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We observed interannual changes in the temperature and salinity of the surface layer of the Adriatic Sea when measured during the period 2005–2020. We observed non-stationarity and a positive linear trend in the series of mixed layer depth, heat storage, and potential energy anomalies. This non-stationarity was related to the climate regime that prevailed between 2011 and 2017. We observed significant changes in the interannual variability of salinity above and below the mixed layer depth and a positive difference in the surface barrier layer. In an effort to reconstruct the cause of this phenomenon, a multi-stage investigation was conducted. The first suspected culprit was the change in wind regime over the Mediterranean and Northeast Atlantic regions in September. Using the growing neural gas algorithm, September wind fields over the past 40 years were classified into nine distinct patterns. Further analysis of the CTD data indicated an increase in heat storage, a physical property of the Adriatic Sea known to be strongly influenced by the inflow of warm water masses controlled by the bimodal oscillating system (BiOS). The observed increase in salinity confirmed the assumption that BiOS activity affects heat storage. Unexpectedly, this analysis showed that an inverse vertical salinity profile was present during the summer months of 2015, 2017, and 2020, which can only be explained by salinity changes being a dominant factor. In addition, the aforementioned wind regime caused an increase in energy loss through latent energy dissipation, contributing to an even larger increase in salinity. While changes in the depth of the mixed layer in the Adriatic are usually due to temperature changes, this phenomenon was primarily caused by abrupt changes in salinity due to a combination of BiOS and local factors. This is the first record of such an event. [ABSTRACT FROM AUTHOR]

The Humboldt Upwelling System is of global interest due to its importance to fisheries, though the origin of its high productivity remains elusive. In regional physical‐biogeochemical model simulations, the seasonal amplitude of mesozooplankton net production exceeds that of phytoplankton, indicating "seasonal trophic amplification." An analytical approach identifies amplification to be driven by a seasonally varying trophic transfer efficiency due to mixed layer variations. The latter alters the vertical distribution of phytoplankton and thus the zooplankton and phytoplankton encounters, with lower encounters occurring in a deeper mixed layer where phytoplankton are diluted. In global model simulations, mixed layer depth appears to affect trophic transfer similarly in other productive regions. Our results highlight the importance of mixed layer depth for trophodynamics on a seasonal scale with potential significant implications, given mixed layer depth changes projected under climate change. Plain Language Summary: The Humboldt Upwelling System is a fishery‐important region. A common assumption is that a certain amount of phytoplankton supports a proportional amount of fish. However, we find that a small seasonal change in phytoplankton can trigger a larger variation in zooplankton. This implies that one may underestimate changes in fish solely based on phytoplankton. Using ecosystem model simulations, we investigate why changes of phytoplankton are not proportionally reflected in zooplankton. The portion of phytoplankton that ends up in zooplankton is controlled by the changing depth of the surface ocean "mixed layer." The "mixed layer" traps both the phytoplankton and zooplankton in a limited amount of space. When the "mixed layer" is shallow, zooplankton can feed more efficiently on phytoplankton as both are compressed in a comparatively smaller space. We conclude that in the Humboldt System, and other "food‐rich" regions, feeding efficiently, determined by the "mixed layer," is more important than how much food is available. Key Points: Environmental factors strongly affect plankton trophodynamics on a seasonal scaleSeasonal trophic amplification in the Humboldt system is driven by mixed layer dynamicsMixed layer depth and food chain efficiency correlate also in other productive regions [ABSTRACT FROM AUTHOR]

The warming of the surface ocean is expected to increase the stratification of the upper water column. This would decrease the efficiency of the wind-induced mixing, reducing the nutrient supply to the euphotic layer and the productivity of the oceans. Climatic projections show that the Mediterranean Sea will experience a strong warming and salting along the twenty first century. Nevertheless, very few works have found and quantified changes in the water column stratification of the Western Mediterranean. In this work, we obtain time series of Mixed Layer Depth (MLD) along the Spanish Mediterranean waters and the Gulf of Cádiz, using periodic CTD profiles collected under the umbrella of the Ocean Observing system of the Instituto Español de Oceanografía (IEO-CSIC). The length of the time series analyzed is variable, depending on the geographical area, but in some cases these time series extend from the beginning of the 1990s decade. Our results show that at present, no statistically significant changes can be detected. These results are confirmed by the analysis of MLD time series obtained from Argo profilers. Some of the meteorological factors that could affect the water column stratification (wind intensity and precipitation rates) did not experience significant changes for the 1990-2021 period, neither were observed long-term changes in the chlorophyll concentration. The hypothesis proposed to explain this lack of trends, is that the salinity increase of the surface waters has compensated for the warming, and consequently, the density of the upper layer of the Western Mediterranean (WMED) has remained constant. As the wind intensity has not experienced significant trends, the stratification of the Spanish Mediterranean waters and those of the Gulf of Cádiz would have not been affected. Nevertheless, we do not discard that our results are a consequence of the short length of the available time series and the large variance of the variables analyzed, evidencing the importance of the maintenance of the ocean monitoring programs.

Global atmospheric circulation experienced drastic changes during the Middle Miocene climate transition (MMCT∼14.7–13.0 Ma) possibly related to the glaciation on West-Antarctica. Palaeoceanographic reconstructions have found that upwelling in the western Arabian Sea due to summer South Asian monsoon (SAM) winds likely occurred since ∼14.7 Ma, with fully modern-like monsoonal wind patterns after the end of the MMCT at around ∼13 Ma. Whether the changes in monsoonal circulation since ∼14.7 Ma are also associated with upper ocean hydrographic changes in the eastern Arabian Sea (EAS) is currently not known for the middle Miocene. To this end, the difference in Mg/Ca-based temperatures (ΔT°C) of surface-dwelling with lower mixed layer/thermocline-dwelling and sub-thermocline-dwelling planktic foraminifera was reconstructed to estimate the upper ocean thermal gradient at Site NGHP-01-01 A in the EAS and thus changes in the upper surface water column structure, i.e., mixed layer depth (MLD) and the depth of thermocline (DOT). The Mg/Ca-temperature estimates from the upper mixed layer down to the sub-thermocline show a prominent cooling trend between ∼14.2 Ma and 13.2 Ma. The upper water column reconstructions reveal weaker mixing, a shallower thermocline, and therefore a well stratified upper water column in the EAS after ∼14.2 Ma. We suggest that the weaker mixing and shallower thermocline in the EAS after ∼14.2 Ma are most likely due to an intensification of the summer SAM. A strong salinity stratification and/or the formation of a barrier layer (BL) because of increased SAM rainfall and advection of low salinity water may also have contributed to a weaker mixing and shallower thermocline in the EAS during the investigated time period. The change in the upper ocean hydrography in the EAS after ∼14.2 Ma fits well into the emerging picture of monsoonal-driven upper ocean hydrographic changes in the equatorial and northern Indian Ocean due to an intensification of the summer SAM since ∼14.7 Ma. • Cooling of the mixed layer and sub-thermocline in the eastern Arabian Sea (EAS) between ∼14.2 Ma and 13.2 Ma • Weaker mixing, a shallower thermocline, and a well stratified upper water column occurred in the EAS after ∼14.2 Ma • A shallower thermocline in the EAS after ∼14.2 Ma is most likely due to an intensification of the summer SAM • A strong salinity stratification due to stronger summer SAM rainfall may also have contributed to a shallower thermocline • Results support monsoonal-driven hydrographic changes in the Arabian Sea due to an intensified summer SAM since ∼14.7 Ma [ABSTRACT FROM AUTHOR]

The seasonality and generation mechanisms of submesoscale processes (SMPs) in the northern Bay of Bengal (nBoB) are investigated by the outputs of a high-resolution model simulation. The results show that the nBoB has abundant energetic SMPs, with significant seasonal features and geographic variability. The head basin (region A) and central basin (region B) of the nBoB are identified as two typical spots of submesoscale motions. Seasonally, SMPs in region A are strongest in spring and are closely correlated with strong mesoscale strain. By contrast, SMPs in region B are more active in winter and late summer due to the combined effects of deep mixed layer and large mesoscale strain. Energy analysis suggests that baroclinic instability is a dominant generation mechanism for energetic SMPs in region B during winter and summer periods. During spring, the prevalent submesoscale kinetic energy (KE) reservoir in region A is fueled by wind forcing, buoyancy conversion, and the forward KE cascades from mesoscale processes, and mainly balanced by the inverse KE cascades from submesoscale to large-scale processes. [ABSTRACT FROM AUTHOR]

In many regions the projected future sea surface temperature (SST) response to greenhouse warming is larger in summer than in winter. What causes this amplification of the SST seasonal cycle has remained unclear. To determine robustness of the projected seasonal cycle intensification and ascertain underlying physical mechanisms we analyze a suite of historical and greenhouse warming simulations conducted with 13 coupled general circulation models in the Coupled Model Intercomparison Project Phase 5. In the Representative Concentration Pathway 8.5 scenario, the amplitude of SST seasonal cycle, defined as the difference between climatological maximum and minimum temperature, increases by 30% ± 20% on average by the end of 21st century. Analysis of a simplified mixed layer heat budget demonstrates that the amplification can be attributed to the increasing upper ocean stratification and hence shoaling of the annual‐mean mixed layer. The projected intensification of SST seasonality may have important implications for future changes in marine ecosystems. Plain Language Summary: One of the robust projected climate changes in response to global warming is the amplification of seasonal cycle of sea surface temperature (SST) with larger warming occurring in summer than in winter, especially in the North Atlantic, North Pacific and South Indian Ocean. Here we investigate the underlying physical mechanisms using a suite of future greenhouse warming simulations. We show that for the high greenhouse gas emission scenario the amplitude of SST seasonality increases over the next 80 years by 30% ± 20% globally. Overall mean ocean warming increases the upper ocean stratification, which leads to a shoaling of the mixed layer. This implies that climatological air‐sea heat fluxes impact a smaller ocean volume, which then leads to an increased SST response. Other heat budget terms play only a secondary role. The increased temperature seasonality could further impact plankton phenology and the climatology of upper ocean CO2. Key Points: Under the high greenhouse gas emission scenario, the seasonal amplitude of sea surface temperature (SST) is projected to increase by 30% ± 20% by the end of 21st centuryThe intensification of SST seasonality is largely due to the shoaling of the annual mean mixed layer depthThe advection term explains about 10%–40% of the change in SST seasonality depending on the region [ABSTRACT FROM AUTHOR]

