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Marine heatwaves (MHWs) in the South China Sea (SCS) have prolonged impacts on local ecosystems and economies, and accurate projection of MHWs under future global warming is crucial for the high-quality development of local society. The future change in the spatial pattern of MHWs, however, is not clear despite the well-known overall intensification of MHWs. Here we find that the Coupled Model Intercomparison Project Phase 6 (CMIP6) models can effectively capture the main distribution of observed SCS MHWs, showing a uniform distribution of frequency and a "north high-south low" distribution of mean intensity and cumulative intensity. However, it is worth noting that the simulated center of long MHW duration is shifted to the southern SCS compared to the central SCS in observations. Under the Shared Socioeconomic Pathway 1–2.6 (SSP126) scenario, the increase in MHW cumulative intensity exhibits a double-center structure in the northern coastal region and southern SCS. This is primarily attributed to the large increase in frequency and mean intensity in the north, and an increase in duration in the south. Both the SSP245 and SSP585 scenarios project similar patterns of MHW intensification, but with larger magnitudes. The climatological distribution of the mixed layer depth (MLD), which is deeper in the south and shallower in the north, contributes to the spatial distribution of SCS MHW changes. The strong seasonal-mean sea surface temperature (SST) warming in the northern SCS, caused by enhanced solar radiation, also contributes to the intensification of MHW frequency and mean intensity in the northern region. [ABSTRACT FROM AUTHOR]

Predictions of tropical cyclone (TC) activity have been a topic of recurrent interest and research in the past. Here we utilize reanalysis datasets of sea surface temperature (SST) and mixed layer depth (MLD) to build a statistical seasonal forecasting model that produces outlooks of expected TC counts in the region of the Southwest Pacific (SWP). Nevertheless, the model applicability can be extended to other regions and basins. A novel TC predictor index is developed at the daily scale and used to obtain an objective classification of synoptic weather patterns. This classification has been performed by clustering the daily index predictor fields, previously transformed into principal components, using a K‐mean algorithm. As a result, 49 daily weather types (DWTs) are presented which inform about the mean representative features and spatial patterns of both predictor and predictand variables. Thus, statistical relationships between TC activity and nonlinear combinations of predictor variables are found to assign daily rates of expected TCs. The cluster‐based model is calibrated from 1982 to 2019 and validated by recent TC season observations, demonstrating the operational application using ensembles of long‐term predictions in the Southwest Pacific. Results have shown which synoptic types of SST and MLD are favourable to cyclogenesis and activity, with additional information related to concurrent sea level pressure and precipitation synoptic patterns, as well as seasonal and interannual climate variability. [ABSTRACT FROM AUTHOR]

The statistical characteristics and mechanism of the South Atlantic Ocean Dipole (SAOD) from 1980 to 2021 are analysed using observational datasets. The spatial pattern of the sea surface temperature anomaly (SSTA) during SAOD is a dipole pattern oriented in the northeast‐southwest direction, and the intensity of the SSTA in the northeast pole (NEP) is stronger than that in the southwest pole (SWP). SAOD has a decadal variability of about 12 years during 1980–2007, along with obvious seasonal phase‐locking, with the anomaly pattern developing in boreal spring (March–May), peaking in summer (June–August) and decaying in autumn (September–November). For a positive SAOD event (positive SSTA in the NEP, negative in the SWP), positive SSTA in the NEP grows due to a decrease in wind speed and thus in latent heat flux loss in boreal spring, as well as an increase in shortwave radiation flux from boreal spring to summer. However, southwesterly wind anomalies drive cold water from high latitudes to the SWP in boreal spring and summer, coupled with strong wind speed anomalies enhancing the loss of latent heat flux, which contributes to a negative SSTA in the SWP. In addition, the intensity of the SSTA in the SWP is weaker than that in the NEP because of the contribution of smaller shortwave radiation flux, sensible heat flux and larger mixed layer depth in the SWP in summer. For a negative SAOD mode (negative SSTA in the NEP, positive in the SWP), the wind, shortwave radiation flux, sensible heat flux and mixed layer depth anomalies are the opposite of those under a positive SAOD event. [ABSTRACT FROM AUTHOR]

Oceanic general circulation models (OGCMs) are important tools used to investigate mechanisms for ocean climate variability and predict the ocean change in the future. However, in most current ocean models, the impact of sea surface waves as one of the most significant dynamic processes in the upper ocean is absent. In this study, the Stokes drift and the vertical mixing induced by nonbreaking surface waves derived from the wave model (WAVEWATCH III) are incorporated into a Climate System Ocean Model, and their effects on an ocean climate simulation are analyzed. Numerical experiments show that both physical processes can improve the simulation of sea surface temperature (SST) and mixed layer depth (MLD) in the Southern Hemisphere. The introduction of Stokes drift effectively reduces the subsurface warm bias in the equatorial tropics, which is caused by the weakening of vertical mixing in the equatorial region. The nonbreaking surface wave mainly reduces the temperature bias in the Southern Ocean by enhancing mixing in the upper ocean. For the MLD, the Stokes drift mainly improves the simulation of the winter MLD, and the nonbreaking surface wave improves the summer MLD. For MLD south of 40° S in summer, the introduction of nonbreaking surface waves resulted in a reduction of 11.86 m in MLD bias and 7.8 m in root mean square errors (RMSEs), respectively. For winter subtropical MLD in the Southern Hemisphere, considering the Stokes drift, the MLD bias and RMSEs were reduced by 2.49 and 5.39 m, respectively. Adding these two physical processes simultaneously provides the best simulation performance for the structure of the upper layer. The introduction of sea surface waves effectively modulates the vertical mixing of the upper ocean and then improves the simulation of the MLD. Thus, sea surface waves are very important for ocean simulation, so we will further couple a sea waves model in the Chinese Academy of Sciences Earth System Model (CAS-ESM) as part of their default model component. [ABSTRACT FROM AUTHOR]

