Chlorophyll Bloom Research Articles

The Burdwood Bank Circulation

AbstractA suite of high‐resolution numerical simulations characterizes the oceanic circulation in the Burdwood Bank, a shallow seamount located in the northeastern end of the Drake Passage. Model analysis shows energetic upwelling and mixing uplifting deep and benthic waters into the photic layer. Tides and the Antarctic Circumpolar Current are the primary drivers of the bank's circulation. Tidal forcing is the main driver for the entrainment of deep waters into the upper layers of the bank and local wind forcing for the detrainment of these waters into the deep ocean. Passive tracer diagnostics suggest that the dynamical processes triggered by the Burdwood Bank could have a significant impact on local ecosystems and the biogeochemical balance of the southwestern Atlantic region, which is one of the most fertile portions of the Southern Ocean. Model results are robust—they are reproduced in a wide array of model configurations—but there is insufficient observational evidence to corroborate them. Satellite color imagery does not show substantial chlorophyll blooms in this region but it shows strong phytoplankton plumes emanating from the bank. There are several potential explanations for the chlorophyll deficit, including lack of light due to persistent cloud cover, deep mixing layers, fast ocean currents, and the likelihood that blooms, while extant, might not develop on the surface. None of these possibilities can be confirmed at this stage.

Cyclone Enhanced Chlorophyll in the Bay of Bengal as Evidenced from Satellite and BGC-Argo Float Observations

Temporal evolution of biophysical properties of Bay of Bengal (BoB) was measured using an Argo float equipped with fluorescence sensor (BGC-float) during the winter monsoon of 2013–2014. This season was subjected to intense vertical mixing due to the passage of several cyclonic storms beginning with ‘Phailin’ in October to ‘Madi’ in December 2013. These events had resulted in anomalous chlorophyll bloom (> 0.9 mg/l) together with an increase of ~ 1 PSU in the surface salinity and a drop-in temperature (~ 1 °C) in the southwestern BoB. The event was well captured by a BGC-float (WMO ID 2902086) present in the region close to the tracks of cyclones ‘Leher’ and ‘Madi’. The presence of a strong cold-core eddy as observed from the sea level anomaly data played a vital role in spreading the bloom over a wider area and maintaining it for nearly two months from November 2013 to January 2014. This was also evident from the satellite observations of surface chlorophyll-a concentration and Ekman pumping velocity (~ 2 × 10–3 m/s). The intense mixing caused by the cyclonic storms has resulted in eroding the stratification and assisting the entrainment of nutrients into the euphotic layer, which was subsequently sustained by the mesoscale eddy. The present study reveals the strong synergistic application potential of BGC-float data together with satellite observations in monitoring the changes in the vertical structure of chlorophyll during the cyclonic storms and other mesoscale processes.

Geostationary Satellite-Based Observations for Ocean Applications

The study presents assessment and potential oceanographic applications of sea-surface temperature (SST), ocean net shortwave radiation (SWR) and chlorophyll concentration (CC) observations obtained from various geostationary platforms. SST and SWR from imager on-board Indian National Satellite (INSAT- 3D) and CC from Global Ocean Color Imager (GOCI) on-board communication ocean and meteorological satellite (COMS) have been used in the analysis. Relative advantages of high temporal resolution obtained from the geostationary platform compared to polar orbiting platforms are demonstrated. Comparison of INSAT-3D SST with observations gives a correlation of 0.85 and RMSE of 0.81 K. These platforms definitely provide a highly reliable source of continuous observations, which is useful in monitoring dynamic oceanic features such as thermal fronts, chlorophyll blooms, air–sea exchange fluxes, etc. on diurnal to daily timescales.

