Observational insights into chlorophyll distributions of subtropical South Indian Ocean eddies (original) (raw)
Related papers
Impact of eddies on surface chlorophyll in the South Indian Ocean
Journal of Geophysical Research: Oceans, 2014
A unique feature of the subtropical South Indian Ocean is the existence of anticyclonic eddies that have higher chlorophyll concentrations than cyclonic eddies. Off Western Australia, this anomalous behaviour is related to the seeding of anticyclonic eddies with shelf water enriched in phytoplankton biomass and nutrients. Further off-shore, two mechanisms have been suggested to explain the eddy/chlorophyll relationship: (i) eddies originating from the Australian coast maintain their chlorophyll anomaly while propagating westward; and (ii) eddy-induced Ekman upwelling (downwelling) enhances (dampens) nutrient supply in anticyclonic (cyclonic) eddies. Here we describe the relationship between eddies and surface chlorophyll within the South Indian Ocean, and discuss possible mechanisms to explain the anomalous behaviour in light of new analyses performed using satellite chlorophyll data. We show that anticyclonic eddies exhibit higher surface chlorophyll concentration than cyclonic eddies across the entire South Indian Ocean basin (from 20 to 28ºS), particularly in winter.
Role of physical processes in chlorophyll distribution in the western tropical Indian Ocean
Physical control of the chlorophyll a (chl a) distribution in the western tropical Indian Ocean (WTIO, 8°N to 18°S along 65°E) was studied during the 2008 winter monsoon (WM) and the 2009 summer monsoon (SM). During both seasons, a prominent deep chlorophyll maximum (DCM, 0.3-0.5 mg m −3 ) was observed at all stations between 8°N and 10°S in the depth range of 50-75 m, but south of 10°S, this phenomenon was observed as deeper (~120 m) and relatively weak (0.15-0.3 mg m −3 ). During the SM, in addition to seasonal forcing, eddies and a freshened surface layer also played major roles in controlling the DCM and the surface chl a concentrations in the southern Arabian Sea and the equatorial Indian Ocean. During the WM, surface freshening controlled the chl a distribution in the Seychelles Chagos Thermocline Ridge (SCTR, 5°S-10°S) region by modulating the static stability and mixed layer depth. It appears that the surface freshening in this region is associated with the core of the South Equatorial Current. South of the SCTR, the chl a distribution was predominantly determined by the anti-cyclonic eddies in both seasons. The spatial patterns of the Sea Level Anomaly (SLA) followed most of the thermocline features observed during the study period.
Chlorophyll Distribution and Variability at the Surface of the Indian Ocean
Journal of Research in Environmental and Earth Sciences, 2022
Surface chlorophyll in the oceans can be a valuable tool for tracing surface water mass movement and analysing water mass mixing, important to the formation of water masses within the global oceanic circulation. It is observed differences in chlorophyll concentration in the oceanic surface layer from one oceanic region to the other. This work establishes a distribution and variability of chlorophyll in the surface layer of the tropical Indian Ocean (25°N to 30°S and 30°E to 120°E). It is based on analysis of semi-annual and annual amplitudes of chlorophyll. It is discussed the different physical processes that can be involved with the variability of chlorophyll. This analysis is strongly restricted to the available data sets. The geographical representation of the variability and distribution of the surface chlorophyll agrees well with the theoretical frame of reference.