Severe typhoon "In-Fa" passed through the northwestern region of East China Sea (ECS) in July 2021, affecting oceanic variables such as seawater temperature, salinity, and chlorophyll-a (Chl-a) concentration over the upwelling area. In this study, we analyzed the influence of the passage of typhoon "In-Fa" on the marine environment over the Upwelling Area off the Yangtze River Estuary (UAYRE) and the Upwelling Area of Zhoushan (UAZS). The results showed a significant decrease in sea surface temperature (SST) during the "In-Fa" typhoon, with maximum SST reductions of 2.98 °C in the UAYRE and 1.46 °C in the UAZS, which showed a "right bias" (indicating a greater cooling effect on the right side of the typhoon path compared to the left side). "In-Fa" influenced the temperature and salinity structure of the study areas and deepened the mixed layer depth (MLD). The MLD varied from the shallowest values of 2.02 m (18 July) to the deepest values of 19.4 m (26 July) in the UAYRE and from 2.43 m (18 July) to 16.79 m (25 July) in the UAZS. Furthermore, "In-Fa" led to an increase in sea surface Chl-a concentration, with a maximum Chl-a concentration enhancement of 285.58% (from 20 July to 28 July) in the UAYRE and 233.33% (from 20 July to 27 July) in the UAZS. The Ekman suction effect of "In-Fa" strengthened the upwelling, facilitating the transport of deep-sea nutrients to the upper ocean and providing favorable conditions for the growth of phytoplankton, thus benefiting the reproduction and survival of zooplankton, fish, and shrimp. This study contributes to understanding the mechanisms by which typhoons impact the ocean environment in upwelling area and provides valuable insights for the sustainable development of marine fisheries resources. [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) anomalies in the Kuroshio Extension (KE) have been suggested to play a crucial role in decadal climate variability of the North Pacific affecting global climate and marine ecosystem variability. By analyzing the mixed layer heat budget, taking into account seasonality and mixed layer depth (MLD) variations, we here show for the first time that the KE SST front undergoes large decadal variations mainly owing to decadal modulations of the effective heat capacity affecting the SST sensitivity to surface heat fluxes during 1982–2015. More specifically, when the mixed layer becomes anomalously thick (shallow) to the south (north) of the front, it becomes less (more) sensitive to wintertime surface cooling. As a result, the SST front is strengthened, with positive (negative) SST anomalies to the south (north). A heat conservation model suggests that MLD anomalies are mainly due to thermocline depth anomalies. Plain Language Summary: Decadal climate variability of the North Pacific is known to affect global climate and marine ecosystem variability. The sea surface temperature (SST) front associated with the Kuroshio Extension (KE) in the northwestern Pacific is considered to play a key role in the turnabout of the North Pacific decadal climate variability. The fundamental question of how the strength of the SST front is modulated on the decadal timescale, however, has yet to be understood. Through quantitative analyses of oceanic and atmospheric data sets during 1982–2015, here we present for the first time that the KE SST front undergoes large decadal variations mainly owing to decadal modulation in thickness of the surface mixed layer that controls sensitivity of SST to surface heat exchange between the ocean and the atmosphere. Furthermore, a highly idealized model suggests that the anomalous mixed layer thickness is mainly due to anomalous oceanic stratification. Key Points: Generation mechanisms of decadal sea surface temperature (SST) anomalies associated with the Kuroshio Extension SST front is examinedA seasonally stratified mixed layer heat budget analysis with variable mixed layer depth (MLD) is conductedDecadal modulation in the MLD that controls sensitivity of SST to surface heat fluxes plays a key role during 1982–2015 [ABSTRACT FROM AUTHOR]

The frequent typhoons in the northwest Pacific drive oceanic responses, for example, changes in sea surface temperature (SST) and chlorophyll‐a (Chl‐a). A composite analysis shows that SST (Chl‐a) around the typhoon center decreases (increases). The SST begins to decrease 3 days before the typhoon's arrival and further decreases until 2 days following the typhoon's passage. The Chl‐a increases rapidly after the typhoon's arrival, and the maximum value is reached 3 days after the typhoon. Large oceanic responses are often associated with typhoons that have high wind speeds and slow translation speeds. From the perspective of the upper ocean structure, increasing changes in SST are identified with a shallower pre‐typhoon mixed layer depth (MLD). However, a significant dependence of the Chl‐a response on the pre‐typhoon MLD emerges only when the depth of typhoon‐induced mixing is greater than the pre‐typhoon MLD. This study helps to quantitatively describe typhoon‐induced changes with consideration of the determinant oceanic environment. Plain Language Summary: Typhoons in the northwestern Pacific induce strong oceanic responses. Using 17 years of satellite observations, the impacts of typhoons on sea surface temperature (SST) and chlorophyll‐a (Chl‐a) are investigated. The SST time series shows that the SST begins to decrease 2 days before the typhoon's arrival and continues to decrease until 2 days following the typhoon's passage. The Chl‐a has a weak peak 2 days prior to the typhoon's arrival, rapidly increases after the typhoon arrives, reaches the strongest response on the third day of the typhoon, and gradually decreases to a value slightly higher than the pre‐typhoon level. Prominent responses are associated with typhoons that have stronger intensity and slower translation speed. The pre‐typhoon upper ocean structure plays a dominant role in determining oceanic responses. Surface cooling is generally stronger where the pre‐typhoon mixed layer depth (MLD) is shallow. However, the change in Chl‐a shows a contrasting response in that the response prominently increases only when the depth of typhoon‐induced mixing exceeds the pre‐typhoon MLD. This study poses a quantitative approach to assess the influence of typhoons on the upper ocean from a statistical perspective with consideration of the upper ocean structure. Key Points: Sea surface temperature and chlorophyll‐a changes are composited to assess their responses to the passage of typhoons in the northwest PacificLarge responses are introduced by strong and slow‐moving typhoons, and surface cooling increases with shallow mixed layer depth (MLD)Responses in chlorophyll‐a increase only when the depth of typhoon‐induced mixing exceeds the pre‐typhoon MLD [ABSTRACT FROM AUTHOR]

In this study, we mainly used in-situ observations from underwater gliders to analyze the ocean response in the northern South China Sea affected by Son-tinh (2018), Mandal et al. (2018) Mangkhut (2018)and Noul (2020). The results showed that these TCs caused 0.6 °C, 1.1 °C and 1.7 °C maximum sea surface temperature cooling respectively, which were weaker than general conditions because of long distance, weak intensity and fast movement speed. Net solar radiation, precipitation, 10-m wind and sea surface heat flux also made contribution in changes of SST. The mixed layer depth (MLD) became shallower after Son-Tinh and Noul passed through, while during Mangkhut it did not change significantly. After TCs passed through, the stratification around MLD became more obvious, with a banded distribution and stronger high-value areas of buoyancy frequency. Within 1 week after the shortest distance, the temperature and salinity responses in the upper ocean were stronger than those at the sea surface, and the gradients of temperature and salinity and their anomalies were more evident in the subsurface layer. The results of this study show that underwater glider observations are important for understanding oceanic responses to tropical cyclones and are useful for studying tropical cyclone activities. [ABSTRACT FROM AUTHOR]

Warm blobs are persistent warm anomalies in the Northeast Pacific (NEP) upper ocean. Here, we assess the role of mixed layer depth (MLD) in their location and development based on ocean‐atmosphere reanalysis data. We find that warm blobs occur more frequently over 165°–130°W and 35°–50°N with shallow MLD. They are largely confined to the mixed layer, although substantial portions exist beneath it in summer when the MLD shoals. Based on a mixed‐layer heat budget analysis, we reveal that anomalous MLD and heat flux contribute dominantly to the surface heat flux term over the study area from May to July and from September to March, respectively, and have a large effect on warm blob development. Therefore, the MLD has important implications for the location and seasonal evolution of warm blobs and temperature diagnosis over the NEP. Plain Language Summary: Warm blobs are anomalous warm waters in the upper ocean of the Northeast Pacific (NEP). Oceanic mixed layer is the warm surface layer that maintains homogeneous temperature or density. This study aims to clarify the role of mixed layer depth (MLD) in the location and development of warm blobs. First, we determine a region of 165°–130°W and 35°–50°N in which warm blobs are most likely to occur due to strong sea surface temperature variability and shallow MLD. In this region, temperature anomalies of the warm blobs are largely confined to the mixed layer, although parts of the anomalies are beneath it when MLD shoals in summer. Then, we separate and quantify the relative contributions of MLD and surface heat flux anomaly to the development of the warm blobs from the surface heat flux term in a mixed‐layer heat budget. We find that the contribution of MLD anomaly is dominant from May to July, while that of surface heat flux anomaly (i.e., less ocean heat loss) is pronounced from September to the following March. Therefore, the effect of MLD on the location and seasonal evolution of the warm blobs gives implications for the temperature diagnosis over the NEP. Key Points: Northeast Pacific warm blobs appear more frequently in areas with a shallow mixed layerTemperature anomalies of warm blobs are not only largely confined to the mixed layer but also stored beneath the mixed layer in summerMixed layer depth anomalies are the dominant contributor to the temperature anomalies over the warm blob area from May to July [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]