At the sub-Antarctic Prince Edward Islands (PEIs) in the Southern Ocean, the Island Mass Effect (IME) plays an important role in maintaining an ecosystem able to support diverse biological communities; however, limited in situ sampling has severely constrained our understanding of it. As such, our study used satellite chlorophyll a (chla) to provide the first detailed characterisation of the spatial extent and seasonal variability of the IME at the PEIs. Seasonal surface chla variations were remarkable, with localised increases observed from mid-austral spring to the end of autumn (October to May). In contrast, during June to September, there were no distinguishable differences between chla at the PEIs and that further afield. Seasonal chla changes were significantly correlated with higher light levels, warmer waters, and shallow upper mixed layer depths reflecting enhanced water column stability during summer and autumn, with the opposite pattern in winter and spring. The IME extended northeast of the islands and remained spatially distinct from elevated chla around the northern branch of the sub-Antarctic Front and the southern branch of the Antarctic Polar Front. From December to February, the IME was spatially connected to the island shelf. In contrast, during March–May and in October, higher chla was observed only to the northeast, some distance away from the islands, suggesting a delayed IME, which has not previously been observed at the PEIs. The clear association of this higher chla with the weak mean geostrophic circulation northeast of the islands suggested retention and accumulation of nutrients and phytoplankton biomass, which was likely aided by wind-driven northeastward transport of water from the shelf. Climatological mean chla to the northeast was generally higher than that on the PEI shelf, and further research is required to determine the importance of this region to ecosystem functioning at the islands. [ABSTRACT FROM AUTHOR]

Diel vertical migration (DVM) is common in zooplankton populations worldwide. Every day, zooplankton leave the productive surface ocean and migrate to deepwater to avoid visual predators and return to the surface at night to feed. This behavior may also help retain migrating zooplankton in biological hotspots. Compared to fast and variable surface currents, deep ocean currents are sluggish, and can be more consistent. The time spent in the subsurface layer is driven by day length and the depth of the surface mixed layer. A subsurface, recirculating eddy has recently been described in Palmer Deep Canyon (PDC), a submarine canyon in a biological hotspot located adjacent to the West Antarctic Peninsula. Circulation model simulations have shown that residence times of neutrally buoyant particles increase with depth within this feature. We hypothesize that DVM into the subsurface eddy increases local retention of migrating zooplankton in this feature and that shallow mixed layers and longer days increase residence times. We demonstrate that simulated vertically migrating zooplankton can have residence times on the order of 30 days over the canyon, which is five times greater than residence times of near‐surface, nonmigrating zooplankton within PDC and other adjacent coastal regions. The potential interaction of zooplankton with this subsurface feature may be important to the establishment of the biological hotspot around PDC by retaining food resources in the region. Acoustic field observations confirm the presence of vertical migrators in this region, suggesting that zooplankton retention due to the subsurface eddy is feasible. Plain Language Summary: Diel vertical migration (DVM) is considered the world's largest migration by biomass. Zooplankton migrate into the sea's surface waters to feed at night when risk of predation by visual predators is low. During the day, zooplankton migrate to deeper waters when risk of predation by predators like seabirds and fish is highest. This behavior may also retain zooplankton in areas of high biological activity, or hotspots. Migration between a rapidly moving surface layer and a sluggish subsurface layer may reduce the net horizontal movement of organisms. Since this behavior is modulated by light intensity, more daylight hours may increase the time spent in the slower subsurface layer and help retain zooplankton in these hotspots. We used simulated zooplankton in a numerical model over Palmer Deep Canyon to test how DVM behavior, and the factors that control the time spent in the subsurface layer, affects zooplankton retention within biological hotspots. Retention was highest for zooplankton when migrations were deepest, days were long, and surface mixed layer was shallow. Performing migrations also increased retention relative to near‐surface nonmigrating zooplankton. Acoustic observations within our study site suggest that the magnitudes of DVM simulated are feasible in this system. Key Points: Simulated diel vertical migrating zooplankton have increased residence time compared to nonmigrators near a biological hotspotShallow surface layers and long days lead to longer residence times of simulated diel vertical migrating zooplanktonField acoustic measurements show the presence of zooplankton diel vertical migration consistent with simulated migration [ABSTRACT FROM AUTHOR]

Obduction is the large‐scale, irreversible upward water transport from the permanent pycnocline to the surface mixed layer. Using Hybrid Coordinate Ocean Model (HYCOM), this study explores the impact of atmospheric intraseasonal oscillations (ISOs) on the Pacific equatorial obduction that plays a fundamental role in regulating the tropical Pacific climate. Parallel HYCOM experiments forced by atmospheric fields with and without ISO variability are compared to assess the effect of ISOs on the ocean. The results suggest that the ISOs can reduce the annual obduction rate of the western and central equatorial Pacific Ocean (WCEPO; 150°E−160°W, 5°S–5°N) by ∼12%, and their impact on the eastern Pacific is rather limited. The ISOs cause prominent intraseasonal variability of mixed layer depth (MLD) in the WCEPO primarily through surface wind forcing, which in turn affects the two key processes controlling the annual obduction rate. First, the intraseasonal MLD deepening events narrow down the total time windows for obduction by ∼30% and thereby give rise to the decrease in obduction rate; second, it leads to high‐frequency entrainment events and enhances the entrainment rate by ∼18%. The increased entrainment rate is overwhelmed by the reduced time windows for effective entrainment, and the net effect of ISOs is to reduce the obduction rate and modify its seasonal cycle. This work highlights the importance of atmospheric ISOs in the equatorial ocean dynamics, with implications for tropical surface temperature and climate variabilities and biogeochemical cycle. Plain Language Summary: The equatorial Pacific Ocean holds the largest upwelling system of the world's ocean. Oceanographers use "obduction" to quantify the amount of water transported from the permanent pycnocline to the surface mixed layer. Change in obduction rate crucially affects seawater properties of the equatorial Pacific and thereby modulates air‐sea interactions. We speculate that tropical atmospheric intraseasonal oscillation (ISOs) can cause strong variability of the mixed layer depth (MLD) and alter the obduction rate of the equatorial Pacific. This hypothesis is tested in this study using ocean model experiments. The results show that the ISOs cause strong intraseasonal variability of MLD in the western and central equatorial Pacific Ocean, mainly through wind forcing. Owing to the MLD variability, the water transported upward into the mixed layer tends to come from the mixed layer of the upstream region rather than from the permanent pycnocline. This effect reduces the obduction rate of the western and central equatorial Pacific Ocean by ∼12% for the 2002–2015 period. Key Points: The atmospheric intraseasonal oscillations (ISOs) reduce the annual obduction rate of western and central equatorial Pacific Ocean by ∼12%The ISOs cause strong intraseasonal variability (ISV) of mixed layer depth primarily through surface wind forcingThe time window for effective entrainment is narrowed down by the ISO‐forced ISV [ABSTRACT FROM AUTHOR]

In open oceans, changes in mixed layer depth (MLD) may affect phytoplankton growth and biomass variations via the regulation of nutrient supply from deep waters. Estimates of relationships between variability in phytoplankton dynamics and the MLD remain limited, especially at different time scales. We compiled and analyzed averaged euphotic‐depth‐integrated chlorophyll‐a (IChl‐a) and surface chlorophyll‐a (SChl‐a) concentrations collected from 27 cruises during the period of 1999–2019 in the tropical northern South China Sea (SCS). Seasonal differences existed in both averaged IChl‐a and SChl‐a concentrations, with significantly high concentrations in the cold season. Inconsistent relationships between the averaged IChl‐a and SChl‐a concentrations between seasons implied that the use of SChl‐a concentration as a common indicator of phytoplankton biomass dynamics should be performed with caution. Over the past decades in the northern SCS, the averaged IChl‐a, SChl‐a, and MLD decreased to a greater extent in the cold season than in the warm season, while sea surface temperature (SST) rose rapidly and dramatically in both seasons. The MLD was observed to have better correlations with the averaged IChl‐a and SChl‐a concentrations than the SST in the time‐series data. Our results highlight the importance of IChl‐a concentration, which is an overall measure of phytoplankton responses to euphotic zone conditions, and the MLD could be used as a good index for changes in phytoplankton biomass under climate change. Plain Language Summary: Given that ocean stratification, corresponding to mixed layer depth (MLD), is critical of phytoplankton growth and biomass variations under climate change, particularly in tropical and subtropical regions, it is important to examine MLD influences on phytoplankton at multiple temporal scales in those areas. Based on long‐term investigation in the northern South China Sea during the 1999–2019 period, the identification of oceanographic links with phytoplankton, especially its biomass represented by chlorophyll‐a (Chl‐a) concentrations, revealed that high Chl‐a concentrations in the cold season and typhoon disturbance could play an important role in increasing oceanic production. The interannual and decadal observations showed a progressive increase in sea surface temperature and decreases in both MLD and Chl‐a concentrations that may become a severe environmental and biotic crisis in the SCS. To better address contemporary climatic threats, measurements of phytoplankton responses in the euphotic zone remain essential and may not be simply replaced by recent satellite remote sensing development. Key Points: We quantify differences between euphotic‐depth‐integrated chlorophyll‐a (IChl‐a) and surface chlorophyll‐a (SChl‐a) concentrations in the South China SeaInconsistent relationships between IChl‐a and SChl‐a concentrations between seasons imply caution to use SChl‐a for phytoplankton dynamicsThe IChl‐a concentration and mixed layer depth could be used as good indices for changes in phytoplankton biomass under climate change [ABSTRACT FROM AUTHOR]

