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Mesoscale eddies are major features in ocean dynamics that significantly influence ocean production depending on their polarity, life-stage, location and time of formation. Eddies in the south-west Indian Ocean have been the focus of many studies, especially in the Mozambique Channel where southward-moving cyclonic and anticyclonic eddies feed into the Agulhas Current. However, eddies formed south of Madagascar that move westwards towards the coast of South Africa have been much less studied, despite their significant contribution to the Agulhas Current. This study aims to understand how the plankton communities (i.e. pico-, nano-, phyto-, microzoo- and mesozooplankton) are distributed in a cyclonic eddy generated off southern Madagascar. We also investigate a possible link between the Madagascar shelf and the eddy. Temperature, salinity, fluorescence and plankton abundance were measured at 25 stations spaced at 18.5 km intervals across a young (~ 1 month old) cyclonic eddy, and at three locations on the Madagascar shelf, in July 2013. A patch of enhanced chlorophyll a concentration was found at four stations in the eddy, homogeneously distributed vertically in the upper mixed layer and characterised by a higher proportion of diatoms compared to the other stations. Abundances of picoplankton, nanoplankton, ciliates and dinoflagellates were very similar within and outside the eddy. Zooplankton followed a contrasting pattern with higher biomass at four stations just to the west of this high diatom patch. The mesozooplankton composition at these four stations revealed greater species similarities with the Madagascar shelf than with the other stations, suggesting a link between the shelf and the western side of the eddy. The periphery of the eddy seems to have been primarily influenced by a strong geostrophic current propagating from the southern shelf of Madagascar which then wrapped itself around the eddy. This filament may be the source of the high zooplankton biomass in the eddy periphery, which would account for the similarities in zooplankton community composition between the eddy periphery and the shelf. [ABSTRACT FROM AUTHOR]

Abstract The synoptic monthly gridded (SMG) world ocean database (WOD) were constructed with 1° × 1° horizontal resolution and 28 standard vertical levels from the surface to 3000 m deep using the optimal spectral decomposition technique. Monthly acceleration potential (AP) relative to 1000-dbar is calculated for the isopycnal surface 26.4 kg m−3 from January 1960 to December 2014 to investigate the spatial and temporal variability of the California Current. A composite analysis was conducted to obtain the total-time mean AP field and the climatological monthly mean AP variability, which is two orders of magnitude smaller than the total-time mean. Residual data were used to examine interannual variations of the AP field. An empirical orthogonal function (EOF) analysis was conducted to analyze the AP anomaly data. The EOF-1 (40% variance) represents strengthening/weakening of the North Pacific Gyre and California Current. EOF-2 (20% variance) represents onshore/offshore geostrophic flow. The first principal component was negatively correlated with the PDO; the second principal component was negatively correlated with both the PDO and ENSO, which implied that the AP anomaly contains climate variability signals. [ABSTRACT FROM AUTHOR]

The National Oceanic and Atmospheric Administration׳s (NOAA) Alaska Fisheries Science Center bottom trawl survey samples demersal fish at over 350 sites on the eastern Bering Sea continental shelf each summer on a 37×37 km 2 grid. Rugged conductivity–temperature-depth sensors (CTDs) were added to the net hauls to obtain gridded data sets of temperature and salinity measurements for 2008–2010. Results reveal the three-dimensional thermohaline structure of the shelf including the Cold Pool and areas of fresher water around St. Matthew Island and in Bristol Bay. Horizontal gradients are often strongest roughly along the 50-m and 100-m isobaths that traditionally separate the inner- and outer-shelf from the middle-shelf centered along the 70-m isobath. The summer mixed layer depth is less than 30 m over much of the region. It reaches the bottom along the Alaska Peninsula in water depths greater than 70 m, showing that the boundary of the well-mixed, inner shelf is not always at the 50-m isobath. The greatest upper-to-lower layer density difference is found across the shelf north of 59°N. The salinity difference is the main contributor to this density difference over most of the region in 2008 and 2010, but the temperature difference dominates in 2009 due to decreased ice melt and reduced freshening near St. Matthew Island. The geostrophic velocity relative to the bottom shows northwestward flow seaward of the 100-m isobath and northwestward transports integrated across the shelf of 0.10–0.25×10 6 m 3 /s. In 2008 and 2010 there was clockwise circulation in a region of less-saline water around St. Matthew Island. In 2009 that fresher lens did not exist, and flow was more concentrated along the 100-m isobath bringing saltier water across the shelf. [ABSTRACT FROM AUTHOR]

