The study, published in Discover Oceans by Madhurima Paul, Rashmi R. Nayak, and Arun Chakraborty of the Indian Institute of Technology Kharagpur, draws on 263 depth-resolved profiles collected between March 2015 and December 2020 by a biogeochemical Argo float (WMO ID 2902174) deployed by the Indian National Centre for Ocean Information Services. Every ten days, the float dove to 2000 meters and ascended, measuring temperature, salinity, dissolved oxygen, and chlorophyll-a fluorescence along the way. Because the float stayed within a compact box between roughly 70.3°E and 73.35°E and 13.2°N and 17.2°N, the researchers could build a clean, location-specific climatology of how plankton biomass is distributed through the water column across the entire annual cycle.

The headline finding is deceptively simple. Surface chlorophyll-a in the CEAS climbs through the southwest monsoon months, peaking at about 0.31 milligrams per cubic meter in August, then plummets sharply in September before surging back to roughly 0.36 milligrams per cubic meter in October. Satellite ocean-color data and the float measurements agree closely, with a correlation of 0.86, and the September dip and October rebound appeared in every single year of the record. That consistency rules out a one-off anomaly and marks the pattern as a genuine climatological signature of the region, one that neither the persistently upwelling southeastern Arabian Sea to the south nor the freshwater-dominated northern waters display in the same way.

Why does September fail? The answer, according to the study, lies in the physics of the retreating monsoon. During the peak monsoon, strong winds associated with the low-level Findlater Jet push wind stress to around 0.13 to 0.14 newtons per square meter, driving turbulent mixing and positive Ekman pumping velocities of 1.5 to 2.5 times ten to the minus six meters per second. This Ekman suction draws cold, salty, nutrient-rich water upward, feeding the phytoplankton and deepening the mixed layer to about 40 meters. In September, however, wind stress collapses to roughly 0.06 newtons per square meter, the wind stress curl weakens, and the Ekman pumping velocity drops sharply. Vertical mixing falters, the upward nutrient conveyor stalls, and the phytoplankton bloom runs out of fuel despite the water still being relatively cool.

The float data add a crucial vertical dimension that satellites simply cannot capture. During the spring intermonsoon from March to May, intense solar heating stratifies the upper ocean, shoaling the mixed layer to around 20 meters. Surface nutrients are rapidly exhausted, and near-surface chlorophyll falls below 0.1 milligrams per cubic meter. Yet below the mixed layer, chlorophyll thrives, reaching about 0.2 milligrams per cubic meter in a pronounced subsurface chlorophyll maximum between 30 and 50 meters, where light from the deeply penetrating euphotic zone, extending to 100 to 140 meters, meets residual nutrients. Integrated biomass during this period is modest at about 17 milligrams per square meter, but most of it sits beneath the layer that satellites would count.

This vertical seesaw between the mixed layer and the water beneath it is the study’s central structural insight. Correlation analysis shows that mixed layer depth is positively linked with chlorophyll in the near-surface and mixed layers (a correlation coefficient of 0.48) and strongly negatively correlated with chlorophyll below the mixed layer (minus 0.87). In other words, when monsoon winds deepen the mixed layer and pump nutrients upward, biomass concentrates near the surface and thins at depth; when the ocean calms and stratifies, the opposite occurs, and phytoplankton migrate downward to a depth where light and nutrients overlap. The subsurface chlorophyll maximum, which the authors note can account for nearly one-fifth of total euphotic-zone biomass basin-wide, is a hidden reservoir that satellite chlorophyll maps systematically miss.

The October rebound has its own mechanism, and it is not simply the mirror image of the September collapse. As the southwest monsoon winds die away, sea surface temperature climbs to a secondary peak of about 29.2 degrees Celsius, the mixed layer shoals to roughly 17 meters, and sea surface salinity drops sharply to about 35.4 practical salinity units. That freshening reflects the poleward reversal of the West India Coastal Current, which begins transporting low-salinity water from the Bay of Bengal along the coast, alongside increased discharge from rivers such as the Periyar, Vaigai, and Tambraparni. These freshwater inputs, combined with northward-flowing surface currents, deliver nutrients and displace nutrient-rich water northward, driving the sharp rise in near-surface chlorophyll, concentrated mainly in the top ten meters of the water column.

The study also highlights how salinity structure shapes the region’s biology in less obvious ways. A barrier layer, the salt-stratified zone between the isothermal layer depth and the mixed layer, thickens markedly during the winter monsoon months of December through February, insulating the mixed layer from cooler subsurface water and influencing sea surface temperature evolution. Meanwhile, the depth of the 20 degree Celsius isotherm, a standard proxy for thermocline depth, deepens during winter, a feature the authors link to heat trapped by the Findlater Jet during the previous summer and redistributed by westward-propagating downwelling Rossby waves. As spring approaches, these waves decay, the thermocline shoals, and the trapped heat is ventilated upward, resetting the upper-ocean structure for the next annual cycle.

Ecologically, the findings help explain why the CEAS sits in an awkward middle ground. To the south, the southeastern Arabian Sea’s vigorous upwelling supports large phytoplankton, rich fisheries, and planktivorous fish communities; to the north, winter convective mixing sustains an extended growing season. The CEAS, by contrast, experiences weaker winds, reduced upwelling, and a deeper thermocline, resulting in lower chlorophyll and integrated biomass during winter and a weaker fishery dominated by different trophic structures. The region also acts as a perennial source of carbon dioxide to the atmosphere in some seasons, while river-fed productivity during the southwest monsoon can temporarily tip it toward being a carbon sink, underscoring how sensitive its biogeochemical balance is to the timing of winds, currents, and runoff.

Perhaps the study’s most important message is methodological. A single float, profiling faithfully every ten days for six years, exposed a seasonal pattern invisible to two decades of satellite ocean color and decades of shipboard sampling. As climate change alters monsoon winds, river discharge, and ocean stratification, the biological response of regions like the CEAS will depend on fine-scale vertical processes that only autonomous biogeochemical sensors can resolve. The September dip and October rebound documented here are more than a regional curiosity; they are a demonstration that the ocean’s hidden interior still holds surprises, and that the robotic observers drifting silently through it are our best hope of catching them before the baseline itself begins to shift.

Subject of Research: Seasonal variability of chlorophyll-a and phytoplankton biomass in the central eastern Arabian Sea observed by biogeochemical Argo floats

Article Title: Variability of chlorophyll-a in the central part of the eastern Arabian Sea using biogeochemical Argo observations

Article References: Paul, M., Nayak, R. R., & Chakraborty, A. (2026). Variability of chlorophyll-a in the central part of the eastern Arabian Sea using biogeochemical Argo observations. Discover Oceans, 3(1), Article 15. https://doi.org/10.1007/s44289-026-00128-2

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00128-2

Keywords: chlorophyll-a, Arabian Sea, BGC-Argo, phytoplankton, monsoon, upwelling, mixed layer depth, subsurface chlorophyll maximum, West India Coastal Current, ocean biogeochemistry, satellite ocean color, marine productivity