Indian Ocean Subtropical Underwater and the Interannual Variability in its Annual Subduction Rate Associated with the Southern Annular Mode

In this study, the Indian Ocean subtropical underwater (IOSTUW) was investigated as a subsurface salinity maximum using Argo floats (2000–2020) for the first time. It has mean salinity, potential temperature and potential density values of 35.54 ± 0.29 psu, 17.91 ± 1.66 °C, and 25.56 ± 0.35 kg m−3, respectively, and mainly extends between 10°S and 30°S along the isopycnal surface in the subtropical south Indian Ocean. The annual subduction rate of the IOSTUW during the period of 2004-2019 was investigated based on a gridded Argo dataset. The results revealed a mean value of 4.39 Sv (1 Sv=106 m3s−1) with an interannual variability that is closely related to the Southern Annular Mode (SAM). The variation in the annual subduction rate of the IOSTUW is dominated by the lateral induction term, which largely depends on the winter mixed layer depth (MLD) in the sea surface salinity (SSS) maximum region. The anomalies of winter MLD is primarily determined by SAM-related air-sea heat flux and zonal wind anomalies through modulation of the buoyancy. As a result, the annual subduction rate of the IOSTUW generally increased when the SAM index showed negative anomalies and decreased when the SAM index showed positive anomalies. Exceptional cases occurred when the wind anomaly within the SSS maximum region was weak or was dominated by its meridional component.

The protist community mirrors seasonality and mesoscale hydrographic features in the oligotrophic Sargasso Sea

Protists represent the majority of the eukaryotic diversity in the oceans. They have different functions in the marine food web, playing essential roles in the biogeochemical cycles. Meanwhile the available data is rich in horizontal and temporal coverage, little is known on their vertical structuring, particularly below the photic zone. The present study applies DNA metabarcoding to samples collected over three years in conjunction with the BATS time-series to assess marine protist communities in the epipelagic and mesopelagic zones. The protist community showed a dynamic seasonality in the epipelagic, responding to hydrographic yearly cycles. Mixotrophic lineages dominated throughout the year; however, autotrophs bloomed during the rapid transition between the winter mixing and the stratified summer, and heterotrophs had their peak at the end of summer, when the base of the thermocline reaches its deepest depth. Below the photic zone, the community, dominated by Rhizaria, is depth-stratified and relatively constant throughout the year, mirroring local hydrographic and biological features such as the oxygen minimum zone. The results suggest a dynamic partitioning of the water column, where the niche vertical position for each community changes throughout the year, likely depending on nutrient availability, the mixed layer depth, and other hydrographic features. Finally, the protist community closely followed mesoscale events (eddies), where the communities mirrored the hydrographic uplift, raising the deeper communities for hundreds of meters, and compressing the communities above.

The Black Sea Physics Analysis and Forecasting System within the Framework of the Copernicus Marine Service

This work describes the design, implementation and validation of the Black Sea physics analysis and forecasting system, developed by the Black Sea Physics production unit within the Black Sea Monitoring and Forecasting Center as part of the Copernicus Marine Environment and Monitoring Service. The system provides analyses and forecasts of the temperature, salinity, sea surface height, mixed layer depth and currents for the whole Black Sea basin, excluding the Azov Sea, and has been operational since 2016. The system is composed of the NEMO (v 3.4) numerical model and an OceanVar scheme, which brings together real time observations (in-situ temperature and salinity profiles, sea level anomaly and sea surface temperature satellite data). An operational quality assessment framework is used to evaluate the accuracy of the products which set the basic standards for the future upgrades, highlighting the strengths and weaknesses of the model and the observing system in the Black Sea.

Seasonal Variability of The Mixed Layer Depth in The Sulawesi Sea

The Sulawesi Sea is a semi-enclosed basin located in the Indonesian Seas and considered as the one of location in the west route of Indonesian Throughflow (ITF). There is less attention on the mixed layer depth investigation in the Sulawesi Sea. Concerning that the mixed layer plays an important role in influencing the ocean in air-sea interaction and affects biological activity, the estimation of mixed layer depth (MLD) in the Sulawesi Sea is important. Seasonal variation of the mixed layer in the Sulawesi Sea between 115°-125°E and 0°-8°N is estimated by using World Ocean Atlas 2013. Forcing elements on the mixed layer in terms of surface-forced turbulent mixing from mechanical forcing of wind stress and buoyancy forcing (from heat flux as well as freshwater flux) in the Sulawesi Sea is provided by using a reanalysis dataset. The MLD is estimated directly on grid profiles with interpolated levels based on chosen density fixed criterion of 0.03 kg.m^-3 and temperature criterion of 0.5°C difference from the surface. The results show that mixed layer depth in the Sulawesi Sea varies both spatially and temporally. Generally, the deepest MLD was occurred during the southwest monsoon (JJA), and the lowest MLD was occurred during the first transition (MAM) and second transition monsoon (SON). Strengthening and weakening MLD are influenced by mechanical forcing from wind stress and buoyancy flux. In the Sulawesi Sea, the mixed layer deepening coincides with the occurrence of a maximum in wind stress, and low buoyancy flux at the surface. This condition is the opposite when mixed layer shallowing occurs.

