Deeper Mixed Layer Depth Research Articles

Improving low-resolution models via parameterisation of the effect of submesoscale vertical advection on temperature: A case study in the East China Sea

Horizontal resolutions of regional ocean models are often inadequate to resolve submesoscale vertical advection (SVA) in the continental shelf seas. This study aims to compare the SVA in two similar model setups in the East China Sea with different horizontal resolutions. A novel method to parameterise the impact of SVA on temperature (SVA-T) has also been proposed, which can significantly improve the low-resolution model (LRM) results. In particular, we focus on the time frame wherein this method can be applied. The suggested method primarily improves the representation of an internal warming process, which is significant in both the observations and high-resolution model (HRM) but is absent in the original LRM. Combined with the observations, we found that the internal warming, which occurred in the barrier layer of the water column during early autumn, is accompanied by submesoscale oscillations of isothermals in the HRM. SVA dominates the internal warming process and increases the total vertical advection by up to an order of magnitude in the HRM compared with the LRM. Therefore, we parameterised SVA-T by increasing the vertical thermal diffusion coefficient in the LRM.To further specify the time frame wherein the parameterisation can be applied, we introduced two empirical conditions related to the major generation mechanisms of frontogenesis, mixed-layer instabilities and turbulent thermal wind in the mixed layer: a deep mixed-layer depth >20 m and a large surface-bottom temperature difference (∆T >2 °C). Finally, we successfully simulated the internal warming in the LRM during early autumn by applying the proposed method. Nevertheless, the parameterisation of the horizontal distribution of SVA-T needs to be considered in the future to achieve better three-dimensional model results.

A comparison of Langmuir turbulence parameterizations and key wave effects in a numerical model of the North Atlantic and Arctic Oceans

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.

Relationship of mixed layer depth and sea surface temperature onlongwave radiation flux up in the west of Sumatra waters

Thermal energywhich stored in upper ocean mixed layer will be released in the form oflongwave radiation flux up (LWUp). LW Up is a longwavethat emitted from sea surface to atmosphere. Variability of upper ocean mixed layerhas influence on ocean thermal capacity and then affected LW Up variation. The areaof interest of this research was west of Sumatra waters. Vertical temperature profiles, sea surface temperature (SST) and LW Updata from 2000 to 2015 were used. Mixed layer depth (MLD) was computedbyThreshold Method based on temperature profiles with 0.31°Ctemperature criterion (ΔT). The results show high LW Up flux occur during shallow MLD and low LW Up occur when MLD become deeper. Correlation between SST and LW Up give a fine value about 0.9 in all station. Good correlation coefficient between MLD and LW Up found in station 1 (-0.81) and 9 (-0.77) with 1 month lag. This results suggest that shallow (deep) MLD followed by high (low) SST so that the result is increase (decrease) LW Up by 1 month lag. Small correlation between MLD and LW Up found in area near equator region. Indian Ocean circulation, westerly wind, and Wyrtki jet are possibly causing the small correlation in equator region.

The spatiotemporal features of submesoscale processes in the northeastern South China Sea

The spatiotemporal features of submesoscale processes (SMPs) in the northeastern South China Sea (SCS) are analyzed based on a high-resolution simulation from 2009 to 2012. The simulation results show that the SMPs with a vertical relative vorticity that matches the local planetary vorticity are ubiquitous in the upper ocean of the northeastern SCS. The SMPs distribution shows an asymmetry due to centrifugal instability, with stronger positive vorticity than negative vorticity. Meanwhile, the SMPs demonstrate an obvious seasonal variation. The SMPs are strong and active in winter but weak and inactive in summer. An investigation of the SMPs generation mechanisms reveals that flow straining and mixed layer depth account for this seasonal variation. The strong flow straining and deep mixed layer depth in winter favor the SMP generation via frontogenesis and mixed layer instability.