The mixed layer depth (MLD) is a widely used parameter for physical, chemical, and biological oceanography. The MLD delimits that region of the ocean, which is directly influenced by the atmosphere. There is a similar length scale referred to as the mixing layer depth, which is determined from profiles of dissipation rate of turbulent kinetic energy, but is less prevalent due to the requirement for specialized instrumentation. Here, we suggest that the MLD can be estimated using density/temperature (henceforth hD) or using dissipation (henceforth hϵ). Utilizing two data sets in the North Atlantic collected with the autonomous Air‐Sea Interaction Profiler, hD and hϵ in the upper 100 m are compared. A new method based on the shape of the dissipation profile for estimating hϵ is presented. The main sources of turbulence in the upper ocean (i.e., wind, waves, and buoyancy fluxes), which tend to deepen the MLD, are more strongly correlated with hϵ variability for both data sets, confirming that the MLD derived from dissipation measurements represents more accurately the variability in the mixed layer. Given that dissipation measurements have become more operational due to the development of ocean microstructure technology, then it is appropriate that hϵ be adopted in the future for estimation of the MLD. This is additionally supported by the fact that hD typically overestimates the MLD when compared to hϵ, thus having consequences for studies where hD is used to represent the extent of mixing in the upper ocean. Plain Language Summary: The mixed layer is the region of the ocean, which is directly influenced by atmospheric processes. The mixed layer plays a large role in regulating air‐sea exchange and climate. The mixed layer depth (MLD) is a critical parameter for physical, chemical, and biological oceanography and is typically estimated using vertical profiles of temperature and density. However, since this layer is being actively mixed by different sources of turbulence (e.g., wind, waves, and buoyancy fluxes), a criterion based on turbulence measurements is more desirable. In this article, we explore different methods to estimate the MLD based on temperature, density, and turbulence data using two data sets in the North Atlantic collected with an autonomous vertical profiler that records these data simultaneously. Significant differences were found among the methods used in this work, showing that a method based on the shape of the turbulent profile proves to be more robust for estimating the MLD. Moreover, by looking at the sources of turbulence during these cruises, it is possible to observe that the MLD based on turbulence measurements represents more accurately the variability in the upper ocean, having consequences for studies where the MLD is used. Key Points: A new method to estimate the mixed layer depth (MLD) based on the shape of the vertical profile of dissipation is presentedThe MLD derived from dissipation offers a more realistic description of turbulent mixing than density, which tends to overestimate the MLDIt is appropriate that dissipation‐based estimates of the MLD be adopted, given that microstructure measurements are becoming increasingly available operationally [ABSTRACT FROM AUTHOR]

High‐resolution underwater glider data collected in the Gulf of Oman (2015–2016), combined with reanalysis data sets, describe the spatial and temporal variability of the mixed layer during winter and spring. We assess the effect of surface forcing and submesoscale processes on upper ocean buoyancy and their effects on mixed layer stratification. Episodic strong and dry wind events from the northwest (Shamals) drive rapid latent heat loss events which lead to intraseasonal deepening of the mixed layer. Comparatively, the prevailing southeasterly winds in the region are more humid, and do not lead to significant heat loss, thereby reducing intraseasonal upper ocean variability in stratification. We use this unique data set to investigate the presence and strength of submesoscale flows, particularly in winter, during deep mixed layers. These submesoscale instabilities act mainly to restratify the upper ocean during winter through mixed layer eddies. The timing of the spring restratification differs by three weeks between 2015 and 2016 and matches the sign change of the net heat flux entering the ocean and the presence of restratifying submesoscale fluxes. These findings describe key high temporal and spatial resolution drivers of upper ocean variability, with downstream effects on phytoplankton bloom dynamics and ventilation of the oxygen minimum zone. Plain Language Summary: Atmospheric forcings, such as wind and solar heating, and small‐scale ocean processes (1–10 km; e.g., eddies, fronts, filaments) modify the properties and the structure of the water column near the surface. These processes regulate the surface layer, creating a well‐mixed surface layer. The variation in these processes determine how the depth of this surface mixed layer changes through both time and space. This study investigates the variability of this layer during winter and spring in the Gulf of Oman using in situ observations and atmospheric data derived from models and observations. Episodic strong and dry winds from the northwest (Shamals) increase mixing and cause shorter‐term variability of the surface mixed layer. Concurrently, we find that the small‐scale ocean processes mainly shoal the mixed layer depth during winter. These processes are also important in determining the timing of the change from the deeper winter mixed layer to the shallower spring mixed layer, as we find a three‐week difference between the two observed years. The observations illustrate previously unquantified processes in the region that can impact coupling between the atmosphere, surface ocean, and deep ocean, with consequences for regional marine ecosystems. Key Points: Ocean glider observations reveal mixed layer variability that cannot be explained by seasonal warming aloneShamal winds dominate intraseasonal variability of the mixed layer in the Gulf of OmanSubmesoscale mixed layer eddies are responsible for 68% of the restratifying buoyancy flux in winter [ABSTRACT FROM AUTHOR]

Abstract In open oceans, changes in mixed layer depth (MLD) may affect phytoplankton growth and biomass variations via the regulation of nutrient supply from deep waters. Estimates of relationships between variability in phytoplankton dynamics and the MLD remain limited, especially at different time scales. We compiled and analyzed averaged euphotic‐depth‐integrated chlorophyll‐a (IChl‐a) and surface chlorophyll‐a (SChl‐a) concentrations collected from 27 cruises during the period of 1999–2019 in the tropical northern South China Sea (SCS). Seasonal differences existed in both averaged IChl‐a and SChl‐a concentrations, with significantly high concentrations in the cold season. Inconsistent relationships between the averaged IChl‐a and SChl‐a concentrations between seasons implied that the use of SChl‐a concentration as a common indicator of phytoplankton biomass dynamics should be performed with caution. Over the past decades in the northern SCS, the averaged IChl‐a, SChl‐a, and MLD decreased to a greater extent in the cold season than in the warm season, while sea surface temperature (SST) rose rapidly and dramatically in both seasons. The MLD was observed to have better correlations with the averaged IChl‐a and SChl‐a concentrations than the SST in the time‐series data. Our results highlight the importance of IChl‐a concentration, which is an overall measure of phytoplankton responses to euphotic zone conditions, and the MLD could be used as a good index for changes in phytoplankton biomass under climate change.

Based on the CTD data from 58 stations in the Bering Sea and the Chukchi Sea in the summer of 2019, the values of mixed layer depth (MLD) were obtained by using the density difference threshold method. It was concluded that the MLD can be estimated more accurately by using a criterion of 0.125 kg/m3 in this region. The average MLD in the Bering Sea basin was larger than that in the Bering Sea shelf, and both of them were smaller than that in the Bering Sea slope. The MLD increased northward in both the Chukchi Sea shelf and the Chukchi Sea slope. The farther northward, the greater the difference between the MLD calculated from temperature (MLDt) and the MLD calculated from density (MLDd). The water masses and their interaction played an important role in the variation of MLD in the northern Bering Sea shelf and Chukchi Sea. The MLD was large due to the vertically homogeneous Anadyr Water in the northwestern Bering Sea shelf. The horizontal advection of Bering Sea Anadyr Water and Alaska Coastal Water in the Bering Sea shelf led to shallower MLD in the central northern Bering Sea shelf. The westward advection of the Alaska Coastal Water caused shallow mixed layers (MLs) in some regions of the Chukchi Sea shelf in the summer of 2019. The observed large MLD at BL01 station near the Aleutian Island was caused by an anticyclonic eddy. The northward increase in the MLD in the Chukchi Sea was related to the low-salinity seawater from sea ice melting in summer. The spatial variation of MLD was also closely related to the surface momentum flux and the sea surface buoyancy flux. Stratification plays an even more important role in determining the variation of MLD. The ML in 2019 was shallower and warmer than those in previous years, especially in the Bering Sea shelf and Chukchi Sea where sea ice volume, thickness, and coverage were significantly larger than the Bering Sea basin, which was related to the small sea ice volume in winter and spring of 2019 compared to previous years.

Based on the measurements from the Oleander Project, the behaviors of submesoscale motions are examined in the area between New Jersey Shelf and Bermuda. The vertical variation of Rossby number, the kinetic energy in the submesoscale range, and the power law of kinetic energy spectra suggest that submesoscale motions are mainly confined within the surface mixed layer with seasonality that is strong in winter and weak in summer. Besides, submesoscale motions with no significant seasonality were also found beneath the surface mixed layer, which could reach 500 m depth. A possible explanation is that the drastically varying flows in the Gulf Stream and mesoscale eddy periphery could generate strong lateral shear throughout their influence depth, which is favorable for breaking the geostrophic balance and causing submesoscale motions beneath the surface mixed layer.

Marine heatwaves (MHWs) are extreme warming events that can result in significant damage to marine ecosystems and local economies. The primary drivers of these events have been frequently studied using an upper ocean heat budget. However, various surface mixed layer (SML) depths have been used with little attention paid to the impact of the depth chosen on heat budget term estimates. We analyse MHW drivers in two dynamically contrasting regions off the east coast of Australia (East Australian Current extension) and the west coast of New Zealand over a 30-year period (1985–2014, inclusive). We compare the magnitude of the air-sea heatflux and advection terms in a volume-averaged heat budget using three different SML depth estimates. We show that the SML depth over which the heat budget is calculated has direct consequences on the identification of MHW dominant drivers. The air-sea heatflux term is amplified when the SML depth is underestimated and dampened when overestimated. The variation in the magnitude of the advection term is dependent on the barotropic or baroclinic structure of the currents. We, also, show that the impact on MHW driver classification is both temporally and regionally dependent. Generally, a deep SML estimate results in more MHWs being classified as advection and less classified as air-sea heatflux-driven. However, during the cool months, a shallow estimate produces the opposite pattern and to a varying degree of intensity depending on the region's dynamics. Use of daily and spatially variable SML depth in a heat budget calculation allows the comparison between regions with different dynamics influencing the mixed layer depth. These results show that when using a heat budget approach to explore marine heatwaves over extended time and space (e.g., regions and seasons), it is imperative to consider the temporal and spatial variability in the SML depth.