Biogeosciences, 2013
The Indian Ocean Dipole (IOD) and the El Niño/Southern Oscillation (ENSO) are independent climate modes, which frequently co-occur, driving significant interannual changes within the Indian Ocean. We use a fourdecade hindcast from a coupled biophysical ocean general circulation model, to disentangle patterns of chlorophyll anomalies driven by these two climate modes. Comparisons with remotely sensed records show that the simulation competently reproduces the chlorophyll seasonal cycle, as well as open-ocean anomalies during the 1997/1998 ENSO and IOD event. Results suggest that anomalous surface and euphoticlayer chlorophyll blooms in the eastern equatorial Indian Ocean in fall, and southern Bay of Bengal in winter, are primarily related to IOD forcing. A negative influence of IOD on chlorophyll concentrations is shown in a region around the southern tip of India in fall. IOD also depresses depthintegrated chlorophyll in the 5-10 • S thermocline ridge region, yet the signal is negligible in surface chlorophyll. The only investigated region where ENSO has a greater influence on chlorophyll than does IOD, is in the Somalia upwelling region, where it causes a decrease in fall and winter chlorophyll by reducing local upwelling winds. Yet unlike most other regions examined, the combined explanatory power of IOD and ENSO in predicting depth-integrated chlorophyll anomalies is relatively low in this region, suggestive that other drivers are important there. We show that the chlorophyll impact of climate indices is frequently asymmetric, with a general tendency for larger positive than negative chlorophyll anomalies. Our results suggest that ENSO and IOD cause significant and predictable regional re-organisation of chlorophyll via their influence on near-surface oceanography. Resolving the details of these effects should improve our understanding, and eventually gain predictability, of interannual changes in Indian Ocean productivity, fisheries, ecosystems and carbon budgets.
Vertical distribution of chlorophyll in dynamically distinct regions of the southern Bay of Bengal
Biogeosciences, 1999
The Bay of Bengal (BoB) generally exhibits surface oligotrophy due to nutrient limitation induced by strong salinity stratification. Nevertheless, there are hotspots of high chlorophyll in the BoB where the monsoonal forcings are strong enough to break the stratification; one such region is the southern BoB, east of Sri Lanka. A recent field programme conducted during the summer monsoon of 2016, as a part of the Bay of Bengal Boundary Layer Experiment (BoBBLE), provides a unique high-resolution dataset of the vertical distribution of chlorophyll in the southern BoB using ocean gliders along with shipboard conductivity-temperature-depth (CTD) measurements. Observations were carried out for a duration of 12-20 days, covering the dynamically active regions of the Sri Lanka Dome (SLD) and the Southwest Monsoon Current (SMC). Mixing and up-welling induced by the monsoonal wind forcing enhanced surface chlorophyll concentrations (0.3-0.7 mg m −3). Prominent deep chlorophyll maxima (DCM; 0.3-1.2 mg m −3) existed at intermediate depths (20-50 m), signifying the contribution of subsurface productivity to the biological carbon cycling in the BoB. The shape of chlorophyll profiles varied in different dynamical regimes; upwelling was associated with sharp and intense DCM, whereas mixing resulted in a diffuse and weaker DCM. Within the SLD, open-ocean Ekman suction favoured a substantial increase in chlorophyll. Farther east, where the thermocline was deeper, enhanced surface chlorophyll was associated with intermittent mixing events. Remote forcing by the westward propagating Rossby waves influenced the upper-ocean dynamics and chlorophyll distribution in the southern BoB. Stabilizing surface freshening events and barrier-layer formation often inhibited the generation of surface chlorophyll. The pathway of the SMC intrusion was marked by a distinct band of chlorophyll, indicating the advective effect of biologically rich Arabian Sea waters. The region of the monsoon current exhibited the strongest DCM as well as the highest column-integrated chlorophyll. Observations suggest that the persistence of DCM in the southern BoB is promoted by surface oligotrophy and shallow mixed layers. Results from a coupled physical-ecosystem model substantiate the dominant role of mixed layer processes associated with the monsoon in controlling the nutrient distribution and biological productivity in the southern BoB. The present study provides new insights into the vertical distribution of chlorophyll in the BoB, emphasizing the need for extensive in situ sampling and ecosystem model-based efforts for a better understanding of the biophysical interactions and the potential climatic feedbacks.