Differential chlorophyll blooms induced by tropical cyclones and their relation to cyclone characteristics and ocean pre-conditions in the Indian Ocean

Variability of an ocean biological response induced by tropical cyclones and the factors responsible for the differential response is relatively unexplored in the Indian ocean. The aim of the current study is to analyse and identify the difference in the amplitude of chlorophyll blooms induced by tropical cyclones during the period of 1999–2016. The relationship of the amplitude of chlorophyll blooms to the cyclone characteristics (cyclone intensity and translation speed) and the oceanographic parameters (mixed layer depth and nutricline depth) is assessed. Analysis of chlorophyll blooms induced by 28 cyclones during the study period indicated that the amplitude of the chlorophyll concentration varies irrespective of the intensity of the cyclones. The results suggest that the translation speed exhibited by the cyclone is the key factor which controls the amplitude of blooms. The slower translation speed of the cyclone enhances the bloom intensity. Another factor which controls the bloom amplitude is the pre-existing shallow mixed layer depth. A shallow mixed layer modulates the light and nutrient availability which is essential in increasing the chlorophyll concentration. It is observed that shallower nutricline depth also favours an increase in the post-cyclone chlorophyll concentration. In-situ chlorophyll observations from Bio-Argo float during cyclones Hudhud and Vardah revealed that with its slower translation speed and shallower mixed layer depth, cyclone Hudhud could induce stronger bloom than cyclone Vardah, though the intensities of both the cyclones are the same at the location of float. This study implies that relatively weaker tropical cyclones can also induce strong chlorophyll blooms under favourable conditions and not all stronger cyclones induce blooms in the Indian Ocean.

Biogeophysical feedback of phytoplankton on Arctic climate. Part II: Arctic warming amplified by interactive chlorophyll under greenhouse warming

It has been shown that the interaction between marine phytoplankton and climate systems may intensify Arctic warming in the future via shortwave heating associated with increased spring chlorophyll bloom. However, the changes of chlorophyll variability and its impact on the Arctic future climate are uncomprehended. Lim et al. (Clim Dyn. https://doi.org/10.1007/s00382-018-4450-6 , 2018a) (Part I) suggested that two nonlinear rectifications of chlorophyll variability play cooling role in present-day climate. In this study, we suggest that the decreased interannual chlorophyll variability may amplify Arctic surface warming (+ 10% in both regions) and sea ice melting (− 13% and − 10%) in Kara-Barents Seas and East Siberian-Chukchi Seas in boreal winter, respectively. Projections of earth system models show a future decrease in chlorophyll both mean concentration and interannual variability via sea ice melting and intensified surface-water stratification in summer. We found that suggested two nonlinear processes in Part I will be reduced by about 31% and 20% in the future, respectively, because the sea ice and chlorophyll variabilities, which control the amplitudes of nonlinear rectifications, are projected to decrease in the future climate. The Arctic warming is consequently enhanced by the weakening of the cooling effects of the nonlinear rectifications. Thus, this additional biological warming will contribute to future Arctic warming. This study suggests that effects of the mean chlorophyll and its variability should be considered to the sensitivity of Arctic warming via biogeophysical feedback processes in future projections using earth system models.

Robust Projected Weakening of Winter Monsoon Winds Over the Arabian Sea Under Climate Change

AbstractThe response of the Indian winter monsoon to climate change has received considerably less attention than that of the summer monsoon. We show here that all Coupled Model Intercomparison Project Phase 5 (CMIP5) models display a consistent reduction (of 6.5% for Representative Concentration Pathways 8.5 and 3.5% for 4.5, on an average) of the winter monsoon winds over the Arabian Sea at the end of 21st century. This projected reduction weakens but remains robust when corrected for overestimated winter Arabian Sea winds in CMIP5. This weakening is driven by a reduction in the interhemispheric sea level pressure gradient resulting from enhanced warming of the dry Arabian Peninsula relative to the southern Indian Ocean. The wind weakening reduces winter oceanic heat losses to the atmosphere and deepening of convective mixed layer in the northern Arabian Sea and hence can potentially inhibit the seasonal chlorophyll bloom that contributes substantially to the Arabian Sea annual productivity.

