Vertical Salinity Gradient Research Articles

Extreme Conditions in the Bering Sea (2017–2018): Record‐Breaking Low Sea‐Ice Extent

AbstractThe lowest winter‐maximum areal sea‐ice coverage on record (1980–2019) in the Bering Sea occurred in the winter of 2017/2018. Sea ice arrived late due to warm southerly winds in November. More typical northerly winds (albeit warm) in December and January advanced the ice, but strong, warm southerlies in February and March forced the ice to retreat. The cold pool (shelf region with bottom water < 2 °C) was the smallest on record, because of two related mechanisms: (1) lack of direct cooling in winter by melting sea ice and (2) weaker vertical stratification (no ice melt reduced the vertical salinity gradient) allowing surface heating to penetrate into the near bottom water during summer. February 2019 exhibited another outbreak of warm southerly winds forcing ice to retreat. The number of >31‐day outbreaks of southerly winds in winter has increased since 2016.

The Influence of Oceanic Barrier Layers on Tropical Cyclone Intensity as Determined through Idealized, Coupled Numerical Simulations

AbstractThe connection relating upper-ocean salinity stratification in the form of oceanic barrier layers to tropical cyclone (TC) intensification is investigated in this study. Previous works disagree on whether ocean salinity is a negligible factor on TC intensification. Relationships derived in many of these studies are based on observations, which can be sparse or incomplete, or uncoupled models, which neglect air–sea feedbacks. Here, idealized ensemble simulations of TCs performed using the Weather Research and Forecasting (WRF) Model coupled to the 3D Price–Weller–Pinkel (PWP) ocean model facilitate examination of the TC–upper-ocean system in a controlled, high-resolution, mesoscale environment. Idealized vertical ocean profiles are modeled after barrier layer profiles of the Amazon–Orinoco river plume region, where barrier layers are defined as vertical salinity gradients between the mixed and isothermal layer depths. Our results reveal that for TCs of category 1 hurricane strength or greater, thick (24–30 m) barrier layers may favor further intensification by 6%–15% when averaging across ensemble members. Conversely, weaker cyclones are hindered by thick barrier layers. Reduced sea surface temperature cooling below the TC inner core is the primary reason for additional intensification. Sensitivity tests of the results to storm translation speed, initial oceanic mixed layer temperature, and atmospheric vertical wind shear provide a more comprehensive analysis. Last, it is shown that the ensemble mean intensity results are similar when using a 3D or 1D version of PWP.

Upper Ocean Response to Rain Observed From a Vertical Profiler

AbstractRainfall induces a vertical salinity gradient directly below the ocean surface, the strength and lifetime of which depend on the size of the rain event, the availability of mixing, and the air‐sea heat fluxes. The presence of rain in turn influences the near‐surface turbulent mixing and air‐sea exchange processes. During a campaign in the midlatitude North Atlantic, the Air‐Sea Interaction Profiler (ASIP) was used to investigate changes in the vertical distribution of salinity (S), temperature (T), and turbulent kinetic energy dissipation rate (ϵ) caused by four rain events. During one of the rain events a strong shallow stratification was formed. The buoyancy effect of this freshwater lens changes the dominant wind‐driven turbulent mixing. The surface momentum flux was limited to a shallow layer, and below it ϵ is reduced by 2 orders of magnitude. For a different rain event of higher‐peak rain rate, the salinity anomaly is smaller and is dispersed deeper into the water column. The difference in ocean response shows that the upper ocean is sensitive to changes in the atmospheric forcing associated with the rain events. The observed salinity anomalies as a function of rain rate and wind speed are compared to relationships from studies with the 1‐D turbulence model GOTM and satellite validation. The observations suggest that the vertical salinity anomaly is best described as a function of total rain. A higher‐resolution prognostic model for sea surface salinity and temperature is shown to perform well in predicting the observed S and T anomalies.