More than 20 years of data from moorings, satellite-tracked drifters and hydrographic surveys are integrated to provide a comprehensive view of currents and transport on the eastern Bering Sea shelf. The major sources of water onto the eastern Bering Sea shelf are North Pacific water flowing through Unimak Pass and Bering Slope water flowing onto the shelf usually via the canyons that intersect the shelf break. Absolute geostrophic transport through Unimak Pass varies from an average of 0.25×10 6 m 3 s −1 (Sv) in the warm months to 0.43 Sv in the cold months. Flow along the 50-m isobath is weak, with a transport of <0.1 Sv (calculated from current meters) in summer and fall. The transport along the 100-m isobath measured at two locations is more than twice that along 50-m isobaths; in the summer at the Pribilof Islands it was 0.2 Sv and during spring and summer at 60°N the northward geostrophic transport (referenced to the bottom) was 0.31 Sv. Northward transport along the 100-m and 50-m isobaths accounts for approximately half of the transport through Bering Strait. A typical transit time from Unimak Pass to Bering Strait is >13 months and from Amukta Pass to Bering Strait via the Bering Slope Current is >8 months. Consequently, the source of most of the heat transported into the Arctic through Bering Strait is a result of air–sea interactions local to the northern Bering Sea. Analysis of the currents and water properties on the southern shelf indicates that ~50% of the shelf water is exchanged with slope water during October–January each year. This exchange elevates the October mid-shelf average nitrate level from 6 μM to 14–16 μM by the end of January. [ABSTRACT FROM AUTHOR]

Abstract: The MESOBIO programme investigated mesoscale dynamics using an integrated ecosystem approach, linking physical and biogeochemical processes with different trophic levels. Observation and modeling were used in combination to explain the main processes occurring in the mesoscale eddy field. The particular shape of the Mozambique Channel, composed of two basins interconnected through a narrow zone, favours the generation of mesoscale eddies and increases the opportunity for eddy-shelf interactions. Phytoplankton abundance peaked in areas of nutrient enrichment that are often found in the core of cyclonic eddies, as well as on the continental shelf. Grazers in zooplankton communities exhibited high biovolume in cyclonic eddies, but their abundance was lower in fronts and divergence zones, with lowest biovolume in anticyclones. Biovolume was highest at shelf stations, but very variable and similar to phytoplankton. Age of eddies, their subsequent maturation stage and the dynamics of the eddy field played a major role effecting zooplankton abundance. Micronekton presented abundance patterns coherent with zooplankton distribution, however this was only demonstrated by acoustic methods, whereas mid-water trawl collection and predators stomach contents (predators being used as biological samplers) did not reveal significant relationships with mesoscale features. For upper trophic levels, the average density of foraging seabirds was lowest in anticyclones, highest in cyclones and at intermediate levels in divergence, shelf and frontal zones. However, multifaceted behavioral responses were observed in such a highly variable environment. Swordfish was clearly associated with divergence zones, and to a lesser extent with fronts, suggesting that the higher density in divergences was related to the presence of its main prey, essentially large squids. Although tunas tended to be more abundant in areas with weak geostrophic currents, their relationship to mesoscale features was not straightforward as adult tunas caught by longline have the ability to explore different foraging habitats over a broad range of depths. Several suggestions for advancing eddy-related research from the current state of knowledge are proposed in the second part of the paper. [Copyright &y& Elsevier]

Abstract: Circulation and the related biological production have been studied during five cruises conducted in the Mozambique Channel (MZC) between 2005 and 2010. The circulation in the MZC is known to be highly turbulent, favouring enhanced primary production as a result of mesoscale eddy dynamics, and connectivity throughout the Channel due to the variable currents associated with migrating eddies. This paper presents the results of in situ measurements that characterize the horizontal and vertical currents in the surface and subsurface layers (0–500m). The in situ data were analysed together with the geostrophic eddy field observed from satellite altimeter measurements. Different circulation regimes were investigated, including the “classical” anticyclonic eddy generated at the Channel narrows (16°S), the enhancement of southward migrating eddies by merging with structures (both cyclonic and anticyclonic) formed in the east of the Channel, and the presence of a fully developed cyclonic eddy at the Channel narrows. Comparison between in situ measurements (S-ADCP and velocities derived from surface drifters) and the geostrophic current derived from sea surface height measurements indicated that the latter can provide a reliable, quantitative description of eddy driven circulation in the MZC, with the exception that these currents are weaker by as much 30%. It is also suggested from in situ observation (drifters) that the departure from geostrophy of the surface circulation might be linked to strong wind conditions. Finally, our observations highlight that a-geostrophic currents need to be considered in future research to facilitate a more comprehensive description of the circulation in this area. [Copyright &y& Elsevier]