Physical control of interannual variations of the winter chlorophyll bloom in the northern Arabian Sea

Abstract. The northern Arabian Sea hosts a winter chlorophyll bloom, triggered by convective overturning in response to cold and dry northeasterly monsoon winds. Previous studies of interannual variations of this bloom only relied on a couple of years of data and reached no consensus on the associated processes. The current study aims at identifying these processes using both ∼ 10 years of observations (including remotely sensed chlorophyll data and physical parameters derived from Argo data) and a 20-year-long coupled biophysical ocean model simulation. Despite discrepancies in the estimated bloom amplitude, the six different remotely sensed chlorophyll products analysed in this study display a good phase agreement at seasonal and interannual timescales. The model and observations both indicate that the interannual winter bloom fluctuations are strongly tied to interannual mixed layer depth anomalies ( ∼ 0.6 to 0.7 correlation), which are themselves controlled by the net heat flux at the air–sea interface. Our modelling results suggest that the mixed layer depth control of the bloom amplitude ensues from the modulation of nutrient entrainment into the euphotic layer. In contrast, the model and observations both display insignificant correlations between the bloom amplitude and thermocline depth, which precludes a control of the bloom amplitude by daily dilution down to the thermocline depth, as suggested in a previous study.

Chlorophyll bloom in response to tropical cyclone Hudhud in the Bay of Bengal: Bio-Argo subsurface observations

Though previous studies have documented substantial increases in chlorophyll concentrations as a result of cyclones, most of them were based on satellite observations dealing with surface chlorophyll blooms. This study documents the subsurface biological response and the subsequent chlorophyll bloom observed in response to the tropical cyclone Hudhud as evident from a Bio-Argo float located at the central Bay of Bengal. Results show high chlorophyll concentrations of up to 4.5mgm−3 which is anomalous in the normally warm, stratified, and oligotrophic Bay of Bengal. The chlorophyll bloom is attributed to the combined effect of subsurface chlorophyll entrainment and nutrient injection. The presence of a pre-existing cyclonic eddy and the decreased translation speed of the cyclone over this region could have played a role in inducing the biological response. This is the first ever report to document the evolution of a subsurface chlorophyll bloom in response to cyclone forcing using Bio-Argo observations.

Satellite assessment of particulate matter and phytoplankton variations in the Santa Barbara Channel and its surrounding waters: Role of surface waves

AbstractSatellite observations of chlorophyll in coastal waters are often described in terms of changes in productivity in response to regional upwelling processes while optical backscattering coefficients are more often linked to episodic inputs of suspended sediments from storm runoff. Here we show that the surface gravity wave resuspension of sediments has a larger role in controlling backscatter than previously considered. Almost 18 years of SeaWiFS, MODIS, MERIS, and VIIRS satellite imagery of the Santa Barbara Channel, California and its surrounding waters spectrally merged with the Garver‐Siegel‐Maritorena bio‐optical model were used to assess the controls on suspended particle distributions. Analysis revealed that chlorophyll blooms in the warmer portions of the domain occur in phase with SST minima, usually in early spring, while blooms in the cooler regions lag SST minima and occur simultaneously to the strongest equatorward winds every year, often in the summer. Tight coupling between the optical variables was seen in offshore areas, as expected for productive waters. However, values of backscatter near the coast were primarily modulated by surface waves. This relationship holds throughout all seasons and is stronger within the 100 m isobath, but often extends tens of kilometers offshore. This forcing of particle resuspension by surface waves is likely a feature ubiquitous in all coastal oceans characterized by fine sediments. The implication of surface wave processes determining suspended particle loads far beyond the surf zone has large consequences for the interpretation of satellite ocean color signals in coastal waters and potentially redefines the extent of the littoral zone.

Sea‐ice retreat controls timing of summer plankton blooms in the Eastern Arctic Ocean

AbstractTwo full‐year mooring records of sea‐ice, physical, and bio‐optical parameters illuminate tight temporal coupling between the retreating seasonal ice edge and the summer phytoplankton bloom on the Laptev Sea shelf. Our records showed no sign of pelagic under‐ice blooms despite available nutrients and thinning sea ice in early summer, presumably because stratification had not yet developed. Chlorophyll blooms were detected immediately after the ice retreated in late May 2014 and late July 2015. Despite radically different timing, the blooms were similar in both magnitude and length, interpreted as community‐level nutrient limitation. Acoustic backscatter records suggest the delayed 2015 bloom resulted in lower zooplankton abundance, perhaps due to a timing mismatch between ice algal and pelagic blooms and unfavorable thermal conditions. Our observations provide classical examples of ice‐edge blooms and further emphasize the complexity of high‐latitude shelves and the need to understand vertical mixing processes important for stratification and nutrient fluxes.