Satellite altimeters provide quasi‐global measurements of sea surface height, and from those the vertically integrated geostrophic velocity can be directly estimated, but not its vertical structure. This study discusses whether the mesoscale (30–400 km) dynamics of three regions in the South Atlantic can be described by the surface quasi‐geostrophic (SQG) theory, both at the surface and in depth, using outputs from an ocean general circulation model. At these scales, the model surface eddy kinetic energy (EKE) spectra show slopes close to k−5/3 (k−3) in winter (summer), characterizing the SQG and quasi‐geostrophic (QG) turbulence regimes. We use surface density and temperature to (a) reconstruct the stream function under the SQG theory, (b) assess its capability of reproducing mesoscale motions, and (c) identify the main parameters that improve such reconstruction. For mixed layers shallower than 100 m, the changes in the mixed‐layer depth contributes nine times more to the surface SQG reconstruction than the EKE, indicating the strong connection between the quality of the reconstruction and the seasonality of the mixed layer. To further explore the reconstruction vertical extension, we add the barotropic and first baroclinic QG modes to the surface solution. The SQG solutions reproduce the model density and geostrophic velocities in winter, whereas in summer, the interior QG modes prevail. Together, these solutions can improve surface correlations (>0.98) and can depict spatial patterns of mesoscale structures in both the horizontal and vertical domains. Improved spatial resolution from upcoming altimeter missions poses a motivating scenario to extend our findings into future observational studies. Plain Language Summary: Altimeters provide sea surface height measurements from which geostrophic velocities can be calculated. However, the measurements are strict to the ocean surface and obtaining its vertical structure is an ongoing challenge. Using outputs from an ocean general circulation model, we focus on describing the dynamics of mesoscale motions (30–400 km) in three regions of the South Atlantic under the surface‐quasi‐geostrophic (SQG) theory. We reconstruct the stream function taking a snapshot of density (and temperature) and assess the capability of the SQG method to correctly reproduce surface and vertical fields. Our results indicate that density may drive mesoscale dynamics under specific environmental conditions, and the role played by the seasonality of mixed‐layer depth and eddy kinetic energy is discussed. To further explore the vertical reconstruction, we include the barotropic and first baroclinic quasi‐geostrophic (QG) modes to the surface solution, yielding fields highly correlated (>0.98) to the model outputs. The upcoming new high‐resolution altimeters poses a motivating scenario to apply the SQG method of reconstruction and extend our findings into future observational studies. Key Points: The surface solution dominates the total stream function in winter. It has a small yet significant contribution in summerThe surface quasi‐geostrophic (SQG) and isQG methods depend more on the seasonality of the mixed‐layer depth than on the content of eddy kinetic energyThe isQG reconstruction on the South Atlantic can reproduce mesoscale motions at depths above 500 m with a threshold of 0.5 correlation [ABSTRACT FROM AUTHOR]

The ecosystem of the sea region adjacent to the Antarctic Peninsula is undergoing remarkable physical and biological changes, in the context of global warming. However, understanding of the dynamics of phytoplankton taxonomic composition in this marginal ice zone remains unclear. In this study, seawater samples collected from 36 stations in the northeastern Antarctic Peninsula were analyzed for nutrients and phytoplankton pigments. Combining with CHEMTAX analysis, remote sensing data, and physicochemical measurements, we investigated the relationships between phytoplankton crops, taxonomic composition, and marine environmental drivers. Integrated chlorophyll a (Chl a) concentrations (200 m) varied from 8.9 mg/m2 to 64.2 mg/m2, with an average of (23.2±12.0) mg/m2 and higher phytoplankton biomass concentrated in the coastal region of South Orkney Island and South Shetland Island. Diatoms were the dominant functional group (63%±21%). Higher proportions of diatoms were associated with higher Chl a (r=0.40, p<0.01), stable water columns (r=0.20, p<0.01), higher Si/P ratios (r=0.34, p<0.01), higher photosynthetically active radiation intensity (r=0.64, p<0.01), and higher sea ice melt water contributions (MWC, r=0.20, p<0.01). Conversely, Phaeocystis antarctica contributed a smaller overall proportion (31%±18%) and was more concentrated in the offshore water masses (e.g., Philip Ridge and South Scotia Ridge) with lower light levels (r=−0.58, p<0.01), deeper mixed layer depths (r=0.17, p<0.05), higher nutrient concentrations (e.g., N, P, and Si, r>0.35, p<0.01), and lower MWC (r=−0.20, p<0.01). In comparison, the total contribution from green flagellates (4%±5%), cryptophyta (1%±3%), dinoflagellates (1%±4%), and cyanobacteria (1% ± 5%) was only 6%. In offshore regions with well-mixed water, less varied taxonomic composition and lower crops with a higher proportion of nanophytoplankton were observed. In contrast, significantly decreasing crops below the mixed layer depth was observed in water columns with strong stratification, where the dominant phytoplankter changed from diatoms to P. antarctica. These findings have important implications for better understanding the future dynamics of marine ecosystems in the sea area adjacent to the Antarctic Peninsula. [ABSTRACT FROM AUTHOR]

The present climate simulation and future projection of the mixed layer depth (MLD) and subduction process in the subtropical Southeast Pacific are investigated based on the geophysical fluid dynamics laboratory earth system model (GFDL-ESM2M). The MLD deepens from May and reaches its maximum (>160 m) near (24°S, 104°W) in September in the historical simulation. The MLD spatial pattern in September is non-uniform in the present climate, which shows three characteristics: (1) the deep MLD extends from the Southeast Pacific to the West Pacific and leads to a "deep tongue" until 135°W; (2) the northern boundary of the MLD maximum is smoothly near 18°S, and MLD shallows sharply to the northeast; (3) there is a relatively shallow MLD zone inserted into the MLD maximum eastern boundary near (26°S, 80°W) as a weak "shallow tongue". The MLD nonuniform spatial pattern generates three strong MLD fronts respectively in the three key regions, promoting the subduction rate. After global warming, the variability of MLD spatial patterns is remarkably diverse, rather than deepening consistently. In all the key regions, the MLD deepens in the south but shoals in the north, strengthing the MLD front. As a result, the subduction rate enhances in these areas. This MLD antisymmetric variability is mainly influenced by various factors, especially the potential-density horizontal advection non-uniform changes. Notice that the freshwater flux change helps to deepen the MLD uniformly in the whole basin, so it hardly works on the regional MLD variability. The study highlights that there are regional differences in the mechanisms of the MLD change, and the MLD front change caused by MLD non-uniform variability is the crucial factor in the subduction response to global warming. [ABSTRACT FROM AUTHOR]

Abstract The Indonesian sea is the only low-latitude pathway connecting two tropical oceans, which plays an important role in the coupled ocean-atmosphere mode in the Indo-Pacific sector. A small change in the sea surface temperature (SST) in the Indonesian sea has a significant influence on the precipitation and air-sea heat flux. During the past 33 years, the SST in the Indonesian sea has indicated a warming trend on the average of 0.19 ± 0.04 decade−1, which is larger than global SST warming trend. Moreover, the warming trend indicates seasonal variations, in which maximum trend occurred during boreal summer season. Investigation on the potential driver for this warming trend, namely, the net surface heat flux, resulted in an opposite trend (cooling trend), while the Ekman pumping and the wind mixing only play a minor role on the SST warming. Here, we proposed the upper layer process associated with an increasing trend in precipitation and decreasing trend in mixed layer depth (MLD) for the SST warming within the Indonesian seas. Shoaling of MLD gives a favorable condition for the surface heat flux to warm the surface ocean. However, shoaling of MLD could not solely explain the total SST warming within the Indonesian seas. The seasonal dependence in the warming trend, highest during boreal summer, was significantly related to the Indo-Pacific climate modes, namely the negative Indian Ocean Dipole (IOD) and La Niña events. Higher warming trend observed in the south of Makassar Strait and in the eastern Indonesian seas, in the vicinity of the Maluku Sea and the northeastern part of the Banda Sea, was significantly associated with the La Niña event. Meanwhile, strong warming trend observed in the Karimata Strait and Java Sea, and in the Flores Sea south of Sulawesi Island seems to be enhanced by the negative IOD event. Our rough quantitative estimate of the possible mechanism leading to the SST warming suggests that other mechanism might be at work in warming the SST within the Indonesian seas. Horizontal heat advection associated with an increasing trend of the heat transport from the Pacific into the Indian Ocean by the Indonesian Throughflow (ITF) might play a role in causing the warming trend within the Indonesian seas. However, to what extend this heat advection could modulate the SST warming is still unresolved in the present study. Further study based on realistic model output as well as long-term observational records is necessary to describe the dynamics underlying the warming trend within the Indonesian seas.