Abstract: A new estimate of the Global Ocean 3D geostrophic circulation from the surface down to 1500m depth (Surcouf3D) has been computed for the 1993–2008 period using an observation-based approach that combines altimetry with temperature and salinity through the thermal wind equation. The validity of this simple approach was tested using a consistent dataset from a model reanalysis. Away from the boundary layers, errors are less than 10% in most places, which indicate that the thermal wind equation is a robust approximation to reconstruct the 3D oceanic circulation in the ocean interior. The Surcouf3D current field was validated in the Atlantic Ocean against in-situ observations. We considered the ANDRO current velocities deduced at 1000m depth from Argo float displacements as well as velocity measurements at 26.5°N from the RAPID-MOCHA current meter array. The Surcouf3D currents show similar skill to the 3D velocities from the GLORYS Mercator Ocean reanalysis in reproducing the amplitude and variability of the ANDRO currents. In the upper 1000m, high correlations are also found with in-situ velocities measured by the RAPID-MOCHA current meters. The Surcouf3D current field was then used to compute estimates of the Atlantic Meridional Overturning Circulation (AMOC) through the 25°N section, showing good comparisons with hydrographic sections from 1998 and 2004. Monthly averaged AMOC time series are also consistent with the RAPID-MOCHA array and with the GLORYS Mercator Ocean reanalysis over the April 2004–September 2007 period. Finally a 15 years long time series of monthly estimates of the AMOC was computed. The AMOC strength has a mean value of 16Sv with an annual (resp. monthly) standard deviation of 2.4Sv (resp. 7.1Sv) over the 1993–2008 period. The time series, characterized by a strong variability, shows no significant trend. [Copyright &y& Elsevier]

Abstract: Inter-annual variability in the magnitude and timing of the spring bloom was investigated for the Oyashio region (40 °-48 °N, 143 °E-152 °E) using 10 years (from 1998 to 2007) of satellite ocean-color data. Geostrophic currents were examined using satellite altimeter data. Early spring blooms (>1.5mgm−3) occurred in early April 2001 and late March 2002. The 2001 bloom continued for one month. Late blooms occurred from mid-May 1999, early June 2004 and late April 2006, continuing for about 1 month, 8 days and 16 days, respectively. A strong bloom (4.7mgm−3) also occurred in mid-April 1998; however, it terminated in early May. We classified the Oyashio region based on the pattern of temporal variation of Chl-a concentration from March to June. The spatio-temporal variability in Chl-a concentration during spring was different among years. The area where Chl-a concentration was highest in April was more extensive in 2001, 2002 and 2006 than usual. In 1999, the area where Chl-a concentration was highest in May was the widest among the 10 years. Mesoscale eddies and currents with high velocity were frequently observed in the area of high Chl-a concentration east of Hokkaido, propagating Coastal Oyashio Water of low salinity and low density into the oceanic region. That strengthened stratification in the surface layer. We suggest that this seaward transfer of coastal water could be one of the important factors for phytoplankton distribution in two ways: (1) horizontal advection of water with high Chl-a concentration and (2) enhancement of stratification in the oceanic region. [Copyright &y& Elsevier]

Abstract: Data from Argo floats indicate that significant salinity changes have occurred in the North Pacific thermocline relative to data collected in the previous two decades, including observations obtained as part of the WOCE hydrographic program. Such a salinity decrease on both isopyncals and isobars implies a freshening scenario in the near-surface source region of this water mass. The frequently repeated meridional section P16 supports this inference. The subsurface salinity freshening likely began in the early 1990s, strengthened through 1997, and continued into the 2000s; the surface salinity freshening had commenced by 1984 and continued through the first decade of the 21st century. The spatial distribution of salinity change on the density surface σθ=25.5 is examined through comparisons of Argo and most of the North Pacific WOCE sections (1985-1994) and between Argo and the Hydrobase climatology, largely composed of data from the late 1970s through the mid-1980s. Both comparisons show a large-scale, basin-wide decrease in subsurface salinity through the Argo time period used in this analysis (2003-2006). The salinity difference is maximum in the northeast area and spreads southward and westward, approximately following geostrophic streamlines. [Copyright &y& Elsevier]