Argo Observations of the Deep Mixing Band in the Southern Ocean: A Salinity Modeling Challenge

AbstractThe Southern Ocean plays an important role in mediating oceanic uptake of CO2 and heat due to a strong meridional overturning circulation. Gridded Argo float data for 2004–2017 were used to evaluate subsurface processes at the mixed layer depth (MLD) that occur in a narrow deep mixing band. Shifts in the value of the Turner Angle at the MLD indicate that early in the season the MLD deepens slowly as it encounters and is stabilized by a subsurface salt maximum. By September mixing has penetrated this salinity feature and the rate of deepening is faster once the MLD is deeper than the depth where the maximum salinity occurs (~150–200 m). This distinctive salinity layer is the result of surface Ekman transport of fresh water from the south and subsurface advection of high‐salinity water from the north. Two configurations of the Community Earth System Model (CESM) ocean‐ice forced hindcast experiments—one with 1° and the other with 0.1° horizontal resolution (Parallel Ocean Program low and high resolutions [POP‐LR and POP‐HR], respectively)—are compared with the Argo data for 2005–2009. POP‐LR has a shallow MLD bias common to many Fifth Coupled Models Intercomparison Project (CMIP5) models, while POP‐HR has a mix of deep and shallow MLD biases. While both models were able to replicate the large‐scale processes leading to formation of a high‐salinity layer, the salinity feature in POP‐HR is too strong and deep. Neither model was able to replicate the vertical mixing processes leading to penetration of the subsurface salt maximum.

Mixed layer depth variability in the Red Sea

Abstract. For the first time, a monthly climatology of mixed layer depth (MLD) in the Red Sea has been derived based on temperature profiles. The general pattern of MLD variability is clearly visible in the Red Sea, with deep MLDs during winter and shallow MLDs during summer. Transitional MLDs have been found during the spring and fall. The northern end of the Red Sea experienced deeper mixing and a higher MLD associated with the winter cooling of the high-saline surface waters. Further, the region north of 19° N experienced deep mixed layers, regardless of the season. Wind stress plays a major role in the MLD variability of the southern Red Sea, while net heat flux and evaporation are the dominating factors in the central and northern Red Sea regions. Ocean eddies and Tokar Gap winds significantly alter the MLD structure in the Red Sea. The dynamics associated with the Tokar Gap winds leads to a difference of more than 20 m in the average MLD between the north and south of the Tokar axis.

The roles of mixed layer depth in affecting chemical and biological characteristics of Makassar straits, Indonesia

Mixed Layer Depth (MLD) is able to determine the amount of light and nutrients abundance for the growth of phytoplankton, its known that phytoplakton pigments are the main absorbers and light scatterers in the ocean. Location wich has deeper MLD and large seasonal variations are the most productive areas, also could have blooming or the maximum growth of large phytoplankton occurring in the existence of a limited number nutrients for several days. The aims of this study are to understand the characteristics of the MLD and it roles to the abundance of Chlorophyll and nutrient in Indonesia Through-Flow/ITF (Indonesia: “ARLINDO”), specifically the area along Makassar Strait. The Chlorophyll and oceanographic parameters were obtained based on measuring by CTD in TIMIT 2016 expedition. The MLD was deep (~43 m) in the northern part of the transect (around Central Sulawesi Island) and shoaled progressively until about 13 m in the southern transect (the Southern Makassar Strait). The Chlorophyll-a (Chl-a) concentration show a decline pattern from the north to the south transects with the maximal Chl-a is 0.37 mg/m3. Nutrient parameters in southern Makassar Strait studied were NODC (National Oceanographic Data Center) data from WOA (World Ocean Atlas), it can indicated a correlation between nutrient (Nitrate) with Chlorophyll. The mean correlation in mixing layer for whole area is 0.78, showing that Nitrate concentration is highly related proportional to Chlorophyll abundance, while below the mixing layer, correlation value is -0.52, indicating the inverse relation between Chlorophyll and nutrient. Based on those correlation, and assumption that the source of Chlorophyll is from phytoplankton, it can be concluded that phytoplankton needs Nitrate in its metabolism process as well as other productivities reason in mixing layer, while in depth below the mixing layer, growth of phytoplankton is no longer depends on concentration of Nitrate nutrient.