Mixed‐layer dynamics exert a first order control on nutrient and light availability for phytoplankton. In this study, we examine the influence of mixed‐layer dynamics on net community production (NCP) in the Southern Ocean on intra‐seasonal, seasonal, interannual, and decadal timescales, using biogeochemical Argo floats and satellite‐derived NCP estimates during the period from 1997 to 2020. On intraseasonal timescales, the shoaling of the mixed layer is more likely to enhance NCP in austral spring and winter, suggesting an alleviation of light limitation. As expected, NCP generally increases with light availability on seasonal timescales. On interannual timescales, NCP is correlated with mixed layer depth (MLD) and mixed‐layer‐averaged photosynthetically active radiation (PAR) in austral spring and winter, especially in regions with deeper mixed layers. Though recent studies have argued that winter MLD controls the subsequent growing season's iron and light availability, the limited number of Argo float observations contemporaneous with our satellite observations do not show a significant correlation between NCP and the previous‐winter's MLD on interannual timescales. Over the 1997–2020 period, we observe regional trends in NCP (e.g., increasing around S. America), but no trend for the entire Southern Ocean. Overall, our results show that the dependence of NCP on MLD is a complex function of timescales. Plain Language Summary: We examine the influence of mixed‐layer dynamics on net community production (NCP) in the Southern Ocean on intra‐seasonal, seasonal, interannual, and decadal timescales, using automated observations and satellite data during the period from 1997 to 2020. On intra‐seasonal timescales, the shoaling of the mixed layer is more likely to enhance NCP in austral spring and winter. As expected, NCP generally increases with light availability on seasonal timescales. On interannual timescales, NCP is correlated with mixed layer depth (MLD) and light within the mixed layer in austral spring and winter, especially in regions with deeper mixed layers. Our results do not show a significant correlation between NCP and the previous‐winter's MLD on interannual timescales. Over the 1997–2020 period, we observe regional trends in NCP, but no trend for the entire Southern Ocean. Overall, our results show that the dependence of NCP on MLD is a complex function of timescales. Key Points: Net community production (NCP) is correlated with mixed layer depth (MLD) and mixed‐layer‐averaged photosynthetically active radiation (PAR) on seasonal timescales, and on interannual timescales for spring and winterOn intraseasonal timescales, the impact of mixed‐layer dynamics on NCP is most pronounced in the austral springOn longer timescales, NCP is changing in various regions (e.g., around S. America), but no trend is evident for the entire Southern Ocean [ABSTRACT FROM AUTHOR]

This study assesses the capability of 12 models in the Phase 6 of the Coupled Models Intercomparison Project (CMIP6) in simulating the Southeast Indian Subantarctic mode water (SEISAMW) by comparing to Argo observations. The results show that all of the analyzed CMIP6 coupled models can reproduce the SEISAMW and its seasonal cycle, albeit with discrepancies of formation region and properties among the models. The SEISAMW subduction rate shows significant interannual variability, which is primarily induced by lateral induction in these models, in agreement with observations. Furthermore, the longterm trends of the SEISAMW show volume loss in accordance with the descending trend of the subduction rate, which are co‐occurred with the ascending trend of the Southern Annular Mode (SAM) indices in most of the analyzed CMIP6 models, similar to those in the CMIP3 and CMIP5 coupled models. Meanwhile, the potential density of the SEISAMWs show decreasing trends in these volume loss models, which could be explained by the warming (SAM0‐UNICON, CESM2‐WACCM, CAS‐ESM2‐0 and CIESM), the freshening (IPSL‐CM6A‐LR, CAMS‐CSM1‐0, MRI‐ESM2‐0, and FGOALS‐f3‐L) or the both (FIO‐ESM‐2‐0, E3SM‐1‐0, and CanESM5) trends of the SEISAMWs in different CMIP6 coupled models. Decreases in the projected subduction rate and volume of SEISAMW imply a slowdown of Southern Indian Ocean circulation in the future, reducing the heat and carbon transport from atmosphere to ocean interior contributed by SEISAMW. Plain Language Summary: Subantarctic mode water (SAMW) is formed between the subantarctic front and the subtropical front. The SAMW that formed in the South Indian Ocean is called Southeast Indian Subantarctic Mode Water (SEISAMW). When it forms, seawater at the base of mixed layer subducted into the ocean interior in late winter, providing subsurface oceanic reservoirs of heat, carbon, and other properties that can significantly affect the global climate. Using data from the historical simulation in 12 CMIP6 coupled models and Argo; we assess the capability of these models in simulating SEISAMW and reveal the interannual and longterm trend in SEISAMW properties (e.g., subduction rate, volume, temperature, salinity, and density). The results indicate that all of the analyzed CMIP6 models are able to reproduce the SEISAMW and its seasonal cycle. The interannual variability of the SEISAMW is attributed to lateral induction in all of the analyzed CMIP6 models, in agreement with observations. Both the subduction rate and volume of the SEISAMW show decreasing trends, which are co‐occurred with enhanced Southern Annular Mode (SAM), in most of the CMIP6 historical simulations. The decrease of subducted SEISAMW implies a slowdown of the Southern Ocean circulation under the anthropogenic forcing, due to surface warming and freshening. Key Points: The analyzed Coupled Models Intercomparison Project Phase 6 (CMIP6) models reproduce Southeast Indian subantarctic mode water (SEISAMW) and its seasonal cycle, albeit with inter‐model differencesThe simulated SEISAMWs show significant interannual variability dominated by lateral induction, consistent with observationsThe simulated SEISAMWs show volume loss during the period of the historical simulation in most of the analyzed CMIP6 models [ABSTRACT FROM AUTHOR]

We observed interannual changes in the temperature and salinity of the surface layer of the Adriatic Sea when measured during the period 2005–2020. We observed non-stationarity and a positive linear trend in the series of mixed layer depth, heat storage, and potential energy anomalies. This non-stationarity was related to the climate regime that prevailed between 2011 and 2017. We observed significant changes in the interannual variability of salinity above and below the mixed layer depth and a positive difference in the surface barrier layer. In an effort to reconstruct the cause of this phenomenon, a multi-stage investigation was conducted. The first suspected culprit was the change in wind regime over the Mediterranean and Northeast Atlantic regions in September. Using the growing neural gas algorithm, September wind fields over the past 40 years were classified into nine distinct patterns. Further analysis of the CTD data indicated an increase in heat storage, a physical property of the Adriatic Sea known to be strongly influenced by the inflow of warm water masses controlled by the bimodal oscillating system (BiOS). The observed increase in salinity confirmed the assumption that BiOS activity affects heat storage. Unexpectedly, this analysis showed that an inverse vertical salinity profile was present during the summer months of 2015, 2017, and 2020, which can only be explained by salinity changes being a dominant factor. In addition, the aforementioned wind regime caused an increase in energy loss through latent energy dissipation, contributing to an even larger increase in salinity. While changes in the depth of the mixed layer in the Adriatic are usually due to temperature changes, this phenomenon was primarily caused by abrupt changes in salinity due to a combination of BiOS and local factors. This is the first record of such an event.

The response of the mixed layer depth (MLD) and subduction rate in the subtropical Northeast Pacific to global warming is investigated based on 9 CMIP5 models. Compared with the present climate in the 9 models, the response of the MLD in the subtropical Northeast Pacific to the increased radiation forcing is spatially nonuniform, with the maximum shoaling about 50 m in the ensemble mean result. The inter-model differences of MLD change are non-negligible, which depend on the various dominated mechanisms. On the north of the MLD front, MLD shallows largely and is influenced by Ekman pumping, heat flux, and upper-ocean cold advection changes. On the south of the MLD front, MLD changes a little in the warmer climate, which is mainly due to the upper-ocean warm advection change. As a result, the MLD front intensity weakens obviously from 0.24 m/km to 0.15 m/km (about 33.9%) in the ensemble mean, not only due to the maximum of MLD shoaling but also dependent on the MLD non-uniform spatial variability. The spatially non-uniform decrease of the subduction rate is primarily dominated by the lateral induction reduction (about 85% in ensemble mean) due to the significant weakening of the MLD front. This research indicates that the ocean advection change impacts the MLD spatially non-uniform change greatly, and then plays an important role in the response of the MLD front and the subduction process to global warming. [ABSTRACT FROM AUTHOR]

Argo floats are one of the eminent ocean observation systems which retrieve frequent measurements of temperature and salinity profiles in the remote marine environment and provide vital information about the world ocean with or without direct access to the human. The Argo floats available in the Red Sea are not well explored by the research community so far. In the present study, the temperature and salinity data from two Argo floats available in the central Red Sea for a period of 2 years are used to examine the mixed layer depth variability in the region. The mixed layer in the region is maximum during February associated with a decrease in static stability and shallowest during August due to an increase in static stability. A noticeable difference is observed in the existing mixed layer structure by the presence of eddies. The analysis of monthly mean thermal and haline structure showed that the warming of the surface layer is intense from March to August followed by cooling from September to February. The surface layer salinity increased from May to October and decreased in the following months. A noticeable east-west difference is observed in the thermal and haline structure, where the eastern side of the region is warmer by ~0.3 °C and less saline by ~0.1 PSU. [ABSTRACT FROM AUTHOR]

At the sub-Antarctic Prince Edward Islands (PEIs) in the Southern Ocean, the Island Mass Effect (IME) plays an important role in maintaining an ecosystem able to support diverse biological communities; however, limited in situ sampling has severely constrained our understanding of it. As such, our study used satellite chlorophyll a (chla) to provide the first detailed characterisation of the spatial extent and seasonal variability of the IME at the PEIs. Seasonal surface chla variations were remarkable, with localised increases observed from mid-austral spring to the end of autumn (October to May). In contrast, during June to September, there were no distinguishable differences between chla at the PEIs and that further afield. Seasonal chla changes were significantly correlated with higher light levels, warmer waters, and shallow upper mixed layer depths reflecting enhanced water column stability during summer and autumn, with the opposite pattern in winter and spring. The IME extended northeast of the islands and remained spatially distinct from elevated chla around the northern branch of the sub-Antarctic Front and the southern branch of the Antarctic Polar Front. From December to February, the IME was spatially connected to the island shelf. In contrast, during March–May and in October, higher chla was observed only to the northeast, some distance away from the islands, suggesting a delayed IME, which has not previously been observed at the PEIs. The clear association of this higher chla with the weak mean geostrophic circulation northeast of the islands suggested retention and accumulation of nutrients and phytoplankton biomass, which was likely aided by wind-driven northeastward transport of water from the shelf. Climatological mean chla to the northeast was generally higher than that on the PEI shelf, and further research is required to determine the importance of this region to ecosystem functioning at the islands.