Contrasting the upper ocean response to two intense cyclones in the Bay of Bengal

The contrasting upper ocean responses of the Bay of Bengal to two intense cyclones, Phailin and Hudhud, which traversed similar paths and occurred at nearly the identical time of year, were analysed using observations from moored buoys, satellite observations, and modelling. Our analyses revealed that the upper ocean was highly stratified prior to Phailin compared to the case for Hudhud. The stratification in the near surface prior to the onset of Phailin (Hudhud) was predominantly driven by vertical gradients in salinity (temperature). The upper ocean response to the passage of Phailin (Hudhud) was characterised by notable changes in sea surface salinity (sea surface temperature). In addition, enhancement of chlorophyll-a was observed over an extensive area following the passage of Hudhud but was confined to a mesoscale eddy after Phailin. The presence of strong near-surface haline stratification sustained temperature inversion and resulted in weak sea surface temperature cooling in the wake of Phailin. The mixing centred along the trajectory of a cyclone in a haline (thermally)-stratified ocean leads to fronts having strong horizontal haline (thermal) gradients in the wake. Cross-front ageostrophic submesoscale circulation features resulted in rapid fluctuations of salinity in the wake of Phailin and temperature in the wake of Hudhud. The cross-front ageostrophic circulation features and post-storm winds played a critical role in controlling mixed layer restratification after the cyclones.

Eddy‐Induced Salinity Changes in the Tropical Pacific

AbstractThe signature of westward propagating mesoscale eddies in sea surface salinity (SSS) is analyzed for the tropical Pacific by collocating 7 years (2010–2016) of Soil Moisture and Ocean Salinity SSS satellite data with coherent mesoscale eddies automatically identified and tracked from altimetry‐derived sea level anomalies. First, the main characteristics of the long‐lived coherent eddies are inferred from sea level anomalies maps. Then, the mean signature of the mesoscale eddies on SSS is depicted for the whole tropical Pacific before focusing in regions centered around the central and eastern parts of the tropical North Pacific. In these areas, composite analyses based on thousands of eddies reveal regionally dependent eddy impacts with opposite SSS anomalies for cyclonic and anticyclonic eddies. In the central region, where the largest meridional SSS large‐scale gradients and smallest eddy amplitudes are observed, results show dipole‐like SSS changes with maximum anomalies on the leading edge of the composite eddy. In contrast, in the eastern region, where the largest near‐surface vertical salinity gradients and largest eddy amplitudes are observed, the composite eddy shows monopole‐like SSS changes with maximum anomalies near the composite eddy center. These distinct dipole/monopole SSS patterns suggest the dominant role of horizontal advection and vertical processes in the central and eastern regions, respectively. Other possible explanations, notably one involving the contrasted eddy amplitudes of the two regions, are discussed.

Measurements of buoyancy flux in a stratified turbulent flow

We present new experimental data on the controls on the buoyancy flux in a stratified turbulent flow. The inner cylinder of an annulus of fluid with vertical axis is rotated to produce a turbulent flow field with Reynolds numbers of up to $10^{5}$, while a flux of saline fluid is supplied to the base of the tank, and an equal flux of fresh fluid is supplied to the top of the tank. In addition, fluid is vented from the base and the top of the tank with the same volume fluxes as the supply. The steady-state vertical flux of salt is explored. When the salt flux supplied to the base of the tank is very small, the tank becomes nearly well-mixed, and the vertical salt flux is approximately equal to one-half the source flux. As the source salt flux increases, a weak stable salinity gradient develops across the tank, and the vertical salt flux increases. As the source flux continues to increase, eventually the vertical salt flux reaches a maximum, and further increases in the source salt flux can lead to an increase in the vertical salinity gradient but not the vertical flux. We interpret the transition in the vertical buoyancy flux as representing a change from a source-limited regime, where the buoyancy flux and buoyancy frequency, $N$, are related, to a mixing-limited regime, in which the buoyancy flux is independent of $N$. In the mixing-limited regime, the effective eddy diffusivity is proportional to $u_{rms}^{3}/LN^{2}$ while in the source-limited regime, the eddy diffusivity is approximately proportional to $u_{rms}^{2}/N$, where $u_{rms}$ and $L$ are the characteristic turbulence speed and length scale. This transition may have implications for the balance between upwelling and diapycnal mixing in the ocean, if the intensity of the turbulence varies in space or the flux of deep water varies in time.

On the Effects of Increased Vertical Mixing on the Arctic Ocean and Sea Ice

AbstractAgainst the backdrop of Arctic sea ice decline, vertical mixing in the interior Arctic Ocean will most likely change, but it is still unclear how the Arctic Ocean and sea ice will respond. In this paper, a sea ice‐ocean model with a simple parameterization for interior background mixing is used to investigate the Arctic Ocean and sea ice response to a scenario of increased vertical mixing. It is found that more vertical mixing reduces sea ice thickness all year round and decreases summertime sea ice concentration. More vertical mixing leads to a cooling of the Arctic halocline layer and Atlantic Water layer below. The increased vertical mixing speeds up vertical heat and salinity exchange, brings the underlying warm and saline water into the surface layer, and contributes to the sea ice decline. Vertical salinity gradient of the Arctic halocline layer reduces together with a much fresher Atlantic Water layer, and more volume of saline water enters the deep ocean below the Atlantic Water layer. As a result, the reduced Arctic Ocean stratification leads to an adjustment of the circulation pattern. Cyclonic circulation anomalies occur in the surface layer shallower than 20‐m depth and in the interior ocean deeper than 700‐m depth, while anticyclonic circulation anomalies occur between these depths. Our study suggests that the extra heat and salinity exchange induced by more vertical mixing will have a noticeable impact on the upper ocean structure, ocean circulation, and sea ice in a changing Arctic Ocean.