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 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]

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]

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]

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 region located in the far northeast of the Gulf of Guinea (NEGG), eastern tropical Atlantic, remains poorly documented due to a lack of available in situ ocean data. Heavy rainfall and intense river discharges observed in this region induce a strong salinity stratification that may have a significant impact on the mixed layer depth and on sea surface temperatures, through the so-called barrier-layer effect. By using recent in situ data and climatological outputs from a numerical simulation, we reveal the existence of a barrier layer in the NEGG and describe its seasonal occurrence. In the NEGG, the barrier layer limits the mixed layer depth. From January to March, significant values for the barrier-layer thickness are observed mostly due to the horizontal advection of fresh water. From April, vertical mixing along with vertical advection increase the sea surface salinity; hence, the barrier-layer thickness decreases and reaches its minimum in July. During the rest of the year, values for the barrier-layer thickness are again high, mostly under the influence of the Niger River discharge and precipitation, with the highest values recorded in October, when the river discharge and precipitation are at a maximum. [ABSTRACT FROM AUTHOR]

Tropical cyclones (TCs) are among the most important factors that influence the upper oceanic environment of the Western North Pacific (WNP). To better understand the effects of TCs on the changes in the physical and biological processes of the WNP, two regions with similar properties but different TC frequencies were studied. Unlike previous studies, the long-term effects of TCs on the biophysical properties of the WNP throughout the TC season were discussed. The biophysical properties of both regions during the TC season were compared using satellite-derived measurements and Argo float data during 2003–2017. During the TC season, short-term rapid enhancement of the wind during TC passages had a direct effect on the cooling of the sea surface temperature and the deepening of the mixed-layer depth. Moreover, surface nitrate concentrations in the TC-prone region were significantly higher than those in the region not heavily influenced by TCs. As a result, the mean surface chlorophyll-a (Chl -a) concentration in the region with a high TC frequency was significantly higher than that with a low TC frequency. The Chl -a concentrations also tended to be higher during the El Niño years because of more strong TC events in both regions, while they were generally lower during the La Niña years when there were fewer strong TCs. Based on a conservative estimate, new primary production in TC season caused by TCs in the region with a high TC frequency accounted for approximately 1–7% of the annual amount in this region during the study years. These findings suggest that the TCs play an important role in the enhancement of biological carbon fixation in parts of the WNP. • The biophysical properties of two marine areas with different tropical cyclone frequencies were compared. • Tropical cyclones produced large differences in Chl- a concentration, SST, and MLD in different marine areas. • Tropical cyclones significantly increased primary productivity in the WNP. [ABSTRACT FROM AUTHOR]

Five different parameterizations of Langmuir turbulence (LT) effect are investigated in a realistic model of the North Atlantic and Arctic using realistic wave forcing from a global wave hindcast. The parameterizations mainly apply an enhancement to the turbulence velocity scale, and/or to the entrainment buoyancy flux in the surface boundary layer. An additional run is also performed with other wave effects to assess the relative importance of Langmuir turbulence, namely the Coriolis-Stokes forcing, Stokes tracer advection and wave-modified momentum fluxes. The default model (without wave effects) underestimates the mixed layer depth in summer and overestimates it at high latitudes in the winter. The results show that adding LT mixing reduces shallow mixed layer depth (MLD) biases, particularly in the subtropics all year-around, and in the Nordic Seas in summer. There is overall a stronger relative impact on the MLD during winter than during summer. In particular, the parameterization with the most vigorous LT effect causes winter MLD increases by more than 50% relative to a control run without Langmuir mixing. On the contrary, the parameterization which assumes LT effects on the entrainment buoyancy flux and accounts for the Stokes penetration depth is able to enhance the mixing in summer more than in winter. This parameterization is also distinct from the others because it restrains the LT mixing in regions of deep MLD biases, so it is the preferred choice for our purpose. The different parameterizations do not change the amplitude or phase of the seasonal cycle of heat content but do influence its long-term trend, which means that the LT can influence the drift of ocean models. The combined impact on water mass properties from the Coriolis-Stokes force, the Stokes drift tracer advection, and the wave-dependent momentum fluxes is negligible compared to the effect from the parameterized Langmuir turbulence. • Five Langmuir mixing parameterizations are compared in a realistic model of the North Atlantic and Arctic oceans. • Warm sea surface temperature biases have been reduced, with significant improvements in the summer. • Shallow mixed layer depth errors are decreased in the subtropics and almost everywhere in summer. • Ocean heat content has increased and been brought closer to observations. • Coriolis-Stokes forcing, Stokes tracer advection and wave-modified stress are included, but only a small impact is found. [ABSTRACT FROM AUTHOR]

Tropical cyclones are form of intense storms which are common in the Bay of Bengal (BOB) and known for their damage potential to human and natural resources especially in the coastal parts of India, Bangladesh and Myanmar. A twoway effect is prominent between BOB’s sea surface temperature (SST) and these cyclonic events. On one side, BOB’s high SST favours formation of cyclones and on the other side it itself gets affected whenever cyclone occurs. The later part along with circulation patterns has been simulated here with the help of high resolution Regional Ocean Modeling System (ROMS) for 'MALA' cyclone. Remote sensing instrument Quick Scatterometer (QuikSCAT)-derived daily wind components are used as atmospheric forcing conditions for ROMS simulations. ROMS circulation is found highly sensitive towards QuikSCAT wind forcing and reveal an anticlockwise rotation with high magnitude currents due to cyclone passage. Near surface as well as deeper isotherm and isohaline surfaces show high degree of spatial variability and overall indication of upwelling happening on near track area. It is found that mixed layer depth reduces at the storm centre due to intense upwelling but increases far away from it because of high winds. Due to intrusion of bottom high saline water and reduction of temperature, density increases from its initial value which affects water column stability from surface to even below 150 m depth. [ABSTRACT FROM AUTHOR]

During the summer monsoon season from June through September, low‐level winds from the Findlater Jet induce strong mesoscale activity and seasonal deepening of the mixed layer. For the first time, we have investigated the impact of mesoscale features on the strong intraseasonal variability of the mixed layer depth in the Arabian Sea using simulations from a reanalysis version of the HYbrid Coordinate Ocean Model and the European Centre for Mid‐range Weather Forecasting ERA‐Interim product. We identify and track mesoscale eddies using an eddy‐tracking algorithm during strong and weak summer monsoon conditions to determine the importance of mesoscale features on mixed layer variability during strong and weak wind forcing. We identify frontal systems in the Arabian Sea and observe how they strengthen with depth throughout the summer monsoon season. To quantify the role of these features on mixed layer variability, we also explore the roles of wind forcing, evaporation, precipitation, significant wave heights, and surface heat fluxes on mixed layer variability. A mixed layer heat budget was computed in the Arabian Sea, finding a strong dominance of vertical advective entrainment followed by compensation by horizontal processes indicating strong mesoscale activity. Plain Language Summary: The uppermost layer of the ocean is full of dynamic features such as frontal systems and quasi‐circular circulations on the order of 10–100 km called eddies. These features actively redistribute waters throughout the oceanic surface layer in which temperature, salinity, and density is almost vertically uniform, referred to as the mixed layer. In the Arabian Sea, the mixed layer deepens twice seasonally: once in the boreal winter and once in the boreal summer. In the wintertime, cool air blows over the Arabian Sea and cools the surface, which then sinks and deepens the mixed layer. In the summer, strong southwesterly winds deepen the mixed layer and provide conditions conducive to strong eddying and frontal development. In this paper, we compare the distribution of these mesoscale features under strong (weak) wind forcing associated with heavy (scanty) rainfall conditions and referred to as strong (weak) monsoon conditions. We find through the application of an eddy‐tracking algorithm, a spatial and temporal analysis of a variety of factors that impact mixed layer variability, and a heat budget analysis that there is an overall dominance of larger features in strong monsoon conditions and a higher frequency of small features in weak monsoon conditions. Key Points: For the first time, we have investigated the impact of mesoscale features on mixed layer variability in the Arabian SeaThe eddy field in the strong (weak) monsoon seasons favors larger‐mesoscale (smaller‐mesoscale) activityAnticyclonic eddies in the Arabian Sea have larger radii and smaller amplitudes for much of the summer monsoon season [ABSTRACT FROM AUTHOR]