Wind-driven upwelling and surface chlorophyll blooms in Greater Cook Strait

ABSTRACTWe present the results of a combined observational and numerical study to investigate cool plumes of nutrient-rich upwelled water that emanate near the Kahurangi Shoals and extend into Greater Cook Strait. Surface temperature and chlorophyll are mapped using satellite observations to produce surface climatologies, to validate a numerical simulation and to show the utility of using spatial temperature differences as a measure of upwelling. We find upwelling near the Kahurangi Shoals is strongly wind-driven in the weather band. Upwelling occurs at all times of the year, but its surface signature is only visible in summer months. The upwelled nutrient-rich water supports increased primary production compared to surrounding waters, particularly in summer when the water column is more stratified and surrounding surface waters are presumably nutrient depleted.

Riverine influence on ocean color in the equatorial South China Sea

Analysis of SeaWiFS data off Northwest Borneo reveals coastal chlorophyll bloom extending more than 200km to deep South China Sea during winter. A combination of remote sensing products is used to untangle its statistical relation with various forcing factors. River discharge rate is estimated from satellite measurements of land rainfall, and surface chlorophyll over the middle shelf is shown to vary with river discharge and lag by one month. Strong (weak) chlorophyll blooms tend to occur in La Niña (El Niño) years. The study provides evidence that river runoff from maritime continent has major influence on equatorial ocean color.

Spatial distributions of polyunsaturated aldehydes and their biogeochemical implications in the Pearl River Estuary and the adjacent northern South China Sea

This study reports the first comprehensive exploration of the spatial patterns of dissolved and particulate polyunsaturated aldehydes (PUAs), their physical and biological controlling factors, and their potential biogeochemical influences in the Pearl River Estuary (PRE) of the northern South China Sea (NSCS). High levels of total particulate PUAs (0–41nM) and dissolved PUAs (0.10–0.37nM) were observed with substantial spatial variation during an intense summer phytoplankton bloom outside the PRE mouth. We found the particulate PUAs strongly correlated with temperature within the high chlorophyll bloom, while showing a generally positive correlation with chlorophyll-a for the entire region. Additionally, the Si/N ratio significantly correlated with the particulate PUAs along the estuary suggesting the important role of silica on PUA production in this region. The dissolved PUAs counterparts exhibited a positive correlation with chlorophyll-a within the high chlorophyll bloom, but a negatively one with temperature outside, reflecting the essential bio-physical coupling effects on the dissolved PUAs distributions in the ocean. Biogeochemical implications of PUAs on the coastal ecosystem include not only the deleterious restriction of high PUAs-producing diatom bloom on copepod population, but also the profound influence of particulate PUAs on the microbial cycling of organic carbon in the NSCS.

Annual cycles of deep‐ocean biogeochemical export fluxes in subtropical and subantarctic waters, southwest Pacific Ocean

AbstractThe annual cycles of particle fluxes derived from moored sediment trap data collected during 2000–2012 in subtropical (STW) and subantarctic waters (SAW) east of New Zealand are presented. These observations are the most comprehensive export flux time series from temperate Southern Hemisphere latitudes to date. With high levels of variability, fluxes in SAW were markedly lower than in STW, reflecting the picophytoplankton‐dominated communities in the iron‐limited, high nutrient‐low chlorophyll SAW. Austral spring chlorophyll blooms in surface STW were near synchronous with elevated fluxes of bio‐siliceous, carbonate, and organic carbon‐rich materials to the deep ocean, probably facilitated by diatom and/or coccolithophorid sedimentation. Lithogenic fluxes were also high in STW, compared to SAW, reflecting proximity to the New Zealand landmass. In contrast, the highest biogenic fluxes in SAW occurred in spring when surface chlorophyll concentrations were low, while highest annual chlorophyll concentrations were in summer with no associated flux increase. We hypothesize that the high spring export in SAW results from subsurface chlorophyll accumulation that is not evident from remote‐sensing satellites. This material was also rich in biogenic silica, perhaps related to the preferential export of diatoms and other silica‐producing organisms, such as silicoflagellates and radiolarians. Organic carbon fluxes in STW are similar to that of other mesotrophic to oligotrophic waters (∼6–7 mg C m−2 d−1), whereas export from SAW is below the global average (∼3 mg C m−2 d−1). Regional differences in flux across the SW Pacific and Tasman region reflect variations in physical processes and ecosystem structure and function.