Oceanographic factors affecting interannual recruitment variability of Pacific saury (Cololabis saira) in the central and western North Pacific

AbstractPacific saury (Cololabis saira) has a short life span of 2 years and tends to exhibit marked population fluctuations. To examine the importance of sea surface temperature (SST) and mixed layer depth (MLD) as oceanographic factors for interannual variability of saury recruitment in early life history, we analyzed the relationship between abundance index (survey CPUE (catch per unit of effort)) of age‐1 fish and the oceanographic factors in the spawning and nursery grounds of the previous year when they were born, for the period of 1979–2006, in the central and western North Pacific. Applying the mixture of two linear regression models, the variability in the survey CPUE was positively correlated with previous year's winter SST in the Kuroshio Recirculation region (KR) throughout the survey period except 1994–2002. In contrast, the survey CPUE was positively correlated with the previous year's spring MLD (a proxy of spring chlorophyll a (Chl‐a) concentration) in the Kuroshio‐Oyashio Transition and Kuroshio Extension (TKE) during 1994–2002. This period is characterized by unusually deep spring MLD during 1994–1997 and anomalous climate conditions during 1998–2002. We suggest that saury recruitment variability was generally driven by the winter SST in the KR (winter spawning/nursery ground), or by the spring Chl‐a concentration (a proxy of prey for saury larvae) in the TKE (spring spawning/nursery ground). These oceanographic factors could be potentially useful to predict abundance trends of age‐1 saury in the future if the conditions leading to the switch between SST and MLD as the key input variable are elucidated further.

Delineation of marine ecosystem zones in the northern Arabian Sea during winter

Abstract. The spatial and temporal variability of marine autotrophic abundance, expressed as chlorophyll concentration, is monitored from space and used to delineate the surface signature of marine ecosystem zones with distinct optical characteristics. An objective zoning method is presented and applied to satellite-derived Chlorophyll a (Chl a) data from the northern Arabian Sea (50–75∘ E and 15–30∘ N) during the winter months (November–March). Principal component analysis (PCA) and cluster analysis (CA) were used to statistically delineate the Chl a into zones with similar surface distribution patterns and temporal variability. The PCA identifies principal components of variability and the CA splits these into zones based on similar characteristics. Based on the temporal variability of the Chl a pattern within the study area, the statistical clustering revealed six distinct ecological zones. The obtained zones are related to the Longhurst provinces to evaluate how these compared to established ecological provinces. The Chl a variability within each zone was then compared with the variability of oceanic and atmospheric properties viz. mixed-layer depth (MLD), wind speed, sea-surface temperature (SST), photosynthetically active radiation (PAR), nitrate and dust optical thickness (DOT) as an indication of atmospheric input of iron to the ocean. The analysis showed that in all zones, peak values of Chl a coincided with low SST and deep MLD. The rate of decrease in SST and the deepening of MLD are observed to trigger the algae bloom events in the first four zones. Lagged cross-correlation analysis shows that peak Chl a follows peak MLD and SST minima. The MLD time lag is shorter than the SST lag by 8 days, indicating that the cool surface conditions might have enhanced mixing, leading to increased primary production in the study area. An analysis of monthly climatological nitrate values showed increased concentrations associated with the deepening of the mixed layer. The input of iron seems to be important in both the open-ocean and coastal areas of the northern and north-western parts of the northern Arabian Sea, where the seasonal variability of the Chl a pattern closely follows the variability of iron deposition.