Southern Ocean mesoscale eddies play an important role in ocean circulation and biogeochemical cycling, but their biological characteristics have not been well quantified at the basin scale. To address this, we combined a 15‐year tracked eddy data set with satellite observations of ocean color, sea surface temperature, and autonomous profiling floats to quantify the surface and subsurface properties of eddies. Anomalies of surface temperature and chlorophyll were examined in eddy‐centric composite averages constructed from thousands of eddies. Normalized surface chlorophyll anomalies (chlnorm) vary seasonally and geographically. Cyclones typically show positive chlnorm, while anticyclones have negative chlnorm. The sign of chlnorm reverses during late summer and autumn for eddies between the Subtropical and Polar Fronts. The reversal is most obvious in the Indian sector, and we attribute this to a combination of eddy stirring (deformation of surface gradients by the rotational velocity of an eddy) and deeper winter mixing in anticyclones. Both chlnorm and sea surface temperature anomalies transition from dipole structures north of the Subtropical Front to monopole structures south of the Subantarctic Front. Sea surface temperature and chlnorm composites provide evidence for eddy trapping (transporting of anomalies) and eddy stirring. This research provides a basin‐scale study of surface chlorophyll in Southern Ocean eddies and reveals counterintuitive biogeochemical signals. Plain Language Summary: Ocean eddies are spinning parcels of water about 100 km across and 1,500‐m deep. They occur everywhere in the ocean. In the Southern Hemisphere, eddies that spin clockwise are cooler than the surrounding ocean because their rotation causes cold, deep water to move upward. This upwelling brings nutrients essential for photosynthesis to the surface and makes clockwise‐rotating eddies more productive. Satellites can measure this productivity by sensing differences in ocean color, which result from the increased plankton. By analyzing thousands of Southern Ocean eddies, we found that in summer and autumn, eddies behave opposite to our expectations. That is, clockwise rotating eddies have lower plankton concentrations compared to neighboring waters and counterclockwise rotating eddies have higher concentrations. To explain this, we examined how deep these eddies mix the ocean in the preceding months. We found that counterclockwise rotating eddies mix the ocean deeper in winter, allowing more nutrients to enter their interiors, leading to higher productivity. This work is important because eddy productivity plays a significant role in the exchange of carbon between the ocean and the atmosphere. Carbon exchange in the Southern Ocean is thought to be changing, and this work helps explain an important piece of that process. Key Points: We quantify the seasonal and geographical variabilities in the physical and biological characteristics of Southern Ocean mesoscale eddiesAtypical chlorophyll anomalies are observed in eddies between the Subtropical Front and the Polar Front during summer and autumnEddy stirring, eddy trapping, and eddy pumping all contribute to physical and biological anomalies, depending on the region and season [ABSTRACT FROM AUTHOR]

Tides play a significant role in the ocean surface circulations and vertical mixing thereby influencing the Sea Surface Temperatures (SST) as well. This, in turn, plays an important role in the global circulation when used as a lower boundary condition in a global atmospheric general circulation model. Therefore in the present study, the dynamics of tides over the Bay of Bengal (BoB) is investigated through numerical simulations using a high resolution (1/12°) Regional Ocean Modeling System (ROMS). Based on statistical analysis it is observed that incorporation of explicit tidal forcing improves the model performance in simulating the basin averaged monthly surface circulation features by 64% compared to the simulation without tides. The model simulates also Mixed Layer Depth (MLD) and SST realistically. The energy exchange between tidal oscillations and eddies leads to redistribution of surface kinetic energy density with a net decrease of 0.012 J m -3 in the western Bay and a net increase of 0.007 J m -3 in the eastern Bay. The tidal forcing also affects the potential energy anomaly and vertical mixing thereby leading to a fall in monthly MLD over the BoB. The mixing due to tides leads to a subsequent reduction in monthly SST and a corresponding reduction in surface heat exchange. These results from the numerical simulation using ROMS reveal that tides have a significant influence over the air-sea heat exchange which is the most important parameter for prediction of Tropical Cyclone frequency and its future variability over the BoB. [ABSTRACT FROM AUTHOR]

A regional ocean circulation model with four-dimensional variational data assimilation scheme is configured to study the ocean state of the Indian Ocean region (65°E-95°E; 5°N-20°N) covering the Arabian Sea (AS) and Bay of Bengal (BoB). The state estimation setup uses 10 km horizontal resolution and 5 m vertical resolution in the upper ocean. The in-situ temperature and salinity, satellite-derived observations of sea surface height, and blended (in-situ and satellite-derived) observations of sea surface temperature alongwith their associated uncertainties are used for data assimilation with the regionally configured ocean model. The ocean state estimation is carried out for 61 days (1 June to 31 July 2013). The assimilated fields are closer to observations compared to other global state estimates. The mixed layer depth (MLD) of the region shows deepening during the period of assimilation with AS showing higher MLD compared to the BoB. An empirical forecast equation is derived for the prediction of MLD using the air-sea forcing variables as predictors. The surface and sub-surface (50 m) heat and salt budget tendencies of the region are also investigated. It is found that at the sub-surface, only the advection and diffusion temperature and salt tendencies are important. [ABSTRACT FROM AUTHOR]

• New high-resolution reanalysis enables a complete 4D view of the US coastal ocean. • Analysis of persistence, trends and distribution of temperature and salinity. • Bathymetry and mixed layer depth influences the bottom water temperature. • Mississippi and Columbia rivers strongly impact the adjacent oceans. Obtaining a long-term perspective on the evolution of the ocean is hindered by a lack of sub-surface observations. Ocean reanalyses, which merge fields from an ocean model with observations using complex data assimilation techniques, provide a dynamically consistent estimate of the ocean in time and space. Recently developed ocean reanalyses, several with resolutions finer than 10 km, provide a three-dimensional view of the ocean, including on the continental shelf. Here, we use monthly fields from the 1/12° Global Ocean Reanalysis and Simulations (GLORYS) over the years 1993–2019 to provide a broad survey of temperature, salinity, and mixed layer depth over the shelf (depth < 400 m) around the continental United States. The analyses reveal noteworthy aspects of US coastal oceanography, including: i) how bathymetry shapes the seasonal cycle of bottom water temperature (BWT) and the role of the mixed layer in linking the BWT and sea surface temperature (SST) anomalies; ii) the persistence, trends and distributions of SST and BWT anomalies, which strongly influence extremes such as marine heatwaves; iii) the influence of the outflow from the Mississippi and Columbia rivers on the adjacent ocean; and iv) how the mean and anomalous vertical structure of temperature and salinity vary between the northeast, southeast, Gulf of Mexico and California Current Large Marine Ecosystems. In addition to documenting coastal ocean conditions, the broader goal of the survey is to encourage more detailed studies using high resolution ocean reanalyses such as GLORYS. [ABSTRACT FROM AUTHOR]