The landings of Indian oil sardine (Sardinella longiceps, Clupeidae) along the south‐eastern Arabian Sea are about 43.8% of total Indian oil sardine production. The annual landings of this species exhibit large‐scale variability with prolonged years of surplus or deficit landings without identified reason. Evaluating Indian oil sardine landings along the Kerala coast during 1961–2017 in relation to environmental variations, we have elucidated a putative link between variability in landings versus environmental parameters and climate indices. The variables examined in this study, such as salinity and temperature along with physical indices such as upwelling and mixed layer depth (MLD) of the ocean help to propose a mechanism to temporal variability in the landings of Indian oil sardine. Colder temperature and timely intense upwelling lead to nutrient enrichment in the surface water, which promotes the growth of phytoplankton (chl‐a) and thereby food availability to Indian oil sardine are found during years with surplus catch. Less saline surface waters and shoaling of MLD at these times could lead to the aggregation of fish at particular depths and thereby a good catches. The reverse mechanism, such as more surface saline water, warm temperature, downwelling or weak upwelling, and less nutrient enrichment, leads to deficit landings. Further, it was noticed that the Pacific decadal oscillation and Atlantic multidecadal oscillation have a more pronounced impact on Indian oil sardine landings over the coast of south‐eastern Arabian Sea than previously reported ENSO associated impacts. All these point towards climate change implications for the Indian oil sardine fishery. [ABSTRACT FROM AUTHOR]

The evolution of thermohaline structure at the upper ocean during three tropical cyclones (TCs) in the Northwest Pacific was studied in this study based on successive observation by two new-style underwater gliders during fall 2018. These remote-controllable gliders with CTD sensor enabled us to explore high frequency responses of temperature, salinity, mixed and barrier layers in the upper ocean to severe TCs in this area. Results showed that three significant cooling-to-warming and stratification destructing-to-reconstructing processes at the mixed layer occurred during the lives of three TCs. The maximal cooling of SST all reached ⩾=0.5°C although TCs with different intensities had different minimal distances to the observed area. Under potential impacts of solar radiation, tide and inertial motions, the mixed layer depth possessed significant high-frequency fluctuations during TC periods. In addition, barrier layers appeared and vanished quickly during TCs, accompanied with varied temperature inversion processes. [ABSTRACT FROM AUTHOR]

We investigated the effects of monsoon onset vortex (MOV) on the mixed layer heat budget in the Bay of Bengal (BOB) in spring 2003 using the reanalysis datasets. The results suggest that the solar radiation flux penetrating the mixed layer and the existence of barrier layer are both able to modulate the effects of MOV on the evolution of sea surface temperature (SST) in the BOB. Prior to the formation of BOB MOV, the local SST raised quickly due to mass of solar radiation reaching the sea surface under the clear-sky condition. Meanwhile, since the mixed layer was shallow before the onset of the Asian summer monsoon (ASM), some solar radiation flux could penetrate to directly heat the deeper water, which partly offset the warming effect of shortwave radiation. On the other hand, the in-situ SST started to cool due to the upwelling of cold water when the MOV generated over the BOB, along with the rapidly increased surface wind speed and its resultant deeper mixed layer. As the MOV developed and moved northward, the SST tended to decrease remarkably because of the strong upward surface latent heat flux over the BOB ascribed to the wind-evaporation mechanism. However, the MOV-related precipitation brought more fresh water into the upper ocean to produce a thicker barrier layer, whose thermal barrier effect damped the cooling effect of entrainment upwelling on the decrease tendency of the BOB SST. In other words, the thermal barrier effect could slow down the decreasing trend of the BOB SST even after the onset of ASM, which facilitated the further enhancement of the MOV. [ABSTRACT FROM AUTHOR]

We test the paradigm that in a future warmer ocean, shallower winter mixing will lead to less net primary production (NPP), by investigating whether warming between 2002 and 2018 led to changes in NPP in the Tasman Sea/New Zealand region. The 2002–18 trend in sea surface temperature (SST) was positive over most of the region, and was driven by increasingly warmer summers and marine heat waves (MHWs) rather than year-round warming. In contrast, the trends in sea surface chlorophyll (SSC) and NPP were generally positive over the Subtropical Front (STF) and in a subtropical band north-east of New Zealand, but negative elsewhere. Regressions between SSC and SST, and between spring SSC and the coldest SST during the preceding winter, show similar spatial patterns to the SSC trend. We suggest these findings reflect different ecosystem functioning in the subtropical and subantarctic biomes that are separated by the STF. We conclude that any future warming is likely to lead to less production in the Tasman Sea, but more production over the STF. Three recent MHWs had different impacts on production, but generally led to less surface biomass north of the STF and more biomass south of the front. [ABSTRACT FROM AUTHOR]

In-situ measurement of physical characteristics of seawater (temperature, salinity, and density) collected with a single fire module CTD (Conductivity, Temperature, Depth) in the northern Bay of Bengal (NBoB), as a part of the Indian Ocean, are given in this article. Due to the scanty of data in the NBoB, such in-situ measurements will certainly play a crucial role in understanding regional oceanic processes. The presented data were collected onboard during two cruises covering 12 stations with high resolution (0.2 m depth) up to 50-meter contour depth from 20.5778 N, 92.3578 E to 21.287 N, 89.696 E. We present raw CTD data as ASCII (.dat) and Comma Separated Value (.csv), and processed CTD data as netCDF (.nc) format in this article. Additionally, the calculated Mixed Layer Depth that was derived from the netCDF (.nc) file was included in the tabular format. This processed in-situ data would be most useful as a baseline for further studies in coastal and offshore physicochemical properties in the NBoB.

Madeira island is a well‐known source of atmospheric and oceanic eddy activity, with relevant downstream impact in both media. Previous studies focused on the dynamics of the island wake environment, suggesting the relevance of different atmosphere–ocean interactions in its maintenance. Here, results from one summer (two months) of fully coupled atmosphere–ocean high‐resolution simulations are used to explore such interactions and to further understand the dynamics of Madeira's wake. Those results, validated against available in situ and remote‐sensing data, indicate that the atmospheric and ocean circulations near Madeira are dominated by the variability of two quasi‐permanent features, its tip‐jets, and more so by the variability of its eastern jet. While both jets are of comparable magnitude and present similar intraseasonal variability at the multi‐week time‐scale, they are associated with qualitatively different forcing. The jets dominate the atmosphere forcing over the upper ocean, leading to enhanced mixing and deeper mixed‐layer depth. Oceanic eddies are more frequent in the east jet region, as shedding anticyclones, confirming observational evidence. A comparison with a similar one‐way coupled atmospheric simulation indicates that atmosphere–ocean feedbacks are relevant to the coastal surface temperature. [ABSTRACT FROM AUTHOR]

This study investigates variability of the South Pacific western subtropical mode water (SPWSTMW), its physical processes, and relationship with El Niño‐Southern Oscillation (ENSO), using a gridded Argo data product from January 2004 to September 2019. On seasonal timescale, the SPWSTMW volume shows a significant variability, which involves three periods: the formation period (June–October), the isolation period (November–February), and the dissipation period (March–May). This seasonal variability is related to seasonal fluctuation of the mixed layer depth. During the Argo period from 2004 to 2019, interannual variability of the SPWSTMW volume is tightly linked to the ENSO, increasing during El Niño periods and decreasing during La Niña periods. Further analyses indicate that ENSO‐related anomalous winds are primarily responsible for interannual variability of the SPWSTMW volume. The anomalous winds first influence the surface heat flux through evaporation and then the mixed layer depth through convection, leaving an imprint of ENSO on the SPWSTMW. This study also shows that the SPWSTMW responds differently to the central Pacific (CP) El Niño and eastern Pacific (EP) El Niño. Plain Language Summary: Using recently available Argo data set and atmospheric reanalysis products, this study investigates the impact of El Niño‐Southern Oscillation (ENSO) on the volume of the South Pacific western subtropical mode water (SPWSTMW) and its underlying mechanisms. The results show that interannual variability of the SPWSTMW volume has a strong ENSO signal during the Argo period from 2004 to 2019, which is mainly forced by ENSO‐induced wind anomalies. During El Niño years, surface winds in the SPWSTMW formation region are stronger than during normal years, resulting in a decrease in sea surface temperature (SST) through the wind‐evaporation‐SST (WES) feedback. The reduced SST can decrease the stratification and deepens the mixed layer depth (MLD), which enhances the horizontal MLD gradients and consequently the lateral induction and subduction of the SPWSTMW, directly contributing to the increase of the SPWSTMW volume. This study provides new insights into our understanding of regional oceanography and climate change in the South Pacific. Key Points: A gridded Argo data set is used to study variability of the South Pacific western subtropical mode water (SPWSTMW) over 2004–2019The SPWSTMW volume varies on various timescales, and this variability is mainly caused by change in winter mixed layer depthAnomalous winds associated with ENSO are largely responsible for the interannual variability of the SPWSTMW volume [ABSTRACT FROM AUTHOR]

The Southern Ocean is chronically undersampled due to its remoteness, harsh environment, and sea ice cover. Ocean circulation models yield significant insight into key processes and to some extent obviate the dearth of data; however, they often underestimate surface mixed layer depth (MLD), with consequences for surface water‐column temperature, salinity, and nutrient concentration. In this study, a coupled circulation and sea ice model was implemented for the region adjacent to the West Antarctic Peninsula, a climatically sensitive region which has exhibited decadal trends towards higher ocean temperature, shorter sea ice season, and increasing glacial freshwater input, overlain by strong interannual variability. Hindcast simulations were conducted with different air‐ice drag coefficients and Langmuir circulation parameterizations to determine the impact of these factors on MLD. Including Langmuir circulation deepened the surface mixed layer, with the deepening being more pronounced in the shelf and slope regions. Optimal selection of an air‐ice drag coefficient also increased modeled MLD by similar amounts and had a larger impact in improving the reliability of the simulated MLD interannual variability. This study highlights the importance of sea ice volume and redistribution to correctly reproduce the physics of the underlying ocean, and the potential of appropriately parameterizing Langmuir circulation to help correct for biases towards shallow MLD in the Southern Ocean. The model also reproduces observed freshwater patterns in the West Antarctic Peninsula during late summer and suggests that areas of intense summertime sea ice melt can still show net annual freezing due to high sea ice formation during the winter. Key Points: Sea ice redistribution has a large impact on the mixed layer depth seasonality and interannual variability in the West Antarctic PeninsulaIn the West Antarctic Peninsula, areas of high summer sea ice melt can show net annual freezing due to high sea ice formation in winterIncluding a parameterization for Langmuir circulation based on entrainment below the mixed layer improves the simulated mixed layer depth [ABSTRACT FROM AUTHOR]