Variability of the mixed layer depth and the ocean surface properties in the Cape Ghir region, Morocco for the period 2002–2014

The mixed layer depth (MLD) is an active part of the marine environment that couples the underlying ocean to the atmosphere. The aim of this original study in Morocco is to investigate for the period 2002–2014 the relationship between the variability of the MLD with the oceanic sea surface properties and the primary productivity in the Cap Ghir region. This area is very productive along the Atlantic coast of Morocco and we examined in this study the monthly variability of the MLD and its relationship with sea surface properties using data from Copernicus—Marine environment monitoring products, coupling between MLD with SST and upwelling activity in the Cape Ghir area. During winter seasons, the MLD is deeper and observed at around 90 m. Compared to others seasons, it is varying between 10 and 25 m. The monthly mean SST show very cold temperature (around 17 °C) during winter season and a warm temperature (around 23 °C), during summer season. In fact, the colder waters in surface coincide with the deepest MLD and the warmer waters coincide with the shallowest MLD. From 2009 to 2011, the MLD was very shallower (40 m) with some observed variability between 30°30N and 31°N, due to the dynamic of Cape Ghir. Regarding to the upwelling activity, the upwelling index shows a clear seasonality (higher activity in summer and weaker in winters) and some relationship between the upwelling index and the MLD were investigated. The characteristic’s feature of the variability of the MLD mixed layer in the Cape Ghir area is based on the fast response of the upwelling activity than the other parameters like SST. When the MLD is shallower, the SST is still cooler for two months more. The oceanographic dynamic of the Cape Ghir area is very complex in nature and, following the MLD, this parameter is an adequate parameter for detecting the activity of the upwelling in this area.

Seasonal variation of the global mixed layer depth: comparison between Argo data and FIO-ESM

The present study evaluates a simulation of the global ocean mixed layer depth (MLD) using the First Institute of Oceanography-Earth System Model (FIOESM). The seasonal variation of the global MLD from the FIO-ESM simulation is compared to Argo observational data. The Argo data show that the global ocean MLD has a strong seasonal variation with a deep MLD in winter and a shallow MLD in summer, while the spring and fall seasons act as transitional periods. Overall, the FIO-ESM simulation accurately captures the seasonal variation in MLD in most areas. It exhibits a better performance during summer and fall than during winter and spring. The simulated MLD in the Southern Hemisphere is much closer to observations than that in the Northern Hemisphere. In general, the simulated MLD over the South Atlantic Ocean matches the observation best among the six areas. Additionally, the model slightly underestimates the MLD in parts of the North Atlantic Ocean, and slightly overestimates the MLD over the other ocean basins.

North and equatorial Pacific Ocean circulation in the CORE-II hindcast simulations

We evaluate the mean circulation patterns, water mass distributions, and tropical dynamics of the North and Equatorial Pacific Ocean based on a suite of global ocean-sea ice simulations driven by the CORE-II atmospheric forcing from 1963-2007. The first three moments (mean, standard deviation and skewness) of sea surface height and surface temperature variability are assessed against observations. Large discrepancies are found in the variance and skewness of sea surface height and in the skewness of sea surface temperature. Comparing with the observation, most models underestimate the Kuroshio transport in the Asian Marginal seas due to the missing influence of the unresolved western boundary current and meso-scale eddies. In terms of the Mixed Layer Depths (MLDs) in the North Pacific, the two observed maxima associated with Subtropical Mode Water and Central Mode Water formation coalesce into a large pool of deep MLDs in all participating models, but another local maximum associated with the formation of Eastern Subtropical Mode Water can be found in all models with different magnitudes. The main model bias of deep MLDs results from excessive Subtropical Mode Water formation due to inaccurate representation of the Kuroshio separation and of the associated excessively warm and salty Kuroshio water. Further water mass analysis shows that the North Pacific Intermediate Water can penetrate southward in most models, but its distribution greatly varies among models depending not only on grid resolution and vertical coordinate but also on the model dynamics. All simulations show overall similar large scale tropical current system, but with differences in the structures of the Equatorial Undercurrent. We also confirm the key role of the meridional gradient of the wind stress curl in driving the equatorial transport, leading to a generally weak North Equatorial Counter Current in all models due to inaccurate CORE-II equatorial wind fields. Most models show a larger interior transport of Pacific subtropical cells than the observation due to the overestimated transport in the Northern Hemisphere likely resulting from the deep pycnocline.