High Frequency (HF) radars, if properly located, can prove to be a valuable tool in areas of specific oceanographic interest, like Straits connecting two contrasting Ocean Basins, as in the case of the Turkish Straits connecting the Mediterranean and the Black Sea. The two basins' exchange generates a surface inflow and current into the North Aegean Sea that determine its characteristics and variability. Thus it is of prime importance to monitor and quantify the lateral advection of Black Sea waters (BSW) after their exit through the Dardanelles Straits. Here, we exploit surface velocity maps obtained through a HF radar in an effort to estimate (a) mixed-layer depth and (b) surface layer volume fluxes with unprecedented high spatiotemporal resolution. Comparison of the results of the hereby introduced method to estimates of the Dardanelles inflow into the Aegean Sea, or to independent estimates of the BSW advection volume fluxes in the interior of the basin, reveal consistently smaller values obtained with the new method. The difference is attributed to the fact that the HF radar method corresponds solely to advection within the surface mixed-layer, which is only a fraction of the total volume influx of Black-Sea waters into the Aegean. Testing this hypothesis on hydrographic measurements from a section further down the flow certifies that the quantity of BSW within the mixed-layer corresponds to a fraction of the total inflow of BSW. [ABSTRACT FROM AUTHOR]

This work introduces a method that enables the production of mixed-layer depth estimates from concurrent measurements of surface current velocities and surface wind stress. The method's basic assumption is that the mixed layer responds to wind forcing as a slab-layer. Here, the surface current velocities are provided by a High Frequency (HF) radar installed at the east coast of Lemnos island, monitoring the Dardanelles outflow of modified Black Sea waters into the Aegean Sea, and the wind stress is provided via the ERA-interim data set. The surface mixed layer is assumed to respond to wind-forcing as a slab. The method is applied both using the full EOF-reconstructed surface velocity field as well as only its wind-forced part. In the current implementation, the former method appears to produce better results than the latter. The method is evaluated using mixed-layer depth values estimated via local CTD profiles as reference. [ABSTRACT FROM AUTHOR]

Sea-to-air fluxes of marine biogenic aerosols have the potential to modify cloud microphysics and regional radiative budgets, and thus moderate Earth's warming. Polar regions play a critical role in the evolution of global climate. In this work, we use a well-established biogeochemical model to simulate the DMS flux from the Greenland Sea (20°W–10°E and 70°N–80°N) for the period 2003–2004. Parameter sensitivity analysis is employed to identify the most sensitive parameters in the model. A genetic algorithm (GA) technique is used for DMS model parameter calibration. Data from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are used to drive the DMS model under 4 × CO 2 conditions. DMS flux under quadrupled CO 2 levels increases more than 300% compared with late 20th century levels (1 × CO 2 ). Reasons for the increase in DMS flux include changes in the ocean state—namely an increase in sea surface temperature (SST) and loss of sea ice—and an increase in DMS transfer velocity, especially in spring and summer. Such a large increase in DMS flux could slow the rate of warming in the Arctic via radiative budget changes associated with DMS-derived aerosols. [ABSTRACT FROM AUTHOR]

Two distinct layers usually exist in the upper ocean. The first has a near-zero vertical gradient in temperature (or density) from the surface and is called the isothermal layer (or mixed layer). Beneath that is a layer with a strong vertical gradient in temperature (or density), called the thermocline (or pycnocline). The isothermal layer depth (ILD) or mixed layer depth (MLD) for the same profile varies depending on the method used to determine it. Also, whether they are subjective or objective, existing methods of determining the ILD do not estimate the thermocline (pycnocline) gradient. Here, we propose a new exponential leap-forward gradient (ELG) method of determining the ILD that retains the strengths of subjective (simplicity) and objective (gradient change) methods and avoids their weaknesses (subjective methods are threshold-sensitive and objective methods are computationally intensive). This new method involves two steps: (1) the estimation of the thermocline gradient G for an individual temperature profile, and (2) the computation of the vertical gradient by averaging over gradients using exponential leap-forward steps. Such averaging can filter out noise in the profile data. Five existing methods of determining the ILD (difference, gradient, maximum curvature, maximum angle, and optimal linear fitting methods) as well as the proposed ELG method were verified using global expendable bathythermograph (XBT) temperature and conductivity-temperature-depth (CTD) datasets. Among all the methods considered, the ELG method yielded the highest skill score and the lowest Shannon information entropy (i.e., the lowest uncertainty). [ABSTRACT FROM AUTHOR]

Detailed mechanisms for frontogenesis/frontolysis of the Agulhas Return Current (ARC) Front, defined as the maximum of the meridional sea surface temperature (SST) gradient at each longitude within the ARC region (40°-50°E, 55°-35°S), are investigated using observational datasets. Due to larger (smaller) latent heat release to the atmosphere on the northern (southern) side of the front, the meridional gradient of surface net heat flux (NHF) is found throughout the year. In austral summer, surface warming is weaker (stronger) on the northern (southern) side, and thus the NHF tends to relax the SST front. The weaker (stronger) surface warming, at the same time, leads to the deeper (shallower) mixed layer on the northern (southern) side. This enhances the frontolysis, because deeper (shallower) mixed layer is less (more) sensitive to surface warming. In austral winter, stronger (weaker) surface cooling on the northern (southern) side contributes to the frontolysis. However, deeper (shallower) mixed layer is induced by stronger (weaker) surface cooling on the northern (southern) side and suppresses the frontolysis, because the deeper (shallower) mixed layer is less (more) sensitive to surface cooling. Therefore, the frontolysis by the NHF becomes stronger (weaker) through the mixed layer processes in austral summer (winter). The cause of the meridional gradient of mixed layer depth is estimated using diagnostic entrainment velocity and the Monin-Obukhov depth. Furthermore, the above mechanisms obtained from the observation are confirmed using outputs from a high-resolution coupled general circulation model. Causes of model biases are also discussed. [ABSTRACT FROM AUTHOR]

Located in the mid-latitudes, the Caspian Sea is the largest enclosed basin in the world. A fully-coupled atmosphere-ocean-wave model of the Caspian Sea at high resolution (8 km) for a period of three years is presented. After validating each component of the modelling platform, the wave state of the Caspian Sea is studied. Results show very different wave regimes between the three different basins, a strong seasonality and an almost swell-free state. It is shown here that waves modify the horizontal eddy viscosity and vertical heat diffusion. However, due to a reasonably weak annual wave state, these effects are restricted to the upper-ocean layer (< 30 m) except during the most severe events (100 m). Three main experiments are conducted: 1) the ROMS ocean model forced by atmospheric reanalysis (CFSR), 2) ROMS coupled with the atmospheric model WRF and 3) the impact of wave-induced processes. The seasonality of the Caspian Sea is accurately captured in each experiment which highlights a rapid warming of the sea surface temperature (SST) in spring while the mixed layer depths (MLD) become very rapidly shallow (shifting from over 100 m to 15 m in two months). Contrarily, a gentle cooling of the SST accompanied with a deepening of the MLD is modelled during autumn and winter. The results also show a significant improvement of the model skill in the representation of the dynamics when ROMS is coupled to WRF. Finally, as ocean surface waves imply feedback at the interface atmosphere-ocean through the transfer of momentum, mass and heat, we investigate their potential effects on the Caspian Sea dynamics. Results are mixed and show a reasonably weak impact of wave-induced processes. While waves have a negligible effect during the winter as wave-induced mixing is confined to the MLD, the summer global SST are less accurately modelled due to the enhancement of mixing in shallow MLDs. However the SST bias, temperature at a subsurface location are improved. [ABSTRACT FROM AUTHOR]