Abstract: Samples obtained in the entrance of the Sea of Cortés were used to study the distribution of euphausiids. This area has a complex hydrography with mixing of the California Current water from the north with Gulf of California and tropical waters arriving from the south probably from the warm pool in southern México, as shown by satellite images and hydrography. The origin of the different water masses is also reflected in their species composition. Three sectors can be recognized according to hydrographic evidence: North, South, and East, with each showing differences in species composition. The dominant species was the neritic low- oxygen-affinity species Euphausia lamelligera, followed by Euphausia eximia, Euphausia tenera, and Euphausia distinguenda. Significant differences exist in postlarval abundance between the three sectors. Early larval stages were found everywhere, but frontal areas of the southern open-ocean sector showed higher abundances, implying greater reproductive activity. E. lamelligera was dominant near the coast in the east sector, and E. eximia, E. distinguenda and E. tenera were dominant in the northern and southern open-ocean sectors. E. tenera was the most abundant species in the western open-ocean transect, but was found mixed with other non-neritic species. Higher diversity indices were found in the northern and southern (open-ocean) sectors; however, diversity indexes were higher at night. This mixing of species was presumably caused by horizontal mixing induced from a large eddy that covers both sectors as shown by the geostrophic currents derived from altimeter data. Complex summer species distributions reflect the complex hydrographic conditions of the area, with water masses mixing, and, consequently, mixing of species with different water mass affinities. [Copyright &y& Elsevier]

Abstract: Northern Norwegian shelf regions are highly productive, supporting fisheries rich in commercially important species such as cod, herring and capelin. It has been long recognized that the mesoscale jets, meanders and eddies associated with interactions between the North Atlantic Current, Norwegian Coastal Current and regional bottom topographic features such as troughs, banks and shelfbreaks play important roles in transporting and retaining zooplankton. To investigate zooplankton distributions and their correspondence with the physical fields, three large-scale surveys with mesoscale resolutions on physical and biological fields were conducted in northern Norwegian shelf regions between latitudes 68°15′N and 70°15′N in springs of 2000–2002. Survey results provide insights into the relationships between zooplankton distributions and the physical features such as fronts, the Norwegian Coastal Current and eddies related to topographic features. The physical and biological data are integrated and analyzed focusing on water types, estimation of geostrophic currents from direct current measurements, along-shelf transport of zooplankton, and retention of zooplankton by the mesoscale meander–eddy over a typical bank area on the shelf. The estimated mean transport in the upper 100m on the shelf in the survey region is approximately 6.4×103 tonneswetweightday−1 northward. High zooplankton abundances were found over both Malangsgrunnen and Sveinsgrunnen banks. The specific accumulation rate from northward–southward transport in the upper 100m over Malangsgrunnen was approximately 0.08day−1, while variable currents with an offshore gradient of zooplankton abundance over Sveinsgrunnen implies an offshore dispersion of coastal-originated zooplankton cohort. [Copyright &y& Elsevier]

Spatial anomalies associated with seamounts and islands have been detected in the South West Indian Ocean using a new surface chlorophyll-a based enrichment index (EI). Calculated from daily L3 4-km MODIS chl-a data from 2003 to 2018, the EI was used to quantify recurrent (seasonal) surface enrichment associated with these features. Most notable were the shallow Walters Shoal (18 m) and nearby deeper WS-2 seamount (480 m), both located on the southern Madagascar Ridge, which showed high EI values of 40% and 15% local enhancement. Tromelin Island, east of Madagascar, exhibited more moderate values of 15% local increase. Other shallower seamounts including La Pérouse (60 m) and MAD-Ridge (240 m) exhibited sporadic or no measurable surface maxima. Regions of strong mesoscale activity such as south of Madagascar where the South East Madagascar Current detaches from the continent also revealed high EI values. A marked seasonality of the EI was observed over the Walters Shoal, with higher values during the oligotrophic season than in the austral winter. The seasonal variability of the MLD in combination with the seamount bathymetry appears to induce the thermal and biological anomalies observed there, possibly favoured by the presence of a Taylor cap at the summit. Ship-collected in situ measurements showed the satellite observed enrichment to be associated with shallowing of the deep chlorophyll maximum, as well as a shift of phytoplankton groups towards diatoms and small flagellates. No net increase of integrated chl-a was measured but a potential doubling in primary productivity is expected. The EI developed in this study, designed to reveal sporadic, small localised chl-a maxima, is likely applicable to other areas in the ocean where there is local enrichments. [ABSTRACT FROM AUTHOR]