Coupled 1-D physical–biological model study of phytoplankton production at two contrasting time-series stations in the western North Pacific

A vertical one-dimensional physical–biological model is applied to clarify the mechanisms controlling the seasonality and interannual variability of primary production in the surface layer at two contrasting time-series stations, K2 (in the subarctic gyre) and S1 (in the subtropical gyre), in the western North Pacific. Using forcing based on realistic atmospheric and oceanic data, the model reproduces seasonal differences in the degree to which different controlling factors affect primary production between these two stations, primarily as a result of differences in the physical environment. At station K2, light intensity is an important factor controlling primary production in summer. After April, the mixed layer depth (MLD) becomes shallow, resulting in higher average light intensity, and the water column remains stratified until September; these sustain high primary production during this period. In contrast, at station S1, the supply of nutrients via entrainment is vital to sustaining production, because light intensity remains sufficient throughout the year. In summer, the relationship between nutricline depth and euphotic layer is a controlling factor. The simulations forced by the different atmospheric conditions for each year, respectively, show different MLD. In the 2012 simulation, the deep winter MLD (200 m) enhances primary production in the surface layer as compared to the other two years (2010 and 2011) simulations.

A study of Indian Ocean Subtropical Mode Water: subduction rate and water characteristics

The annual subduction rate in the South Indian Ocean was calculated by analyzing Simple Ocean Data Assimilation (SODA) outputs in the period of 1950–2008. The subduction rate census for potential density classes showed a peak corresponding to Indian Ocean subtropical mode water (IOSTMW) in the southwestern part of the South Indian Ocean subtropical gyre. The deeper mixed layer depth, the sharper mixed-layer fronts and the associated relatively faster circulation in the present climatology resulted in a larger lateral induction, which primarily dominants the IOSTMW subduction rate, while with only minor contribution from vertical pumping. Without loss of generality, through careful analysis of the water characteristics in the layer of minimum vertical temperature gradient (LMVTG), the authors suggest that the IOSTMW was identified as a thermostad, with a lateral minimum of low potential vorticity (PV, less than 200×10–12 m–1·s–1) and a low dT⁄dz (less than 1.5°C/(100 m)). The IOSTMW within the South Indian Ocean subtropical gyre distributed in the region approximately from 25° to 50° E and from 30° to 39°S. Additionally, the average characteristics (temperature, salinity, potential density) of the mode water were estimated about (16.38 ± 0.29)°C, (35.46 ± 0.04), (26.02 ± 0.04) σ θ over the past 60 years.

Impact of different solar penetration depths on climate simulations

Three different estimates of shortwave attenuation depth (SWAD) of photosynthetically active radiation (PAR) derived from remotely sensed ocean colour data have been tested in an ocean general circulation model (OGCM) forced with interannual atmospheric forcings. Two estimates (referred to as and ) are calculated from different algorithms based on the diffusive attenuation coefficient at 490 nm and the third one () is just an average of and . is larger than almost everywhere in the tropical oceans. Our results show that the OGCM with produces warmer sea surface temperature (SST) in the eastern equatorial Pacific and Atlantic and leads to reduce a cold bias in the equatorial cold tongue regions. It has warmer subsurface temperatures in the low latitude, a slower meridional velocity and Pacific equatorial undercurrent (EUC) than the model with . These results are similar to previous studies, although we use a different model and different methods. This study has further analysis and firstly reveals that slower EUC and meridional velocity in the model with are mainly related to the changes of the acceleration due to zonal density gradient. This acceleration driving the EUC eastward in the subsurface becomes smaller in the subsurface along the equatorial Pacific. However, near the sea surface, the zonally averaged accelerations over the different ocean basins are larger in the model with than that with , which pushes back the poleward meridional transport. The interannual variability in the model with is generally weaker than that in the experiment with due to a deeper mixed layer depth. The vertical temperature errors averaged horizontally within the domain of 30°S to 30°N in the experiment with are almost in the middle of errors of the other two experiments. This indicates that the effect of the SWAD on the simulation of the vertical temperature profile is largely linear.