Composite eddy structures on both sides of the Luzon Strait and influence factors

Combining Argo observations with satellite remote sensing data during the period of 2002–2014, the mean three-dimensional structures of mesoscale eddies on both sides of the Luzon Strait (LS) were obtained via a composite method and analyzed to statistically examine the influences of background marine environment and the Kuroshio current on the eddy structures. The significant signals of temperature and salinity anomalies within the composite eddies extend much deeper in the region east of the LS (zone E) than those in the region west of the strait (zone W) because of stronger eddy intensity and larger vertical gradients of background temperature and salinity in the deep layer in zone E. In the vertical structure of temperature anomaly within the eddies, two cores occur at around 200 and 400 dbar depths, respectively, in zone E and only one core is centered at about 100 dbar in zone W. There is a clear three-core sandwich pattern in the vertical structure of salinity anomaly within the eddies in zone E. The Kuroshio water trapped in the eddy is responsible for abnormally positive salinity anomaly in the surface layer of the anticyclonic eddy center in zone W. On both sides of the LS, an asymmetric dipole structure in the surface layer gradually turns into a monopole one at depths, which resulted from the competition between horizontal advection effect and eddy pumping effect. The Kuroshio current influences the distribution patterns of isotherms and isohalines and enhances background temperature and salinity horizontal gradients on both sides of the LS, determining the orientations of dipole temperature and salinity structures within eddies.

Drastic thickening of the barrier layer off the western coast of Sumatra due to the Madden-Julian oscillation passage during the Pre-Years of the Maritime Continent campaign

The drastic thickening of the barrier layer in the marginal sea off the western coast of Sumatra during the passage of the Madden-Julian oscillation (MJO) observed during December 2015 is investigated. Before the MJO arrival, the halocline above a depth of 20 m was very strong, and the barrier layer thickness was 5–10 m based on R/V Mirai observations. During the MJO forcing of 13–16 December, the isothermal layer drastically deepened from 20 to 100 m. Meanwhile, the mixed layer deepening lagged behind the isothermal layer deepening by 1 day, and the barrier layer underwent dramatic thickening to 60 m within 24 h. An evaluation of the vertical salinity gradient tendency showed that the dramatic thickening of the barrier layer was due to the vertical oceanic mixing by the atmospheric MJO forcing and the vertical stretching by the oceanic downwelling coastal Kelvin wave intruding from the open ocean. In addition, an evaluation of the vertical temperature gradient tendency showed that the temperature inversion in the barrier layer formed by losing heat to the atmosphere due to the MJO forcing and downward advection of the temperature gradient due to the downwelling Kelvin wave resulting in the dramatic isothermal layer deepening. One of the important factors in the drastic barrier layer thickening was the atmospheric external forcing and the oceanic internal wave being in phase. The downwelling oceanic Kelvin wave continuously lowered the thermocline from the middle of November to the end of December, and the salinity stratification in the vicinity of the thermocline was continuously mitigated by the vertical stretching. Under such conditions, the MJO forcing caused vertical mixing of the freshwater with strong salinity stratification and temperature stratification near the surface. The combination of the two distinct processes caused the drastic thickening of the barrier layer, and the barrier layer thickness reached a maximum of 85 m 5 days after the MJO arrival.ᅟ

Modeling of sediment transport in a saltwater lake with supplemental sandy freshwater.