This work is part of the MADRidge Project special issue which aims to describe pelagic ecosystems in the vicinity of three prominent shallow seamounts in the South West Indian Ocean: one here named MAD-Ridge (240 m below the surface) plus Walters Shoal (18 m) on the Madagascar Ridge, and La Pérouse (60 m) on the abyssal plain east of Madagascar. The three span latitudes 20°S and 33°S, some 1500 km. The study provides the background oceanography for the once-off, multidisciplinary snapshot cruise studies around the seamounts. As life on seamounts is determined by factors such as summit depth, proximity to the light layers of the ocean, and the ambient circulation, a first description of regional spatial-field climatologies (16–22 years) and monthly along-ridge gradients of surface wind (driving force), water column properties of sea surface temperature, mixed layer depth, chlorophyll- a and eddy kinetic energy, plus ocean currents is provided. Being relevant to many applications in the study domain, these properties in particular reveal contrasting environments along the Madagascar Ridge and between the three seamounts that should drive biological differences. Relative to the other two seamounts, MAD-Ridge is in the more extreme situation, being at the end of the East Madagascar Current, where it experiences sturdy, albeit variable, currents and the frequent passing of mesoscale eddies. [ABSTRACT FROM AUTHOR]

Stirring of passive tracers in the Bay of Bengal driven by altimetry derived daily geostrophic surface currents, is studied on subseasonal timescales. To begin with, Hovmöller plots, wavenumber-frequency diagrams and power spectra confirm the multiscale nature of the flow. Advection of latitudinal and longitudinal bands highlights the chaotic nature of stirring in the Bay via repeated straining and filamentation of the tracer field. An immediate finding is that stirring is local, i.e. of the scale of the eddies, and does not span the entire basin. Further, stirring rates are enhanced along the coast of the Bay and are relatively higher in the pre- and post-monsoonal seasons. Indeed, Finite Time Lyapunov Exponent (FTLE) and Finite Size Lyapunov Exponent (FSLE) maps in all the seasons are patchy with minima scattered through the interior of the Bay. Further, these maps bring out a seasonal cycle wherein rapid stirring progressively moves from the northern to southern Bay during pre- and post-monsoonal periods, respectively. The non-uniform stirring of the Bay is reflected in long tailed probability density functions of FTLEs, that become more stretched for longer time intervals. Quantitatively, advection for a week shows the mean FTLE lies between 0.13 ± 0.07 day−1, while extremes reach almost 0.6 day−1. Averaged over the Bay, Relative dispersion initially grows exponentially, followed by a power-law at scales between approximately 100 and 250 km, which finally transitions to an eddy-diffusive regime. Quantitatively, below 250 km, a scale dependent diffusion coefficient is extracted that behaves as a power-law with cluster size, while above 250 km, eddy-diffusivities range from 6 × 10 3 − 1.6 × 10 4 m2s−1 in different regions of the Bay. These estimates provide a useful guide for resolution dependent diffusivities in numerical models that hope to properly represent surface stirring in the Bay. [ABSTRACT FROM AUTHOR]

We investigate two-dimensional advective stirring by satellite-derived ocean surface currents in the north Bay of Bengal during December 2015–March 2016 using the framework of Lagrangian Coherent Structures (LCS). The forward and backward finite-time Lyapunov exponent (fFTLE and bFTLE, respectively) fields indicate that freshwater parcels from the river run-off are subjected to intense stirring by the ocean surface currents, which contain both geostrophic and wind-forced Ekman currents. Large-scale thermal fronts present in the satellite-based sea surface temperature (SST) fields are then shown to closely resemble attracting LCS (aLCS) that govern horizontal advective stirring on subseasonal time scales. Statistical estimates of the angle between the thermal fronts (of all spatial scales present in the SST data) and the aLCS from the entire north Bay of Bengal show a preference to align with each other. Significantly, in the presence of sufficiently strong aLCS in their neighbourhood, thermal fronts are shown to have a strong preference to align with them. Finite-time advection of fluid particles over around a week to compute the bFTLE fields results in the strongest alignment between the thermal fronts and neighbouring aLCS. The robustness of our results are established by presenting analyses of (i) geostrophic currents from altimeter data, and (ii) a reanalysis-based fine-scale salinity field. Attracting LCS, identified as sufficiently strong ridges in the bFTLE fields, are argued to represent reasonable estimates of the location and orientation of fronts (thermal or salinity) formed by advective stirring over subseasonal time scales. [ABSTRACT FROM AUTHOR]