An upper ocean response to Typhoon Bolaven analyzed with Argo profiling floats

In situ observations from Argo profiling floats combined with satellite retrieved SST and rain rate are used to investigate an upper ocean response to Typhoon Bolaven from 20 through 29 August 2012. After the passage of Typhoon Bolaven, the deepening of mixed layer depth (MLD), and the cooling of mixed layer temperature (MLT) were observed. The changes in mixed layer salinity (MLS) showed an equivalent number of increasing and decreasing because the typhoon-induced salinity changes in the mixed layer were influenced by precipitation, evaporation, turbulent mixing and upwelling of thermocline water. The deepening of the MLD and the cooling of the MLT indicated a significant rightward bias, whereas the MLS was freshened to the left side of the typhoon track and increased on the other side. Intensive temperature and salinity profiles observed by Iridium floats make it possible to view response processes in the upper ocean after the passage of a typhoon. The cooling in the near-surface and the warming in the subsurface were observed by two Iridium floats located to the left side of the cyclonic track during the development stage of the storm, beyond the radius of maximum winds relative to the typhoon center. Water salinity increases at the base of the mixed layer and the top of the thermocline were the most obvious change observed by those two floats. On the right side of the track and near the typhoon center when the typhoon was intensified, the significant cooling from sea surface to a depth of 200×104 Pa, with the exception of the water at the top of the thermocline, was observed by the other Iridium float. Owing to the enhanced upwelling near the typhoon center, the water salinity in the near-surface increased noticeably. The heat pumping from the mixed layer into the thermocline induced by downwelling and the upwelling induced by the positive wind stress curl are the main causes for the different temperature and salinity variations on the different sides of the track. It seems that more time is required for the anomalies in the subsurface to be restored to pretyphoon conditions than for the anomalies in the mixed layer.

Biological response to physical processes in the Indian Ocean sector of the Southern Ocean: a case study in the coastal and oceanic waters.

The spatial variation of chlorophyll a (Chl a) and factors influencing the high Chl a were studied during austral summer based on the physical and biogeochemical parameters collected near the coastal waters of Antarctica in 2010 and a zonal section along 60°S in 2011. In the coastal waters, high Chl a (>3mgm(-3)) was observed near the upper layers (∼15m) between 53°30'E and 54°30'E. A comparatively higher mesozooplankton biomass (53.33ml 100m(-3)) was also observed concordant with the elevated Chl a. Low saline water formed by melting of glacial ice and snow, as well as deep mixed-layer depth (60m) due to strong wind (>11ms(-1)) could be the dominant factors for this biological response. In the open ocean, moderately high surface Chl a was observed (>0.6mgm(-3)) between 47°E and 50°E along with a Deep Chlorophyll Maximum of ∼1mgm(-3) present at 30-40m depth. Melt water advected from the Antarctic continent could be the prime reason for this high Chl a. The mesozooplankton biomass (22.76ml 100m(-3)) observed in the open ocean was comparatively lower than that in the coastal waters. Physical factors such as melting, advection of melt water from Antarctic continent, water masses and wind-induced vertical mixing may be the possible reasons that led to the increase in phytoplankton biomass (Chl a).