Recent observations have shown that increases in climate change‐related coral mortality cause changes in shallow coral reef community structure through phase shifts to alternative taxa. As a result, sponges have emerged as a potential candidate taxon to become a "winner," and therefore a numerically and functionally dominant member of many coral reef communities. But, in order for this to occur, there must be sufficient trophic resources to support larger populations of these active filter‐feeding organisms. Globally, climate change is causing an increase in sea surface temperatures (SSTs) and a decrease in salinity, which can lead to an intensification in the stratification of shallow nearshore waters (0–200 m), that affects both the mixed layer depth (MLD) and the strength and duration of internal waves. Specifically, climate change‐driven increases in SSTs for tropical waters are predicted to cause increased stratification, and more stabilized surface waters. This causes a shallowing of the MLD which prevents nutrients from reaching the euphotic zone, and is predicted to decrease net primary production (NPP) up to 20% by the end of the century. Lower NPP would subsequently affect multiple trophic levels, including shallow benthic filter‐feeding communities, as the coupling between water column productivity and the benthos weakens. We argue here that sponge populations may actually be constrained, rather than promoted, by climate change due to decreases in their primary trophic resources, caused by bottom‐up forcing, secondary to physical changes in the water column (i.e., stratification and changes in the MLD resulting in lower nutrients and NPP). As a result, we predict sponge‐dominated tropical reefs will be rare, or short‐lived, if they occur at all into the future in the Anthropocene. [ABSTRACT FROM AUTHOR]

The Indonesian sea is the only low-latitude pathway connecting two tropical oceans, which plays an important role in the coupled ocean-atmosphere mode in the Indo-Pacific sector. A small change in the sea surface temperature (SST) in the Indonesian sea has a significant influence on the precipitation and air-sea heat flux. During the past 33 years, the SST in the Indonesian sea has indicated a warming trend on the average of 0.19 ± 0.04 decade−1, which is larger than global SST warming trend. Moreover, the warming trend indicates seasonal variations, in which maximum trend occurred during boreal summer season. Investigation on the potential driver for this warming trend, namely, the net surface heat flux, resulted in an opposite trend (cooling trend), while the Ekman pumping and the wind mixing only play a minor role on the SST warming. Here, we proposed the upper layer process associated with an increasing trend in precipitation and decreasing trend in mixed layer depth (MLD) for the SST warming within the Indonesian seas. Shoaling of MLD gives a favorable condition for the surface heat flux to warm the surface ocean. However, shoaling of MLD could not solely explain the total SST warming within the Indonesian seas. The seasonal dependence in the warming trend, highest during boreal summer, was significantly related to the Indo-Pacific climate modes, namely the negative Indian Ocean Dipole (IOD) and La Niña events. Higher warming trend observed in the south of Makassar Strait and in the eastern Indonesian seas, in the vicinity of the Maluku Sea and the northeastern part of the Banda Sea, was significantly associated with the La Niña event. Meanwhile, strong warming trend observed in the Karimata Strait and Java Sea, and in the Flores Sea south of Sulawesi Island seems to be enhanced by the negative IOD event. Our rough quantitative estimate of the possible mechanism leading to the SST warming suggests that other mechanism might be at work in warming the SST within the Indonesian seas. Horizontal heat advection associated with an increasing trend of the heat transport from the Pacific into the Indian Ocean by the Indonesian Throughflow (ITF) might play a role in causing the warming trend within the Indonesian seas. However, to what extend this heat advection could modulate the SST warming is still unresolved in the present study. Further study based on realistic model output as well as long-term observational records is necessary to describe the dynamics underlying the warming trend within the Indonesian seas. [ABSTRACT FROM AUTHOR]

Wind-induced mixing forms the surface mixed layer (ML) above the stratified interior oceans. The ML depth (MLD), a key quantity for several upper ocean processes such as the air-sea interaction and primary production of phytoplankton biomass, is often scaled as U ∗ / N f , where U∗ is the friction velocity, N is the Brunt-Väisälä frequency, and f is the Coriolis parameter. Here, we performed large-eddy simulations (LESs) to evaluate this scaling. It was found that the ML deepens rapidly until one-half inertial period (0.5Tf) by which the MLD becomes 1.6 − 1.7 U ∗ / N f . Thereafter, the ML deepening slows down but never stops, and the MLD keeps increasing gradually. The MLDs at Tf, 1.5Tf, and 5Tf become greater than those at 0.5Tf by 6.2 %, 16 %, and 40 %, respectively. Therefore, time-dependent scaling of the MLD is necessary for more quantitative estimates than the classical theory. LESs performed with several U∗, N, and f showed that the deepening rate of the ML depends on the Rossby number and the Froude number. The present study proposes time-dependent scalings of the ML deepening rate and the MLD as a function of the Rossby number and the Froude number, which cover the classical scaling but can be extended even after 0.5Tf. [ABSTRACT FROM AUTHOR]

Momentum input from westerly winds blowing over the Southern Ocean can be modulated by mesoscale surface currents and result in changes in large‐scale ocean circulation. Here, using an eddy‐resolving 1/20 degree ocean model configured near Drake Passage, we evaluate the impact of current‐wind interaction on vertical processes. We find a reduction in momentum input from the wind, reduced eddy kinetic energy, and a modification of Ekman pumping rates. Wind stress curl resulting from current‐wind interaction leads to net upward motion, while the nonlinear Ekman pumping term associated with horizontal gradients of relative vorticity induces net downward motion. The spatially averaged mixed layer depth estimated using a density criteria is shoaled slightly by current‐wind interaction. Current‐wind interaction, on the other hand, enhances the stratification in the thermocline below the mixed layer. Such changes have the potential to alter biogeochemical processes including nutrient supply, biological productivity, and air‐sea carbon dioxide exchange. Plain Language Summary: Momentum transfer between winds blowing over the Southern Ocean depends on the relative speed of the winds and surface currents. Mesoscale eddies with a scale of 100 km or less are very vigorous and thus can modulate momentum transfer. Here, we use an ocean model with sufficiently high horizontal resolution that it can resolve the mesoscale and hence capture the modulation. We find a reduction in the momentum transfer from the wind to the ocean and a reduction in eddy kinetic energy, together with a modification of wind‐driven vertical motion. Structural changes in the wind stress field modify patterns of upwelling and downwelling in a manner that can be understood from nonlinear Ekman theory. Moreover, current‐wind interaction results in an increase in the stratification below the mixed layer and hence a reduced communication between the surface and the interior ocean. There is thus a potential impact on biogeochemical processes and the climate of the Southern Ocean. Key Points: High‐resolution Southern Ocean simulations suggest that current‐wind‐stress interaction on the mesoscale reduces eddy kinetic energy by 25%Current‐wind interaction induces a net upward linear but downward nonlinear modulation of Ekman pumping ratesCurrent‐wind interaction enhances stratification near the bottom of the mixed layer by up to 10% [ABSTRACT FROM AUTHOR]

El Niño/Southern Oscillation (ENSO) has far reaching global climatic impacts and so extending useful ENSO forecasts would have great societal benefit. However, one key variable that has yet to be fully exploited within coupled forecast systems is accurate estimation of near‐surface ocean salinity. Satellite sea surface salinity (SSS), combined with temperature, help to improve the estimates of ocean density changes and associated near‐surface mixing. For the first time, we assess the impact of satellite SSS observations for improving near‐surface dynamics within ocean reanalyses and how these initializations impact dynamical ENSO forecasts using NASA's coupled forecast system (GEOS‐S2S‐2). For all initialization experiments, all available sea level and in situ temperature and salinity observations are assimilated. Separate observing system experiments additionally assimilate Aquarius, SMAP, SMOS, and these data sets combined. We highlight the impact of satellite SSS on ocean reanalyses by comparing experiments with and without the application of SSS assimilation. Next, we compare case studies of coupled forecasts for the big 2015 El Niño, the 2017 La Niña, and the weak El Niño in 2018 that are initialized from GEOS‐S2S‐2 spring reanalyses that assimilate and withhold along‐track SSS. For each of these ENSO‐event case studies, assimilation of satellite SSS improves the forecast validation with respect to observed NINO3.4 anomalies (or at least reduces the forecast uncertainty). Satellite SSS assimilation improved characterization of the mixed layer depth leading to more accurate coupled air/sea interaction and better forecasts. These results further underline the value of satellite SSS assimilation into operational forecast systems. Plain Language Summary: Improving the prediction of El Niño/Southern Oscillation (ENSO) is important because of the global impacts of ENSO and the associated socioeconomic implications. Only recently has satellite sea surface salinity (SSS) become available for improving our characterization of the global hydrological cycle. SSS, combined with temperature, helps to improve the estimates of near‐surface density changes and associated ocean mixing. Here we show results of experiments designed to highlight the impact of SSS on ENSO forecasts. In the control experiment, we assimilate a comprehensive set of in situ oceanographic information and satellite altimetry, as typically done in operational ocean data assimilation, but exclude satellite SSS. In the second set of reanalyses, we add different satellite SSS products to our assimilation. Air/sea coupled model hindcasts are then initialized for various case studies including the big El Niño (2015), the moderate La Niña (2017), and a weak El Niño (2018). For each example, satellite SSS assimilation improves coupled forecasts by adjusting the large‐scale equatorial waves that are integral to ENSO development. For 2015, SSS damps these waves resulting in a more realistic ENSO prediction. In 2017 and 2018, SSS assimilation acts to change the sign of ENSO forecasts, again leading to more realistic ENSO forecasts. Key Points: Assimilation of satellite sea surface salinity (SSS) into ocean reanalyses improves near‐surface density structure and mixed layer depthMore realistic MLD improves air/sea interaction and acts to modulate the oceanic Kelvin waves of ENSO leading to improved forecastsCase studies for the El Nino and La Nina initialized in spring showed that SSS assimilation led to more realistic ENSO forecasts [ABSTRACT FROM AUTHOR]