A winter chlorophyll bloom analyzed using Lagrangian diagnoses in the northeastern South China Sea

The Finite Time Lyapunov Exponent technique is a highly effective way to visualize and analyze flow fields. Its ridges are referred to as Lagrangian Coherent Structures which reveal the transport barriers of fluid systems, or their hidden skeletons. This allows us to investigate the complex mesoscale and submesoscale ocean processes on a Lagrangian view. In this article, we use the Lagrangian technique to investigate one chlorophyll bloom event in the northeastern South China Sea and discuss the impact of horizontal transport on the distribution of surface chlorophyll. Our results demonstrate that horizontal transport barriers uncovered by Lagrangian Coherent Structures can shape the vortex and filament in a simple and distinct way.

Inferring source regions and supply mechanisms of iron in the Southern Ocean from satellite chlorophyll data

Primary productivity is limited by the availability of iron over large areas of the global ocean. Changes in the supply of iron to these regions could have major impacts on primary productivity and the carbon cycle. However, source regions and supply mechanisms of iron to the global oceans remain poorly constrained. Shelf sediments are considered one of the largest sources of dissolved iron to the global ocean, and a large shelf sediment iron flux is prescribed in many biogeochemical models over all areas of bathymetry shallower than 1000m. Here, we infer the likely location of shelf sediment iron sources in the Southern Ocean, by identifying where satellite chlorophyll concentrations are enhanced over shallow bathymetry (<1000m). We further compare chlorophyll concentrations with the position of ocean fronts, to assess the relative role of horizontal advection and upwelling for supplying iron to the ocean surface. We show that mean annual chlorophyll concentrations are not visibly enhanced over areas of shallow bathymetry that are located more than 500km from a coastline. Mean annual chlorophyll concentrations >2mgm−3 are only found within 50km of a continental or island coastline. These results suggest that sedimentary iron sources only exist on continental and island shelves. Large sedimentary iron fluxes do not seem present on seamounts and submerged plateaus. Large chlorophyll blooms develop where the western boundary currents detach from the continental shelves, and turn eastward into the Sub-Antarctic Zone. Chlorophyll concentrations are enhanced along contours of sea surface height extending off the continental shelves, as shown by the trajectories of virtual water parcels in satellite altimetry data. These analyses support the hypothesis that bioavailable iron from continental shelves is entrained into western boundary currents, and advected into the Sub-Antarctic Zone along the Dynamical Subtropical Front. Our results indicate that upwelling at fronts in the open ocean is unlikely to deliver iron to the ocean surface from deep sources. Finally, we hypothesise how a reduction in sea level may have altered the distribution of shelf sediment iron sources in the Southern Ocean and increased export production over the Sub-Antarctic Zone during glacial intervals.

Thermocline regulated seasonal evolution of surface chlorophyll in the Gulf of Aden.

The Gulf of Aden, although subject to seasonally reversing monsoonal winds, has been previously reported as an oligotrophic basin during summer, with elevated chlorophyll concentrations only occurring during winter due to convective mixing. However, the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) ocean color data reveal that the Gulf of Aden also exhibits a prominent summer chlorophyll bloom and sustains elevated chlorophyll concentrations throughout the fall, and is a biophysical province distinct from the adjacent Arabian Sea. Climatological hydrographic data suggest that the thermocline, hence the nutricline, in the entire gulf is markedly shoaled by the southwest monsoon during summer and fall. Under this condition, cyclonic eddies in the gulf can effectively pump deep nutrients to the surface layer and lead to the chlorophyll bloom in late summer, and, after the transition to the northeast monsoon in fall, coastal upwelling driven by the northeasterly winds produces a pronounced increase in surface chlorophyll concentrations along the Somali coast.