Considering the highly complex flow structure of saltwater lakes during freshwater supplementation, a three-dimensional numerical model was developed to simulate suspended sediment transport in saltwater lakes. The model was validated using measurements of the salinity and sediment concentration during a pumping test at Yamdrok Lake. The simulation results were in quantitative agreement with the measured data. The observed and simulated results also indicated that the wind stress and vertical salinity gradient have a significant influence on salinity and sediment transport in a saltwater lake. The validated model was then used to predict and analyze the contributions of wind, the supplement flow rate and salinity stratification to the sediment transport process in Yamdrok Lake during continuous river water supplementation. The simulation results showed that after the sandy river water was continuously discharged into the saltwater lake, the lateral diffusion trends of the sediment exhibited three stages: linear growth in the inflow direction, logarithmic growth in the wind direction, and stabilization. Furthermore, wind was the dominant factor in driving the lake flow pattern and sediment transport. Specifically, wind can effectively reduce the area of the sediment diffusion zone by increasing the lateral sediment carrying and dilution capacities. The effect of inflow on the lake current is negligible, but the extent of the sediment turbidity zone mainly depends on the inflow. Reducing the inflow discharge can decrease the area of the sediment turbidity zone to proportions that far exceed the proportions of inflow discharge reductions. In addition, the high-salinity lake water can support the supplemented freshwater via buoyancy forces, which weaken vertical mixing and sediment settlement and increase lake currents and sediment diffusion near the surface.

Fast Physics and Slow Physics in the Nonlinear Dansgaard–Oeschger Relaxation Oscillation

AbstractThe Dansgaard–Oeschger (D-O) relaxation oscillation that governed glacial climate variability during marine isotope stage 3 has been accurately simulated using a high-resolution coupled climate model. Here the authors present additional detailed analyses of both the slow physics transition between warm and cold states and the fast physics transition between cold and warm states of the D-O cycle. First, the authors demonstrate that the mechanisms active during the slow transition from interstadial to stadial conditions involves the continuous flux of thick and old sea ice from the Arctic basin into the North Atlantic subpolar gyre region along the East Greenland Current. During this slow physical process, the freshwater input from sea ice melting as it moves over the surface of the warm ocean restratifies the high-latitude North Atlantic and leads to a significant reduction in the rate of North Atlantic Deep Water formation. A detailed freshwater budget and hydrography analysis is also presented to demonstrate that the D-O cycle is a low-latitude–high-latitude salt oscillator as the authors have previously argued. Second, the authors provide a more detailed analysis than previously of the fast-time-scale processes that govern the extremely rapid transition from cold stadial conditions back to the warm interstadial state. These are associated with the onset of a sub-sea ice thermohaline convective instability, which opens a massive polynya to the north of the southern boundary of the extensive North Atlantic sea ice lid that is characteristic of stadial conditions. This instability is enabled by the continuous increase of salinity above the sub-sea ice pycnocline, which eliminates the vertical salinity gradient that prevents convective destabilization of the water column under full stadial conditions. This reduction in the vertical salinity gradient beneath the sea ice lid results from the continuing northward salt transport by the North Atlantic gyre circulation once the expansion of the stadial sea ice lid has ceased. The onset of instability occurs in the Irminger basin to the south of Denmark Strait, and the authors discuss the reason for this localization of instability onset.

Effects of salinity gradients on larval growth and otolith asymmetry of austral hake Merluccius australis

Austral hake Merluccius australis sustain an important demersal fishery in South Pacific and Atlantic oceans. The main spawning and nursery areas are in the Chilean Patagonia, which is characterized by large inputs of freshwater from rivers and ice melting. The spatial distribution of larval austral hake M. australis was studied during spring, and the potential effects of oceanographic conditions (temperature, salinity, vertical stability and turbidity) on early life traits, derived from sagitta otolith microstructure analysis, size-at-age (SAR), individual larval growth (IGR) and bilateral asymmetry (FA6). Larval abundance ranged between 0.86 and 41.55 ind 1000 m -3 (median: 4.08 ind 1000 m -3 ) and population larval growth estimated by micro-increment counts of sagitta otoliths was 0.129 ± 0.010 mm d -1 , with age varying from 4 to 24 days old. All three indicators derived from otolith analysis (SAR, IGR, and FA6) showed positive and significant correlations with vertical salinity gradients; IGR and FA6 were also positively correlated to Brunt-Vaisala frequency (vertical stability) and FA6 was correlated to turbidity. Therefore, larval M. australis were larger-at-age, grew faster and showed more asymmetry in areas with more influence of freshwater runoff in south Pacific Patagonia.