Phet super cyclone (31 May–7 June 2010) was the most intense and also the rarest of the rare track in Arabian Sea as per the recorded history during 1877–2009. The present study focuses on the ocean physical responses to Phet cyclone using satellite and Argo observations. The sea surface temperature is decreased to 6 °C with an approximately 350 km long and 100 km width area in the Arabian Sea after the cyclone passage. The translation speed of cyclone is 3.86 m/s, the mixed layer is 79 m, and thermocline displacement is 13 m at the cooling area. With the relationship of wind stress curl and Ekman pumping velocity (EPV), the author found that the speed of EPV was increased after the passage of cyclone. So the extent of the SST drop was probably due to the moving speed of cyclone and the depth of the mixed layer. [ABSTRACT FROM AUTHOR]

The effects of the mixing of wave transport flux residual (Bvl) on the upper ocean is studied through carrying out the control run (CR) and a series of sensitive runs (SR) with ROMS model. In this study, the important role of Bvl is revealed by comparing the ocean temperature, statistical analysis of errors and evaluating the mixed layer depth. It is shown that the overestimated SST is improved effectively when the wave-induced mixing is incorporated to the vertical mixing scheme. As can be seen from the vertical structure of temperature 28°C isotherm changes from 20 min CR to 35 m in SR3, which is more close to the observation. Statistic analysis shows that the root-mean-square errors of the temperature in 10 m are reduced and the correlation between model results and observation data are increased after considering the effect of Bvl. The numerical results of the ocean temperature show improvement in summer and in tropical zones in winter, especially in the strong current regions in summer. In August the mixed layer depth (MLD) which is defined as the depth that the temperature has changed 0.5°C from the reference depth of 10 m is further analyzed. The simulation results have a close relationship with undetermined coefficient of Bvl, sensitivity studies show that a coefficient about 0.1 is reasonable value in the model. [ABSTRACT FROM AUTHOR]

Based on 26 days of Chinese Sea-wing underwater glider measurements and satellite microwave data, we documented cooling of the upper mixed layer of the ocean in response to changes in the wind in the Northern South China Sea (NSCS) from September 19, 2014, to October 15, 2014. The Sea-wing underwater glider measured 177 profiles of temperature, salinity, and pressure within a 55 km×55 km area, and reached a depth of 1000 m at a temporal resolution of ∼4 h. The study area experienced two cooling events, Cooling I and Cooling II, according to their timing. During Cooling I, water temperature at 1-m depth ( T 1 ) decreased by ∼1.0 °C, and the corresponding satellite-derived surface winds increased locally by 4.2 m/s. During Cooling II, T 1 decreased sharply by 1.7 °C within a period of 4 days; sea surface winds increased by 7 m/s and covered the entire NSCS. The corresponding mixed layer depth (MLD) deepened sharply from 30 m to 60 m during Cooling II, and remained steady during Cooling I. We estimated temperature tendencies using a ML model. High resolution Sea-wing underwater glider measurements provided an estimation of MLD migration, allowing us to obtain the temporal entrainment rate of cool sub-thermocline water. Quantitative analysis confirmed that the entrainment rate and latent heat flux were the two major components that regulated cooling of the ML, and that the Ekman advection and sensible heat flux were small. [ABSTRACT FROM AUTHOR]

In this study a coupled air-sea-wave model system, containing the model components of GRAPES-TCM, ECOM-si and WAVEWATCH III, is established based on an air-sea coupled model. The changes of wave state and the effects of sea spray are both considered. Using the complex air-sea-wave model, a set of idealized simulations was applied to investigate the effects of air-sea-wave interaction in the upper ocean. Results show that air-wave coupling can strengthen tropical cyclones while air-sea coupling can weaken them; and air-sea-wave coupling is comparable to that of air-sea coupling, as the intensity is almost unchanged with the wave model coupled to the air-sea coupled model. The mixing by vertical advection is strengthened if the wave effect is considered, and causes much more obvious sea surface temperature (SST) decreases in the upper ocean in the air-sea coupled model. Air-wave coupling strengthens the air-sea heat exchange, while the thermodynamic coupling between the atmosphere and ocean weakens the air-sea heat exchange: the air-sea-wave coupling is the result of their balance. The wave field distribution characteristic is determined by the wind field. Experiments are also conducted to simulate ocean responses to different mixed layer depths. With increasing depth of the initial mixed layer, the decrease of SST weakens, but the temperature decrease of deeper layers is enhanced and the loss of heat in the upper ocean is increased. The significant wave height is larger when the initial mixed layer depth increases. [ABSTRACT FROM AUTHOR]

The primary effect of symmetric instability (SI) is the shoaling of the mixed layer. This has been shown to improve the vertical mixing of a passive tracer between the surface mixed layer and the ocean interior in several ideal models. In this study, we apply an SI parameterization scheme on a resolved front in a realistic, high-resolution global ocean model, and evaluate global-scale SI effects. We analyze the model outputs for two typical front regions: the Kuroshio Extension (KE) shows conditions favorable for SI generation whereas in the Subantarctic Front (SAF), east of Australia, SI is weak. We demonstrate that the SI parameterization has a detectable effect on the global-scale sea surface temperature (SST), mixed layer depth (MLD), especially in the strong front and current-topography interactions regions, in winter in the northern and southern hemispheres. The variation of MLD changes by 30% in a local region when SI parameterization scheme is considered. These spatial and temporal distribution characteristics are consistent with those expected for the SI mechanism. However, the SI parameterization only results in submesoscale positive and negative staggered patch structures in the MLD and SST, analogous to turbulence structures. Therefore, its global-scale direct impacts on the MLD and SST are not systematic. The submesoscale disturbance induced by the SI parameterization is of the same order of magnitude as the errors in the current SST observation data; as such, it is assumed to be statistically insignificant in the scope of the current study in a global model. The submesoscale MLD and SST disturbance induced by SI is probably important for local air-sea exchange, but this requires further investigation with the use of long-time-integral coupled models in the future. [ABSTRACT FROM AUTHOR]

We analyzed the marine environmental setting to characterize the mass nesting habitat of Olive ridley sea turtles from three locations of the river mouth region i.e. , Gahirmatha, Devi, and Rushikulya along the coast, of the Bay of Bengal. The environmental variables such as sea surface temperature (SST), ocean current, mixed layer depth, and chlorophyll- a (Chl-a) were analyzed to examine its influence on the mass nesting of sea turtles. The clear pattern of the mass population shows the nearshore and river mouth region of the nesting site throughout the inter-nesting range. The population was dense at the river mouth of Gahirmatha followed by Devi and Rushikulya. We found a significant linear relationship between the Chl-a and nesting population (r = 0.98, p ≤ 0.0001); and the SST and nesting population (r = 0.78, p ≤ 0.0001) at all river mouths. The conducive environmental threshold limit of SST and Chl-a was ≤29 0C and ≤ 0.5 mg m-3 respectively. The mass nesting, breeding, and migration of Olive ridley are attributed to the geomorphologic basin structure, physicochemical parameters, and feeding potential grounds in the coastal water. Inter nesting of movements in relation to the influential environment variables, the present and proposed regions for Olive ridley need to be implemented to provide comprehensive protection to both nesting populations and locations. The present study provides new knowledge on the linkage between the marine ecosystem and the mass nesting habitat of the sea turtle population for long-term monitoring, conservation application, and sustainable ecosystem management. • Olive ridley mass population shows at near shore and river mouth region of the nesting site throughout the inter-nesting range. • Mass nesting population was dense at the river mouth of Gahirmatha followed by Devi and Rushikulya. • Significant linear relationship shows between the Chl-a and nesting population; and between the SST and nesting population • Olive ridley conservation need to be implemented to provide comprehensive protection to both nesting populations and locations. [ABSTRACT FROM AUTHOR]