Wind-induced upwelling in the Kerguelen Plateau region

Abstract. In contrast to most of the Southern Ocean, the Kerguelen Plateau supports an unusually strong spring chlorophyll (Chl a) bloom, likely because the euphotic zone in the region is supplied with higher iron concentrations. This study uses satellite wind, sea surface temperature (SST), and ocean color data to explore the impact of wind-driven processes on upwelling of cold (presumably iron-rich) water to the euphotic zone. Results show that, in the Kerguelen region, cold SSTs correlate with high wind speeds, implying that wind-mixing leads to enhanced vertical mixing. Cold SSTs also correlate with negative wind-stress curl, implying that Ekman pumping can further enhance upwelling. In the moderate to high eddy kinetic energy (EKE) regions surrounding Kerguelen, we find evidence of coupling between winds and SST gradients associated with mesoscale eddies, which can locally modulate the wind-stress curl. This coupling introduces persistent wind-stress curl patterns and Ekman pumping around these long-lived eddies, which may modulate the evolution of Chl a in the downstream plume far offshore. Close to the plateau, this eddy coupling breaks down. Kerguelen has a significant wind shadow on its downwind side, which changes position depending on the prevailing wind and which generates a wind-stress curl dipole that shifts location depending on wind direction. This leads to locally enhanced Ekman pumping for a few hundred kilometers downstream from the Kerguelen Plateau; Chl a values tend to be more elevated in places where wind-stress curl induces Ekman upwelling than in locations of downwelling, although the estimated upwelling rates are too small for this relationship to derive from direct effects on upward iron supply, and thus other processes, which remain to be determined, must also be involved in the establishment of these correlations. During the October and November (2011) KErguelen Ocean and Plateau compared Study (KEOPS-2) field program, wind conditions were fairly typical for the region, with enhanced Ekman upwelling expected to the north of the Kerguelen Islands.

Impact of eddies on surface chlorophyll in the South Indian Ocean

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 behavior 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 behavior 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 28°S), particularly in winter. Using Self Organizing Maps we analyze the chlorophyll patterns within anticyclonic eddies and cyclonic eddies and highlight their complexity. Our analysis suggests that multiple mechanisms may underlie the observed eddy/chlorophyll relationship. Based on Argo float data, we postulate the relationship may be partly related to seasonal adjustment of the mixed layer depth within eddies. Deeper mixing in anticyclonic eddies is expected to enhance nutrient supply to the mixed layer, while shallower mixing in cyclonic eddies is expected to reduce it. This could explain why the observed winter surface chlorophyll bloom is stronger in anticyclonic eddies than in cyclonic eddies.

Polar Front around the Kerguelen Islands: An up‐to‐date determination and associated circulation of surface/subsurface waters

AbstractThe circulation of iron‐rich shelf waters around the Kerguelen Islands plays a crucial role for a climatically important, annually recurrent phytoplankton spring bloom over the sluggish shelf region and its downstream plume area along the Antarctic circumpolar flow. However, there is a long‐standing confusion about the Polar Front (PF) in the Kerguelen region due to diverse suggestions in the literature for its geographical location with an extreme difference over 10° of latitude. Based on abundant historical hydrographic data, the in situ hydrographic and current measurements during the 2011 KEOPS2 cruise, satellite chlorophyll images, and altimetry‐derived surface velocity fields, we determine and validate an up‐to‐date location of the PF around the Kerguelen Islands. Artificial Lagrangian particle trajectories computed from altimetric velocity time series are analyzed for the possible pathways and sources of different surface/subsurface waters advected into the chlorophyll bloom area east off the islands studied during the KEOPS2 cruise. The PF location determined as the northernmost boundary of the Winter Water colder than 2°C, which is also associated with a band of strong currents, appears to be primarily controlled by topography. The PF rounds the Kerguelen Islands from the south to deflect northward along the eastern escarpment up to the northeastern corner of the Kerguelen Plateau before making its southward retroflection. It is shown that the major surface/subsurface waters found within the deep basin east of the Kerguelen Islands originate from the shelf around the Heard Island, rather than from the shallow shelf north of the Kerguelen Islands.