Identifying metabolic pathways for production of extracellular polymeric substances by the diatom Fragilariopsis cylindrus inhabiting sea ice

Diatoms are significant primary producers in sea ice, an ephemeral habitat with steep vertical gradients of temperature and salinity characterizing the ice matrix environment. To cope with the variable and challenging conditions, sea ice diatoms produce polysaccharide-rich extracellular polymeric substances (EPS) that play important roles in adhesion, cell protection, ligand binding and as organic carbon sources. Significant differences in EPS concentrations and chemical composition corresponding to temperature and salinity gradients were present in sea ice from the Weddell Sea and Eastern Antarctic regions of the Southern Ocean. To reconstruct the first metabolic pathway for EPS production in diatoms, we exposed Fragilariopsis cylindrus, a key bi-polar diatom species, to simulated sea ice formation. Transcriptome profiling under varying conditions of EPS production identified a significant number of genes and divergent alleles. Their complex differential expression patterns under simulated sea ice formation was aligned with physiological and biochemical properties of the cells, and with field measurements of sea ice EPS characteristics. Thus, the molecular complexity of the EPS pathway suggests metabolic plasticity in F. cylindrus is required to cope with the challenging conditions of the highly variable and extreme sea ice habitat.

Structure and functioning of the acid–base system in the Baltic Sea

Abstract. The marine acid–base system is relatively well understood for oceanic waters. Its structure and functioning is less obvious for the coastal and shelf seas due to a number of regionally specific anomalies. In this review article we collect and integrate existing knowledge of the acid–base system in the Baltic Sea. Hydrographical and biogeochemical characteristics of the Baltic Sea, as manifested in horizontal and vertical salinity gradients, permanent stratification of the water column, eutrophication, high organic-matter concentrations and high anthropogenic pressure, make the acid–base system complex. In this study, we summarize the general knowledge of the marine acid–base system as well as describe the peculiarities identified and reported for the Baltic Sea specifically. In this context we discuss issues such as dissociation constants in brackish water, different chemical alkalinity models including contributions by organic acid–base systems, long-term changes in total alkalinity, anomalies of borate alkalinity, and the acid–base effects of biomass production and mineralization. Finally, we identify research gaps and specify limitations concerning the Baltic Sea acid–base system.

Free-living Neoparamoeba perurans depth distribution is mostly uniform in salmon cages, but reshaped by stratification and potentially extreme fish crowding

Understanding the spatio-temporal positioning of hosts and their parasites in free-living states is useful in devising methods to diminish parasite encounters in animal production systems. We explored the potential for depth-based control methods of the amoebic gill disease (AGD) agent, Neoparamoeba perurans, in salmon mariculture systems by conducting: (1) depth-stratified N. perurans water sampling surveys in 2 years, and (2) a mensurative experiment comparing depth distributions of N. perurans and salmon hosts in commercial salmon sea-cages. From water sampling mostly at marine salinities, N. perurans abundance (quantitative PCR-derived cells l-1) varied among years, but overall, neither depth, time since freshwater bathing, temperature and salinity were predictors of N. perurans abundance. However, at 1 survey time, depth patterns in N. perurans abundance appeared during strong vertical salinity gradients following rainfall (at 1 site, salinity ranged between 14 and 35 g l-1), with greater numbers of cells below a less saline surface layer. This suggested that salinity mediates N. perurans depth distribution during intermittent halocline development. Fish depth distribution monitoring revealed intense fish crowding, with local swimming densities up to 5 times stocking densities, typically at the surface at night. Simultaneously collected daytime water samples during low levels of fish crowding, with stock scattered amongst upper and lower cage sections, revealed no relationship between N. perurans and fish depth distributions. If intense fish crowding in narrow depth bands leads to high concentrations of N. perurans in cage environments and increased AGD risk, behavioural manipulations that vertically spread fish could be a successful AGD mitigation strategy.

Meteorological Aspects of the Eastern North American Pattern with Impacts on Long Island Sound Salinity

The eastern North American sea level pressure dipole (ENA) pattern is a recently identified teleconnection pattern that has been shown to influence mid-Atlantic United States (U.S) streamflow variability. Because the pattern was only recently identified, its impacts on U.S. precipitation and estuaries on daily to seasonal timescales is unknown. Thus, this paper presents the first seasonal investigation of ENA relationships with global atmospheric fields, U.S. precipitation, and mid-Atlantic estuarine salinity. We show that the ENA pattern explains up to 25–36% of precipitation variability across Texas and the western U.S. We also show that, for the Northeast U.S, the ENA pattern explains up to 65% of precipitation variability, contrasting with previous work showing how well-known climate indices can only explain a modest amount of precipitation variability. The strongest ENA-precipitation relationships are in the spring and fall. The relationships between the ENA pattern and precipitation across remote regions reflect the upper-atmospheric Rossby wave pattern associated with the ENA pattern that varies seasonally. The El-Nino/Southern Oscillation (ENSO) is related to the spring ENA pattern, indicating that extended outlooks of the ENA pattern may be possible. We also show that the ENA index is strongly correlated with salinity and vertical haline stratification across coastal portions of the mid-Atlantic Bight so that hypoxia forecasts based on the ENA index may be possible. Statistical connections between vertical salinity gradient and ENSO were identified at lags of up two years, further highlighting the potential for extended hypoxia outlooks. The strong connection between anomalies for precipitation and mid-Atlantic Bight salinity suggests that the ENA pattern may be useful at an interdisciplinary level for better understanding historical regional climate variability and future impacts of climate change on regional precipitation and the health of estuaries.