International Journal of Remote Sensing, 2019
Surface chlorophyll-a (chl-a) variation in the Southeastern Tropical Indian Ocean (SETIO) shows different patterns in response to the various types of the Indian Ocean Dipole (IOD) events. Thirteen years of remotely sensed surface chl-a data from the Moderateresolution Imaging Spectroradiometer (MODIS) were used to evaluate interannual surface chl-a variation in the SETIO. During the period of analysis (January 2003-December 2015), there were three canonical positive IOD (pIOD) and four pIOD Modoki events. It is found that the spatial patterns of surface chl-a variation were coherent with the pattern of surface wind anomaly, and the sea surface temperature anomaly (SSTA). During canonical pIOD events, high chl-a concentrations were observed in the vicinity of the Sunda Strait and along the coast of western tip of the Java Island around the Cilacap region. Meanwhile, during pIOD Modoki event, surface chl-a concentration was relatively higher and distributed wider than those observed during canonical pIOD event. The analysis shows that relatively weak upwelling event indicated by a deep isothermal layer depth (ILD) during pIOD Modoki events combined with thin barrier layer thickness (BLT) and deep mixed layer provides a favourable condition for an increase in surface chla in the SETIO region. Meanwhile, strong upwelling as indicated by shallow ILD combined with thick BLT and shallow mixed layer prevents surface chl-a to increase during canonical pIOD events.
Deep-sea Research Part Ii-topical Studies in Oceanography, 2007
Deep chlorophyll maxima (DCM) have the capacity to fuel substantial fractions of total water column production. The ecological importance of a ubiquitous DCM layer ranging from 50 to 120 m deep within Leeuwin Current (LC) and offshore waters of Western Australia is addressed here using data from a regional oceanographic field study conducted during the austral summer of 2000. Phytoplankton communities from surface and DCM layers were compared by examining pigments (chlorophyll a), phytoplankton carbon, photosynthetic characteristics and productivity rates estimated using 14C-based photosynthesis versus irradiance relationships. In the DCM layer, both extracted pigments (up to 0.83 mg m−3) and phytoplankton carbon (6.4–54.4 mg C m−3) were maximal, and were on average 6 and 5 times larger than in the surface layer, respectively. Sensitivity analyses were performed on production estimates using regionally relevant ranges of light attenuation (Kd=0.050–0.066 m−1) and photoinhibition (β*=0.00–0.01 mg C (mg chl a)−1 h−1 [μmol m−2 s−1]−1). These analyses provide upper and lower limits on previously reported estimates of primary production for the region, and show that small differences in light attenuation and photoinhibition can significantly affect computations of primary production and cause a shift from surface-dominated to DCM-dominated production scenarios. The contribution of the DCM layer to total water-column production ranged from a maximum of 30–70% under the scenarios examined. A regional overview of nitrate and stratification conditions in relation to the depth of the phytoplankton biomass maximum indicated that the critical balance between light and nutrients was a key factor driving DCM structure. We show that changing oceanographic conditions in both the along-shore and cross-shore directions, which included latitudinal variation in the strength of the LC, are accompanied by changes in the depth (and in turn production) of the DCM. The previously unrecognized significance of these DCM layers in the coastal eastern Indian Ocean has important implications for satellite-based estimates of production within the region.
A quantitative method for describing the seasonal cycles of surface chlorophyll in the Indian Ocean
Proceedings of SPIE - The International Society for Optical Engineering
The seasonal cycles of surface chlorophyll (SCHL) in the Indian Ocean (IO) are regionally described by means of 6 parameters: the timing of the bloom onset and of the bloom peak, and the integrated SCHL value in between these two extrema for both winter and summer blooms. This description, based on a climatology constructed from 7 years of SeaWiFS data, provides a regional image of the influence of the two monsoons on phytoplankton blooms. Over a large part of the basin, the seasonal cycle is characterized by two distinct growth periods, one in summer during the South West Monsoon (SWM), the other in winter during the North East Monsoon (NEM). However, in some specific areas such as the southwestern coast of India, there is no maxima during the NEM. The bloom areas during the SWM and the NEM show totally different regional patterns. Important lags in the timing of the blooms are identified, and are also associated with distinct regional patterns. The next step in the understanding o...