column. The P removed by SPM in the mixed layer showed inverse relation with mixed layer integrated primary production in the open sea region but its impact is negligible in the coastal waters. The laboratory experiment by measuring primary production in the presence of different quantities of SPM concentrations confirmed decrease in primary production due to removal of phosphate in the BoB.


      $\bullet$ Significant fraction of phosphate adsorbs on to the suspended particles and it is bio-non-available.

      $\bullet$ Phosphate severely controls primary production in the northern Bay of Bengal.

      $\bullet$ The removal of phosphate is more in the coastal waters, it does not control primary production due to higher input than removal.

      $\bullet$ High N/P ratios is caused by phosphate removed on association with suspended matter.

    • Influence of phytoplankton pigment composition and primary production on pCO$_{2}$ levels in the Indian Ocean


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      The tropical Indian Ocean is a net sink for the carbon dioxide (CO$_{2}$) in the atmosphere and phytoplankton production plays a crucial role in CO$_{2}$ fixation and determines the direction of CO$_{2}$ flux at the sea-to-air interface. In order to assess the influence of phytoplankton composition and primary production on pCO$_{2}$ levels in the Indian Ocean, sampling was conducted at 25 stations during the spring intermonsoon period under the auspices of the Indian GEOTRACES program. The pCO$_{2}$ was significantly correlated with salinity due to the discharge of low pCO$_{2}$ water by major rivers to the Bay of Bengal (BoB). The stronger negative correlation observed between pCO$_{2}$ and major phytoplankton marker pigments, net primary production and oxygen saturation levels suggesting significant influence of biological processes on pCO$_{2}$ levels in the Indian Ocean. This study indicates that pCO$_{2}$ levels are strongly modulated by biological processes than hitherto hypothesized as solubility pump in the Indian Ocean.


      $\bullet$ Surface pCO$_{2}$ levels are undersaturated in the Indian Ocean compared to atmosphere, except Arabian Sea.

      $\bullet$ Oligotrophic conditions prevailed in the entire tropical Indian Ocean.

      $\bullet$ Picophytoplankton (cyanobacteria) is the dominant phytoplankton in the Indian Ocean.

      $\bullet$ Significant relation between phytoplankton groups and pCO$_{2}$ indicates strong biological control on surface pCO$_{2}$.

    • Relations between marine biogeochemical indices and cloud properties over the Arabian Sea


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      Knowledge of relations among ocean biogeochemical and cloud properties will help to plan experiments necessary to understand the mechanisms and processes underlying the links between ocean and atmosphere interactions. Here, we explored the associations between ocean biogeochemical and cloud properties in a region that seasonally experiences polluted and pristine atmospheric conditions in winter and summer, respectively. The implications of ocean surface chlorophyll-a and biogeochemical fluxes (sea salt, dimethyl sulphide and organic fraction in sea spray) to cloud properties (cloud effective radii (R$_{e}$), cloud optical thickness, and cloud droplet number concentration(CDNC)) were studied using MODIS (Terra, Collection 6, L3) monthly data from 2001 to 2015 along with reanalysis information. We have adopted a climatological averaging approach in time (monthly, seasonal and annual) and space (coastal, open and total (basin) Arabian Sea). This approach was used to minimize incompatibility, if any, between ocean and cloud properties arising from spatio-temporal lags due to different dynamics in the respective boundary layers. The trends in monthly means suggest decreases in chlorophyll-a and CDNC, while R$_{e}$ increased over the Arabian Sea basin during 2001–2015. Variability at the basin scale (expressed as standard deviation in each month, SD) exceeded mean values of respective months for chlorophyll-a, whereas it was nearly half of the mean values for CDNC. An increase in R$_{e}$ seems facilitated more during warmer 2011–2015 than in the 2001–2010 period, which coincided with the decrease in CDNC. Fifteen-year monthly mean climatologies suggest considerable associations among ocean biogeochemical indices and cloud properties, which is more conspicuous during summer monsoon. Increase in sea salt flux appears to account for the higher values of R$_{e}$ in June–July over the basin due to strong monsoon wind. Inverse relations between chlorophyll-a and R$_{e}$ are indicative of smaller droplets that resulted from new particles formed from and or facilitated by marine biogeochemical emissions. Decline in new particle production due to decrease in surface chlorophyll-a and the growth of particles facilitated by increase in warming, seem responsible for increase in R$_{e}$ and decrease in CDNC from 2001 to 2015. Using chlorophyll-a as the main proxy for ocean biogeochemical indices, we demonstrated that connections between ocean biogeochemistry and clouds are sustained in both small and large scales in space and time over the Arabian Sea.

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