The physical evolution of water-column stratification following the implementation of widespread artificial upwelling across the North Pacific Subtropical Gyre (NPSG) is examined in a numerical experiment using the ocean general circulation model MIT-gcm. The upwelling process is represented by distributed pairs of fluid sinks and sources located at different depths to mimic the withdrawal of cool nutrient-rich water at about 400 m below the surface and its discharge in the upper water column. The overarching conclusion from the simulations is that within the NPSG, the upwelling process as modeled appears to enhance relatively strong stratification and to weaken relatively weak stratification. This generally holds for the short term as well as when a new steady-state is reached after a few decades. In the latter case, it applies temporally, with deeper mixed layers in the winter and shallower ones in the summer, as well as geographically with a broad separation between less stratified northern regions and more stratified southern regions within the NPSG. The global model allows an estimation of possible unintended consequences well outside the upwelling implementation area. It is predicted, for example, that surface cooling in the NPSG allows heat to penetrate the ocean interior until the overall oceanic surface heat flux is restored to zero (in a yearly averaged sense). As a result, changes in the thermal structure of the water column occur. Regions with a net surface warming develop outside the NPSG, most notably along the East coast of Japan and the West coasts of the Americas. [ABSTRACT FROM AUTHOR]

Argo float data supplemented with satellite measurements was used to study the seasonal and inter-annual variation in wind speed, sea surface temperature (SST) and mixed layer depth (MLD) of the southern Bay of Bengal from 2003 through 2010. Due to persistence of wind, clear sky and high insolation an increase in SST by about 2°C is evident during summer months (March-May) and is followed by shallowed MLD with a minimum depth of 9.3 m during summer 2004. MLD reached the maximum depth during monsoon season (November-December) and often extends to post monsoon (February) owing to strong monsoon wind, cloudy sky and SST plummeted by 3°C. During the inter-monsoon period (August-October) the MLD shallowed and maintained a depth of 20–30 m all through the study period. High wind accompanied with moderate temperature (SST) due to the south west monsoon leads to decreased MLD with an average depth of 44 m in July. Analysis of wind speed, SST and MLD suggested that out of various meteorological parameters wind speed and induced mixing are highly influential in MLD formation. Reduced occurrence and amplitude of MLD deepening noticed in recent years can be attributed to the evident climate change scenarios. Large scale upper ocean variability observed from the present study has innumerable antagonistic consequences on the marine ecosystem which is evident from various events of seagrass burns and coral bleaching which have occurred in the last decade. [ABSTRACT FROM AUTHOR]

This study evaluates the ability of the Hawaii Regional Navy Coastal Ocean Model to accurately predict the depth of the surface mixed layer in the lee of the Hawaiian Islands. Accurately modeling the depth of the surface mixed layer in this complex wake island environment is important to naval operations because the area hosts numerous training exercises. The simulated data were compared to CTD data collected from sea gliders, and tests for correlation were conducted. For mixed layer depths that did show correlation, match-paired t tests were used to determine the significance of the correlations. It was determined that the Hawaii Regional Navy Coastal Ocean Model has difficulty accurately predicting the depth of the surface mixed layer. It was also determined that the model has difficulty with unusual oceanographic features such as mode water eddies. These features are too uncommon and short-lived to be depicted in the climatology data. The climatology data are a major component of the synthetic profiles that the model generates, and these profiles tend to smooth out the unusual subsurface isothermal layer. List of Abbreviations BT - bathythermographs CCE - cold core eddy COAMPS - Coupled Ocean/Atmosphere Mesoscale Prediction System CTD - conductivity, temperature, and depth GDEM - Generalized Digital Environmental Model IR - infrared MLD - mixed layer depth MODAS - Modular Ocean Data Assimilation System MOODS - Master Oceanographic Observation Dataset NCODA - Navy Coupled Ocean Data Assimilation NC0M1 - Hawaii Regional Navy Coastal Ocean Model with in s/ia assimilation NC0M2- Hawaii Regional Navy Coastal Ocean Model without in situ assimilation PAVE - Profile Analysis and Visualization Environment SSHa - sea surface height anomaly derived from altimetry SST - sea surface temperature WCE - warm core eddy [ABSTRACT FROM AUTHOR]

Abstract: This paper is devoted to a study on the effects of different parameterisations of vertical turbulence – with a 3D hydrodynamic model COHERENS – on the accuracy of calculated temperature and salinity fields in a hydrodynamically complex test area – the Baltic Sea, Gulf of Finland. Two algebraic parameterisations and k–ε and k-models were used. For k-model four different sets of stability functions were used. Calculated vertical profiles of temperature and salinity were compared against CTD-profiles collected during a measurement campaign in the Gulf of Finland in summer 1996. The dataset has an outstanding spatial and temporal coverage including over 300 measured CTD profiles. The thermocline depth was underestimated throughout summer by all the vertical turbulence schemes. The selection of stability functions had significant effect on the accuracy of the k-model. Generally k-model performed better when the limiting conditions for mixing length were not applied. The k-model with stability functions based on the Munk–Anderson relation without limiting condition for mixing length showed best accuracy in the calculated profiles of temperature and in the thermocline depth. The improvement of the meteorological forcing had an impact on the exactness of the calculated thermocline depth. However, sensitivity tests showed that this impact was relatively small. Generally, calculated salinity was overestimated in the surface layer and underestimated in the bottom layers. Algebraic parameterisations had highest accuracy in the vertical salinity profiles. In the eastern Gulf of Finland the calculated values of salinity were overestimated. The accuracy of initial conditions, river runoff and bathymetry had significant effect on the accuracy of calculated salinity fields. [Copyright &y& Elsevier]

The South China Sea is an oligotrophic marginal sea located in the tropical-subtropical Northwestern Pacific Ocean. Under the influences of monsoon winds, both the physical and biogeochemical processes exhibit distinct seasonal variability in the upper waters. In order to study the seasonal variations of surface phytoplankton, a one-dimensional coupled physical-biogeochemical model was developed and applied to the deep basins of the Northern South China Sea, away from the coastal upwelling regions. Forced under real-time surface monsoon winds and heat flux, the model reproduced the mixed layer depth, sea surface temperature and surface chlorophyll-a compared with satellite observations and previous reported values. In seasonal mean, the mixed layer depth was highest in winter (∼61.62 m) and lowest in spring (∼12.07 m). The sea surface temperature was lowest (∼25.05°C) in winter and highest (∼29.20°C) in summer. Furthermore, conspicuous phytoplankton blooms occurred in winter with the highest chlorophyll-a concentration up to ∼0.21 mg m−3. In other seasons, the concentration remained relatively low, especially in summer (∼0.05 mg m−3). The spatial distributions of phytoplankton were closely related with patterns of surface nutrient availability, as well as mixed layer depth and sea surface temperature. These relationships indicate that surface phytoplankton primary production was mainly controlled by nutrient availability, which was dominated by vertical turbulent diffusion in the deep basin of the northern Sea which is away from the coastal upwelling regions. Overall, our model results indicated that the seasonal variability of surface phytoplankton was modulated by coupled effects of physical and biogeochemical processes in the Northern South China Sea. [ABSTRACT FROM AUTHOR]