Mixed layer thermodynamics of the Southern South China Sea

Seasonal and inter-annual variability of the mixed layer temperature in the Southern South China Sea (SSCS) is investigated using a regional ocean circulation model simulation. The mixed layer depth (MLD) over the SSCS exhibits a strong seasonal signal with deeper MLDs during the northeast and southwest monsoons. The main factor that drives the mixed layer temperature variation in the SSCS is the air-sea heat fluxes, with vertical ocean processes acting as a relatively weak negative feedback. In general, the budget analysis demonstrates a net balance between the vertical ocean processes and surface heat flux during the pre-monsoon and southwest monsoon. Northeast monsoon period is noted by an offsetting of surface heat flux, horizontal and vertical ocean processes. The first dominant mode of mixed layer temperature inter-annual variability in the SSCS shows significant correlation (0.34) with the El Nino phenomenon in the Pacific Ocean and is best correlated (0.67) with a lag of 5 months.

Comparison and implication of TEX86 and U37K' temperature records over the last 356 kyr of ODP Site 1147 from the northern South China Sea

Proxy records from the South China Sea (SCS) have played an important role in understanding late Quaternary SCS environmental changes and the East Asian Monsoon (EAM) variations, but few long sea surface temperature (SST) records have been generated from the SCS. The recently established TEX86 index provides an opportunity to generate long SST records, especially for tropical oceans, but the applicability and climatic interpretation of the TEX86 index for the SCS need further study. We report and compare orbital scale resolution (ca. 5.3kyr) TEX86 and U37K' temperature records of the past 356kyr for ODP Site 1147 from the northern SCS. Both records display clear and similar glacial–interglacial patterns. The U37K' SST fluctuated between 22.3°C and 28.3°C, with a glacial–interglacial difference of 4–5°C; while the TEX86 temperature fluctuated between 20.3°C and 25.4°C, with a slightly smaller glacial–interglacial difference of ca. 4°C. The TEX86 temperature has been consistently lower than the U37K' temperature over the past 356kyr, consistent with previous studies suggesting that the U37K' index is a proxy for SST but the TEX86 index is a proxy for subsurface temperature. Thus, it is proposed that the temperature difference (ΔT) between the U37K' and TEX86 records reflects mixed layer depth (MLD) changes in the northern SCS, which fluctuated between 0.3 and 4.0°C with high values mostly during interglacials and lower values mostly during glacial stages. MLD changes in the northern SCS are interpreted to be driven by EAM changes, with stronger winter monsoon resulting in a deeper MLD and a smaller ΔT. Thus, the combination of U37K' and TEX86 records and their ΔT can be used to reconstruct East Asian winter monsoon variations, as well as their influences on the hydrographic structure of the SCS.

Linking synoptic forcing and local mesoscale processes with biological dynamics off Ningaloo Reef

AbstractA hydrographic survey offshore Ningaloo reef, north‐west Australia, in austral autumn 2010 revealed relatively stable subsurface water masses in the region, despite the influence of interannual variability of the Leeuwin Current (LC). The surface water mass seems slightly more variable at seasonal time scale, probably due to various contributions of the geographically distinct source waters of the LC. A subsurface nitrate maximum (~110–230 m), a prominent feature of the Ningaloo area during autumn, was consistently observed within different water masses. Tightly coupled variations of subsurface nitrate and oxygen at small vertical scale suggest it is due to local in situ remineralization of organic matter likely to accumulate along sharp physical interfaces and possibly favored by injections of oxygenated subsurface waters. Offshore, enhanced levels of surface chlorophyll a within the downwelling‐favorable LC are associated with deeper mixed layer depth, eroding the shallow source of nutrients. Close to the continental shelf, these nutrients are observed to be efficiently uplifted within the core of quasi‐persistent topographically trapped submesoscale cyclonic eddies dominated by nonlinear effects. A wind‐driven coastal upwelling event occurred in autumn and was characterized by a relatively deep source (~100–150 m), coinciding with the subsurface maximum of nutrients, thus promoting coastal productivity locally and farther north by alongshore advection within the Ningaloo current.