The effect of river runoff over the northern Indian Ocean (NIO) especially over the Bay of Bengal (BoB) has been studied using global Nucleus for European Modelling of the Ocean (NEMO). Two sensitivity experiments, with and without river runoff are conducted and the influence of river runoff on the Indian Ocean hydrography, stratification and circulation features are studied. It is found that due to river runoff surface salinity over the northern BoB decreases by more than 5 and the East India Coastal Current strengthens by 2 cm/s during post monsoon season. The fresh river water reaches up to 15°N in the BoB and is the main cause for low salinity there. Sea surface temperature in the northwestern BoB increases by more than 0.2°C due to the river runoff in summer monsoon while surface cooling upto 0.2°C is seen in north-west part of BoB in winter season. The seasonal mixed layer depth in the region is found to be dependent on river runoff. The effect of vertical shear and Brunt Vaisala frequency on stratification is also examined. The ocean water becomes highly stratified up to 3 035 m due to the river runoff. It is found that the energy required for mixing is high in the northern and coastal BoB. [ABSTRACT FROM AUTHOR]

In the Indian Ocean regions under the influence of monsoons, two phytoplankton blooms characterize the seasonal cycle of surface chlorophyll, one during summer, and the other during winter. In the Northern Mozambique Channel, however, where the wind regime is an extension of the northern Indian Ocean monsoons, the annual cycle of chlorophyll concentrations is characterized by a single winter bloom. Wind stress and surface net heat flux modulate the seasonal cycle of the mixed layer depth with impacts on the surface chlorophyll. In order to evaluate the importance of these forcing fields on the seasonality of the mixed layer depth, and consequently, the surface chlorophyll variability, we used a suite of physical-biogeochemical model sensitivity experiments. Our results show that the seasonal cycle of surface chlorophyll is primarily modulated by the net heat flux while the wind field controls the amplitude. The winter bloom is triggered by negative surface heat fluxes, where cooling at the surface induces mixing and entrainment of nutrients at the base of the nutricline and light is not limiting. Winds enhance the winter bloom by uplifting additional nutrients and diluting subsurface chlorophyll into the surface layer. In the summertime, weaker wind stress and positive heat flux inhibit vertical mixing. As a consequence, the surface layer is depleted in nutrients and a deep chlorophyll maximum is formed. Analysis of top-down control on phytoplankton biomass reveals that zooplankton abundance increases in a near-linear proportion with phytoplankton biomass despite the deepening of the mixed layer depth. This suggests that the phytoplankton stock in the Northern Mozambique Channel is also controlled by grazing, given that zooplankton biomass is not directly affected by the deepening of the mixed layer during wintertime. [ABSTRACT FROM AUTHOR]

To better understand the role of iron in driving marine ecosystems, the Southern Ocean Iron Experiment (SOFeX) fertilized two surface water patches with iron north and south of the Antarctic Polar Front Zone (APFZ). Using 1-D coupled biological-physical simulations, we examine the biogeochemical dynamics that occurred both inside and outside of the fertilized patches during and shortly after the SOFeX field campaign. We focus, in particular, on three main issues governing the biological response to deliberate iron fertilization: the interaction among phytoplankton, light, macronutrient and iron limitation; dilution and lateral mixing between the fertilized patch and external, unfertilized waters; and the effect of varying mixed layer depth on the light field. At the patch south of the APFZ, sensitivity simulations with no dilution results in the maximum bloom magnitude, whereas dilution with external water extends the development of the north patch bloom by relieving silicon limitation. In model sensitivity studies for both sites, maximum chlorophyll concentration and dissolved inorganic carbon depletion inside the fertilized patches are inversely related to mixed layer depth, similar to the patterns observed across a number of iron fertilization field experiments. Our results suggest that Southern Ocean phytoplankton blooms resulting from natural or deliberate iron fertilization will tend to become iron-light co-limited unless the mixed layer depth is quite shallow. © 2007 Elsevier B.V. All rights reserved.

Long-term trend and interannual variability of heat content down to 300 m in the Arabian Sea during the period 2000–2017 are analyzed to understand the physical forcings that lead to the significant warming of the Arabian sea. The warming trend during spring and summer are primarily due to heat accumulated below the mixed layer (ML) while the heat accumulated in the ML contributes to the warming during the fall and winter. The study reveals that the combined effect of El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) drives the interannual variability of heat content below the ML and the corresponding thermocline variability during spring. During summer, advection of heat plays a key role. Air-sea fluxes mainly drive the variability of heat content in the ML. However, ENSO and IOD also add to the variability during fall and winter. This study indicates that the warming over the upper 300 m in the Arabian Sea is influenced by the number of positive IOD and El Niño years during the study period, while the air-sea fluxes are responsible for the warming of the surface ML. This analysis sheds new light on the Arabian Sea seasonal heat content trends, and underscores the need for evaluation of their implications for the regional climate variability, trend and extremes. The findings have implications for process understanding needed to better predict impacts on marine heatwaves, cyclones and the regional climate. • Increasing trend is observed in Arabian sea heat content during all the seasons. • Subsurface heat content contributes to warming during spring and summer. • Heat accumulated in mixed layer contributes to the warming during fall and winter. • Air-sea fluxes, thermocline variations influence the interannual variability. [ABSTRACT FROM AUTHOR]

We investigated the characteristics of seasonal variations in sea surface salinity (SSS) in the subtropical North Pacific and North Atlantic oceans by analyzing observational datasets. As noted in previous studies, the seasonal variation in SSS markedly differs between the western and eastern parts of the basins. These contrasting seasonal variations in SSS have nonnegligible impacts on surface density for each basin and translate into changes in temperature at the outcropping isopycnals of about 1.0 °C. Further analysis of the mixed-layer salinity budget reveals that the zonally contrasting seasonality is attributed to a combination of seasonal variations in the freshwater flux and mixed layer (ML) depth. In the western basins, the wintertime increase in evaporation and reduction in precipitation, as well as the vertical entrainment of salty water below the ML contribute to salinification, whereas the seasonal freshening in summer is mostly attributed to increase in precipitation, reduction in evaporation, and strengthening of geostrophic currents. In contrast, evaporation exceeds precipitation throughout a year in the eastern basins, and seasonal variations in freshwater flux cannot fully explain the seasonal cycle of SSS in the region. Rather, the wintertime freshening there is aided by the deepening of the ML, which acts to mitigate salinification by the evaporation and sustain vertical entrainment of fresher water below the ML. Summertime salinification there can be understood as a mirror image of the wintertime condition, but seasonal strengthening of Ekman currents and associated salinity advection also play a role. (241 words) [ABSTRACT FROM AUTHOR]

[Display omitted] • High-frequency oscillation of SSH-induced pressure gradient forces can trigger offshore transport. • Offshore transport of coastal waters has a significant relationship with PB occurrences. • The influence of atypical wind patterns on long-term ecosystem impacts should not be ignored. The East China Shelf Sea (ECSS) is subject to high-frequency storms during winter and spring, with these storm processes serving as a significant driving factor for initiating the outward diffusion of materials from the inner shelf. Inner shelf waters tend to be rich in nutrients, thus their horizontal diffusion is crucial to the marine primary productivity of the outer shelf. However, our understanding of the processes that govern the offshore water mass transport under winter storm oscillations and the mechanisms driving their impact on phytoplankton is currently limited. In this study, we used satellite and reanalysis data from 2003 to 2020 to investigate the modulation mechanism of winter offshore currents (WOC) on phytoplankton bloom (PB) stimulated by storms in Northern Taiwan (NTW). The results revealed that at synoptic scales, winter storms result in higher sea surface height (SSH) and potential energy on the inner side of the front. However, during periods of weakened storms (for example, during north wind relaxation or a shift to southerly winds), a strong WOC is produced in the top 30–50 m of the ocean due to continuous SSH adjustments caused by changed pressure gradients and induced by topography. PB occurs in the offshore waters of the front, driven by a continuous nutrient supply and an increase in photosynthetically available radiation (PAR). On an interannual scale, offshore transport is controlled by the storm's duration and the extent of its weakening. As the climate warms and winter winds in NTW have consistently weakened over the past 20 years, lower turbulent kinetic energy (TKE) and a shallower mixed layer depth (MLD) will promote phytoplankton growth. The results suggest that water level fluctuations caused by high-frequency changes in winter storms are strongly linked to the offshore sediment transport from near-shore waters, impacting the material transport and the health of the ecosystem on the continental shelf. [ABSTRACT FROM AUTHOR]

In this study impact of assimilating Sea Surface Salinity (SSS) from multi-satellites (SMOS, Aquarius and SMAP) on numerical ocean model simulations in the north Indian Ocean has been analysed under the observing system experiment (OSE) framework. Daily data sets of Aquarius, SMAP and SMOS, which were available for a common period of April–May 2015, are used to constrain the ocean model using ensemble optimal interpolation technique. Apart from the control simulation in which satellite data were not assimilated, a total of seven assimilation experiments using different combinations of satellite SSS were conducted. The impact of assimilation experiments is analysed by comparing the model-simulated variables with in situ observations. Assimilating satellite SSS results in a reduction in Root Mean Square Error (RMSE) in SSS (∼ 54%) and also in subsurface salinity (∼ 21%) over the control run. The impact of assimilating SMAP observations is maximum on model simulations with the errors reducing by ∼ 54%. Subsurface salinity improvement is better with three satellites with ∼31% improvement in RMSE in the halocline region, which was ∼11% more than single satellite assimilation. Assimilation of SSS also resulted in improved simulations of the model surface, subsurface temperature and mixed layer depth. Model results show the ability of SSS observations to complement other ocean observation networks. One important observation from this study is that while the impact of assimilating SSS observations from a single satellite was on par with the impact of assimilating SSS observations from two or three satellites in correcting simulated surface salinity, assimilation from more than one satellite had a larger impact in the salinity of deeper layers of the ocean. • Ocean OSE has been performed for the Indian Ocean using SSS from Aquarius, SMAP and SMOS satellites. • Single satellite SSS assimilation is at par with 2 or 3 satellite SSS assimilation in improving surface salinity fields. • Upto 54% improvement at model surface salinity and 8% improvement at model MLD with SSS assimilation. • 2 or 3 satellite SSS data assimilation is more effective in correcting salinity fields in the deeper layers of ocean. [ABSTRACT FROM AUTHOR]