It is investigated how the changes of winter sea surface temperature (SST) and mixed layer depth (MLD) under climate change projections are predicted differently in the North Pacific depending on the coupled general circulation models (CGCMs), and how they are related to the dynamical property of the simulated ocean mixed layer. For this purpose the dataset from eleven CGCMs reported to IPCC's AR4 are used, while detailed analysis is given to the MRI and MIROC models. Analysis of the CGCM data reveals that the increase of SST and the decrease of MLD in response to global warming tend to be smaller for the CGCM in which the ratio of ocean heat transport (OHT) to surface heat flux (SHF), R (=|OHT/SHF|), is larger in the heat budget of the mixed layer. The negative correlation is found between the changes of OHT and SHF under global warming, which may weaken the response to global warming in the CGCM with larger R. It is also found that the models with low horizontal resolution tend to give broader western boundary currents, larger R, and the smaller changes of SST and MLD under global warming. [ABSTRACT FROM AUTHOR]

Based on the satellite observed sea surface temperature (SST), the recovery of SST cooling induced by the tropical cyclones (TCs) over the northwestern Pacific Ocean is investigated. The results show that the passage of a TC induces a mean maximum cooling in the SST of roughly −1.25 °C. It was also found that most of this cooling (~87%) is typically erased within 30 days of TC passage. This recovery time depends upon the degree of cooling, with stronger (weaker) SST cooling corresponding to longer (shorter) recovery time. Further analyses show that the mixed layer depth (MLD) and the upper layer thermocline temperature gradient (UTTG) also play an important role in the SST response to TCs. The maximum cooling increases ~0.1 °C for every 7 m decrease in the MLD or every 0.04 °C/m increase in the UTTG. The combined effects of MLD and TC intensity and translation speed on the SST response are also discussed. [ABSTRACT FROM AUTHOR]

Taking advantage of 37-year-long (1982–2018) of high-quality satellite datasets, we examined the role of direct atmospheric forcing on the high and low sea surface temperature (SST) extremes over the Red Sea (RS). Considering the importance of SST in regulating ocean physics and biology, the associated impacts on chlorophyll (Chl-a) concentration were also explored, since a small change in SST can cause a significant impact in the ocean. After describing the climate features, we classified the top 5% of SST values (≥31.5 °C) as extreme high events (EHEs) during the boreal summer period and the lowest SST values (≤22.8 °C) as extreme low events (ELEs) during the boreal winter period. The spatiotemporal analysis showed that the EHEs (ELEs) were observed over the southern (northern) basin, with a significant warming trend of 0.027 (0.021) °C year−1, respectively. The EHEs were observed when there was widespread less than average sea level pressure (SLP) over southern Europe, northeast Africa, and Middle East, including in the RS, leading to the cold wind stress from Europe being relatively less than usual and the intrusion of stronger than usual relatively warm air mass from central Sudan throughout the Tokar Gap. Conversely, EHEs were observed when above average SLP prevailed over southern Europe and the Mediterranean Sea as a result of the Azores high and westward extension of the Siberian anticyclone, which led to above average transfer of cold and dry wind stress from higher latitudes. At the same time, notably less wind stress due to southerlies that transfer warm and humid air masses northward was observed. Furthermore, physical and biological responses related to extreme stress showed distinct ocean patterns associated with each event. It was found that the Chl-a concentration anomalies over the northern basin caused by vertical nutrient transport through deep upwelling processes are the manifestation of the superimposition of ELEs. The situation was the opposite for EHEs due to the stably stratified ocean boundary layer, which is a well-known consequence of global warming. [ABSTRACT FROM AUTHOR]

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

Biogenic dimethylsulfide (DMS) sea-to-air flux and its closely linked environment factors are studied in the East Marginal Seas of China (EMSC) (25°N-40°N, 120°E-130°E) from 2011 to 2015. Higher chlorophyll-a (CHL) up north during blooming season is due to the southeasterly wind brought more nutrients from the Changjiang River waters. The elevated CHL during autumn and winter periods down south is more related with the higher sea surface temperature (SST). The lower CHL in the middle region in the later half year was mainly influenced by terrestrial runoff and the diluted Changjiang River waters. The enriched Changjiang River discharges brought more nutrients to the Zhejiang coastal waters and enriched the local phytoplankton, where aerosol optical depth (AOD) had the highest correlation with CHL (0.96). Peak SST was in August and had a negative correlation with wind speed (WIND). With increased WIND up north, AOD reduced accordingly. Lower CHL in 2015 was mainly related to the southeastern wind brought less nutrient water from Open Sea during blooming season. DMS sea-to-air fluxes are calculated and compared with OUC (Ocean University of China) field data. The reasonable agreements are made. DMS concentrations ranged from 0.32 to 5.64 nM with a mean of 2.19 nM, it had high positive correlations with CHL in the summer and autumn periods. DMS sea-to-air fluxes ranged within 2.14–59.06 μmolm−2d−1 with a mean of 18.96 μmolm−2d−1. The elevated DMS sea-to-air fluxes in ECMS indicate the contributions of DMS emission in the study region cannot be ignored on the global climate evaluation. • Chlorophyll distributions are higher up north and lower down south in the East Marginal Seas of China. • Variation of Sea Surface Temperature, Wind, Mixing and Aerosol had great impact on the Phytoplankton biomass. • Changjiang River discharge input had positive and negative impact on the local phytoplankton. • DMS and DMS flux had reasonable agreement with field data. • Elevated DMS fluxes located in east coast have impact on local climate. [ABSTRACT FROM AUTHOR]

The mixed layer depth (MLD) is generally estimated using in situ or model data. However, MLD analyses have limitations due to the sparse resolution of the observed data. Therefore, this study reconstructs three-dimensional (3D) ocean thermal structures using only satellite sea surface measurements for a higher spatial and longer temporal resolution than that of Argo and diagnoses the decadal variation of global MLD variability. To simulate the ocean thermal structures, the relationship between the ocean subsurface temperature and the sea surface fields was computed based on gridded Argo data. Based on this relationship, high spatial resolution and extended periods of satellite-derived altimeter, sea surface temperature (SST), and wind stress data were used to estimate the 3D ocean thermal structures with 0.25° spatial resolution and 26 standard depth levels (5–2000 m) for 24 years (1993–2016). Then, the MLD was calculated using a temperature threshold method (∆T = 0.2 °C) and correlated reasonably well (>0.9) with other MLD datasets. The extended 24-year data enabled us to analyze the decadal variability of the MLD. The global linear trend of the 24-year MLD is −0.110 m yr−1; however, from 1998 to 2012, the linear trend is −0.003 m yr−1 which is an order of magnitude smaller than that of other periods and corresponds to a global warming hiatus period. Via comparisons between the trends of the SST anomalies and the MLD anomalies, we tracked how the MLD trend changes in response to the global warming hiatus. [ABSTRACT FROM AUTHOR]