Physical controls of chlorophyll distribution in the North Indian Ocean

Abstract

The north Indian Ocean is one of the highly productive regions among the tropical oceans. Being a prominent monsoon regime, the basin exists as a highly dynamic region charac- terised by intense mixing and upwelling and strong surface currents. This provides a favourable background for the formation of strong surface chlorophyll blooms through ef- cient vertical supply of biologically rich subsurface waters. Earlier studies have suggested the determinant role of upper-ocean physical processes associated with the monsoons in controlling the biological productivity of the north Indian Ocean. A strong seasonality is observed in the surface chlorophyll distribution of the north Indian Ocean, with two blooming seasons, the summer and winter monsoons, in which the former is the most productive season. The seasonally reversing monsoon winds favour vertical mixing and upwelling, whereas the surface freshwater in flux, mainly through the monsoonal river dis- charge, weakens the vertical processes by increasing density strati fication. The competing impacts of the monsoonal wind and freshwater forcings on the density structure of the upper ocean determine the vertical transport of nutrients and chlorophyll from the subsur- face layers to the surface sunlit layers. Additionally, the seasonality in surface chlorophyll is often regulated by the basin-wide variability in surface wind patterns associated with interannual events, like the Indian Ocean Dipole (IOD) and the El Nino. Despite being a highly productive basin with complex dynamical characteristics, the north Indian Ocean remains undersampled, unlike the other tropical oceans. Extensive research efforts are needed incorporating both observations and biogeochemical modelling, in order to ll the gaps in our understanding of the phytoplankton dynamics of the basin. This thesis investigates the role of upper-ocean physical processes, acting at seasonal and interannual timescales, in controlling the chlorophyll distribution of the north Indian Ocean, focusing on some of its biological hotspots that develop during summer. The selected study regions are the major upwelling zones around the Indian subcontinent, the northwestern BoB (NWBoB), the southern BoB (SBoB), and the southeastern Ara- bian Sea (SEAS), which contribute signi cantly to the marine sheries production of the country. In particular, the study focuses on the relative importance of surface wind and freshwater forcings in determining the formation and evolution of the surface chlorophyll blooms. Special attention is given in documenting the vertical distribution of chlorophyll, especially the deep chlorophyll maxima (DCM). The study makes use of a wide range of datasets, including satellite observations, in situ measurements from Seagliders deployed during the Bay of Bengal Boundary Layer Experiment (BoBBLE) eld program, and sim- ulations from a coupled physical-biogeochemical model. The Seagliders provide vertical pro les of chlorophyll at high-resolution, which is suitable to study the DCM with large spatio-temporal variability. The biogeochemical model, the Tracers of Phytoplankton with Allometric Zooplankton (TOPAZ) model, includes an advanced representation of oceanic biogeochemical cycles, providing realistic simulations of chlorophyll distribution in the Indian Ocean. The model was used to isolate the effect of wind and freshwater forcings on the upper-ocean density strati cation and identify the relative importance of different mixed layer processes in controlling the surface nutrient distribution. Biological productivity in the surface layers of the BoB is generally weak, owing to nutrient limitation imposed by strong vertical strati cation due to the presence of large freshwater in flux. However, formation of chlorophyll blooms at the surface can be ob- served in regions like the NWBoB, where the monsoonal wind forcing is favourable for upwelling. During the peak phase of the summer monsoon, the NWBoB exhibits an intense surface chlorophyll bloom, extending from the northeast coast of India to the off- shore regions. The vertical distribution of upper ocean properties and nutrient budget analysis obtained from the coupled physical-biogeochemical model reveals the dominant role of coastal upwelling driven by alongshore winds in triggering this seasonal bloom. Horizontal advection plays a secondary role by increasing the spatial extent of the surface bloom, through the supply of upwelled water from the coastal to offshore regions. The bloom decays with the weakening of surface winds and coastal upwelling by the end of the summer monsoon. Surface chlorophyll concentrations remain largely unaffected by the riverine freshwater since the river plumes are inhibited from reaching the location of the bloom due to the strong coastal upwelling. This shows that the timing and intensity of surface wind and freshwater forcings are important in determining the seasonal evolution of chlorophyll blooms in strati ed basins like the BoB. The SBoB is a highly dynamic sector, located away from the direct in uence of the riverine freshwater discharge. Surface chlorophyll distribution in the SBoB is determined by vertical mixing, strong open ocean upwelling in the region of the Sri Lanka Dome (SLD), advection by the summer monsoon current (SMC) as well as the remote in fluences associated with the westward propagating Rossby waves. The large spatial heterogeneity in the upper ocean dynamics makes the region suitable to examine the differential re- response of the vertical distribution of chlorophyll, particularly the DCM, to various mixed layer processes. The vertical shape of chlorophyll pro les is largely determined by the in- tensity and depth distribution of DCM. The Seaglider observations revealed the presence of prominent DCM in the SBoB, with high sensitivity to the variations in the upper-ocean density structure. Upwelling favoured sharp and intense DCM, leading to a pronounced vertical structure in chlorophyll. On the other hand, mixing resulted in a weaker DCM, leaving the shape of chlorophyll pro les rather diffused. Surface freshening and the as- sociated near-surface strati cation favoured the intensi cation of DCM, along with the weakening of surface chlorophyll. DCM intensi cation can be attributed to the inhibition of phytoplankton dilution under reduced vertical mixing and the relaxation in light lim- itation, as the self-shading effect of surface phytoplankton weakens. It is suggested that the persistence of DCM in the BoB is promoted by surface oligotrophy and shallow mixing. Unlike the BoB, the Arabian Sea is only weakly in uenced by freshwater from rivers, and at the same time, the surface winds are relatively stronger here. Consequently, the vertical mixing and upwelling processes are stronger in the Arabian Sea, favouring high biological productivity. The SEAS is one such region, exhibiting intense surface chloro- phyll blooms triggered by coastal upwelling off the coast of Kerala, during the peak phase of the summer monsoon. In 2016, this climatological seasonal pattern of surface chloro- phyll in the SEAS was disrupted owing to the development of an extreme negative IOD (nIOD). The region exhibited unusually high surface chlorophyll during the late boreal summer and fall intermonsoon of 2016, the highest concentrations ever reported in the region in the past two decades of the satellite era. The anomalous bloom was triggered by two cold-core eddies, along the periphery of the coastal upwelling zone. Observational evidence suggest that the eddy formation can be attributed to the local wind forcing and remote effects originating around Sri Lanka and the southern tip of India in association with the extreme nIOD. Pre-conditioning of the upper ocean induced by the seasonal shoaling of thermocline (nutricline) due to coastal upwelling during the peak monsoon phase is suggested to have a non-negligible role in favouring efficient nutrient supply to the surface layers during the eddy activity. Biological productivity in the north Indian Ocean has signi ficant climatic feedbacks, through its control on the air-sea exchanges of heat and greenhouse gases. Also, the bi- ologically productive regions of the basin support one among the world's largest marine fi shing sectors, contributing to the country's economic stability. Hence, understanding the processes underlying the chlorophyll distribution in the north Indian Ocean is climatically and economically signi ficant. The present thesis provides new insights into several un- explored aspects of chlorophyll distribution in the north Indian Ocean, pointing towards the need for extensive in situ data collection and biogeochemical modelling to better un- derstand its biophysical interactions and the associated climatic feedbacks.