Winter Convective Mixing in the northern Arabian Sea during Contrasting Monsoons

Along-track Argo observations in the northern Arabian Sea during 2017 – 19 showed by far the most contrasting winter convective mixing; 2017 – 18 was characterized by less intense convective mixing resulting in a mixed layer depth of 110 m, while 2018 – 19 experienced strong and prolonged convective mixing with the mixed layer deepening to 150 m. The response of the mixed layer to contrasting atmospheric forcing and the associated formation of Arabian Sea High Salinity Water (ASHSW) in the northeastern Arabian Sea are studied using a combination of Argo float observations, gridded observations, a data assimilative general circulation model and a series of 1-D model simulations. The 1-D model experiments show that the response of winter mixed layer to atmospheric forcing is not only influenced by winter surface buoyancy loss, but also by a preconditioned response to freshwater fluxes and associated buoyancy gain by the ocean during the summer that is preceding the following winter. A shallower and short-lived winter mixed layer occurred during 2017 – 18 following the exceptionally high precipitation over evaporation during the summer monsoon in 2017. The precipitation induced salinity stratification (a salinity anomaly of -0.7 psu) during summer inhibited convective mixing in the following winter resulting in a shallow winter mixed layer (103 m). Combined with weak buoyancy loss due to weaker surface heat loss in the northeastern Arabian Sea, this caused an early termination of the convective mixing (February 26, 2018). In contrast, the winter convective mixing during 2018 – 19 was deeper (143 m) and long-lived. The 2018 summer, by comparison, was characterized by normal or below normal precipitation which generated a weakly stratified ocean pre-conditioned to winter mixing. This combined with colder and drier air from the land mass to the north with low specific humidity lead to strong buoyancy loss, and resulted in prolonged winter convective mixing through March 25, 2019.

Parameterization of submesoscale mixed layer restratification under sea ice

Commonly used parameterization of mixed layer instabilities in general circulation models (Fox-Kemper and Ferrari 2008a) was developed for temperate oceans and does not take into account the presence of sea ice in any way. However, the ice-ocean drag provides a strong mechanical coupling between the sea ice and the surface ocean currents and hence may affect mixed layer restratification processes. Here we use idealized simulations of mixed layer instabilities to demonstrate that the sea ice dramatically suppresses the eddy-driven overturning in the mixed layer by dissipating the eddy kinetic energy generated during instabilities. Considering the commonly-used viscous-plastic sea ice rheology, we developed an improvement to the existing mixed layer overturning parameterization, making it explicitly dependent on sea ice concentration. Below the critical sea ice concentration of about 0.68, the internal sea ice stresses are very weak and the conventional parameterization holds. At higher concentrations, the sea ice cover starts acting as a nearly-immobile surface lid, inducing strong dissipation of submesoscale eddies and reducing the intensity of the restratification streamfunction up to a factor of 4 for a fully ice-covered ocean. Our findings suggest that climate projection models might be exaggerating the restratification processes under sea ice, which could contribute to biases in mixed layer depth, salinity, ice-ocean heat fluxes, and sea ice cover.

Upper ocean responses in the central western equatorial Indian Ocean During the southwest monsoon season

Upper ocean (200 m) response under the pre-onset, and active regimes of southwest (SW) monsoonal forcing at 0°N. 60°E in the Indian Ocean was analysed utilising time series data collection during Indo-Soviet Monsoon Experiment, 1973 (ISMEX- 73). Oceanic response under the pre-onset domination of the wind stress momentum and onset domination of buoyancy flux (B0) was apparent in shoaling/warming and deepening/cooling (12 m/0.50 C in 4 days) of Mixed Layer Depth (MLD). The pre-onset increase was followed by an onset decrease in below layer thermohaline/density gradient and disappearance of Sub-surface Salinity Maximum (SSM). Corespondingly, MLD and its heat content (HCMLD ) were more correlated to (B0) and QN . Upper ocean response during active regime manifested in deepening/colling (20 m/1C in 6 days) of MLD under dominant production of turbulent kinetic energy by wind stress except for the convectively dominant mixing at the beginning and end. With reduction in below-layer thermohaline/density gradient and absence of SSM the correlations between MLD B0 wind stress, QN and HCMLD became insignificant due to increased advective flux during active regime. One dimensional simulation of mixed layer paramerters showed agreement.

A note about density staircases in the Gulf of Naples : 20 years of persistent weak salt-fingering layers in a coastal area

This is a short communication about the inter-annual recurring presence at the coastal site in the Gulf of Naples of density staircases visible below the mixed surface layer of the water-column, from the end of summer to the beginning of winter, each year during nearly two decades of survey (2001 to 2020). We repetitively observe sequences from 1 to 4 small vertical staircases structures (~ 3 m thick) in the density profiles (~ Δ0.2 kg.m-3), located between 10 m to 50 m deep below the seasonal mixed layer depth. We interpret these vertical structures as the result of double diffusive processes that could host salt-fingering regime (SF) due to warm salty water parcels overlying on relatively fresher and colder layers. This common feature of the Mediterranean basin (i.e., the thermohaline staircases of the Tyrrhenian sea) may sign here for the lateral intrusions of nearshore water masses. These stably stratified layers are characterized by density ratio Rρ 5.0 to 10.0, slightly higher than the critical range (1.0 - 3.0) generally expected for fully developed salt-fingers. SF mixing, such as parameterized (Zhang et al., 1998), appears to inhibit weakly the effective eddy diffusivity with negative averaged value (~ - 1e-8 m2.s-1). A quasi 5-year cycle is visible in the inter-annual variability of the eddy diffusivity associated to SF, suggesting a decadal modulation of the parameters regulating the SF regime. Even contributing weakly to the turbulent mixing of the area, we hypothesis that SF could influence the seasonal stratification by intensifying the density of deep layers. Downward transfer of salt could have an impact on the nutrient supply for the biological communities, that remains to be determined.