Synechococcus Assemblages across the Salinity Gradient in a Salt Wedge Estuary.

Synechococcus are the most abundant and widely distributed picocyanobacteria in the ocean. The salt-wedge type of estuary possesses the complete horizontal and vertical gradient of salinity together with other physical and chemical parameters. In order to reveal whether such a complex environmental gradient harbors a high diversity of Synechococcus, we investigated the abundance, taxonomic composition and pigment genetic diversity of Synechococcus in surface and bottom waters across the salinity gradient in a salt-wedge estuary by flow cytometric analysis and pyrosequencing of the rpoC1 gene and cpcBA operon (encoding phycocyanin). Synechococcus were ubiquitously distributed in the studied region, with clear spatial variations both horizontally and vertically. The abundance and diversity of Synechococcus were low in the freshwater-dominated low salinity waters. By pyrosequencing of the rpoC1 gene, we have shown that with the increase of salinity, the dominant Synechococcus shifted from the freshwater Synechococcus to the combination of phylogenetic subcluster 5.2 and freshwater Synechococcus, and then the strictly marine subcluster 5.1 clade III. Besides, the composition of Synechococcus assemblage in the deep layer was markedly different from the surface in the stratified waters (dissimilarities: 40.32%-95.97%, SIMPER analysis). High abundance of clade III Synechococcus found in the brackish waters may revise our previous understanding that strains of this clade prefers oligotrophic environment. Our data also suggested that both the phylogenetic subcluster 5.3 Synechococcus, a lineage that was not well understood, and subcluster 5.1 clade I, a typical cold water lineage, were widely distributed in the bottom layer of the estuary. Clade I detected in the studied region was mainly contributed by subclade IG. Analysis of the cpcBA operon sequences revealed niche partitioning between type 1 and type 3 Synechococcus, with type 2 distributed broadly across the whole environmental gradients. Our results suggest that the salt wedge estuary provides various niches for different lineages of Synechococcus, making it an environment with high Synechococcus diversity compared with adjacent freshwater and shelf sea environments.

Near‐surface salinity and temperature structure observed with dual‐sensor drifters in the subtropical South Pacific

AbstractThree surface drifters equipped with temperature and salinity sensors at 0.2 and 5 m depths were deployed in April/May 2015 in the subtropical South Pacific with the objective of measuring near‐surface salinity differences seen by satellite and in situ sensors and examining the causes of these differences. Measurements from these drifters indicate that water at a depth of 0.2 m is about 0.013 psu fresher than at 5 m and about 0.024°C warmer. Events with large temperature and salinity differences between the two depths are caused by anomalies in surface freshwater and heat fluxes, modulated by wind. While surface freshening and cooling occurs during rainfall events, surface salinification is generally observed under weak wind conditions (≤4 m/s). Further examination of the drifter measurements demonstrates that (i) the amount of surface freshening and strength of the vertical salinity gradient heavily depend on wind speed during rain events, (ii) salinity differences between 0.2 and 5 m are positively correlated with the corresponding temperature differences for cases with surface salinification, and (iii) temperature exhibits a diurnal cycle at both depths, whereas the diurnal cycle of salinity is observed only at 0.2 m when the wind speed is less than 6 m/s. The amplitudes of the diurnal cycles of temperature at both depths decrease with increasing wind speed. The mean diurnal cycle of surface salinity is dominated by events with winds less than 2 m/s.

Climatological Annual Cycle of the Salinity Budgets of the Subtropical Maxima

AbstractSix subtropical salinity maxima (Smax) exist: two each in the Pacific, Atlantic, and Indian Ocean basins. The north Indian (NI) Smax lies in the Arabian Sea while the remaining five lie in the open ocean. The annual cycle of evaporation minus precipitation (E − P) flux over the Smax is asymmetric about the equator. Over the Northern Hemisphere Smax, the semiannual harmonic is dominant (peaking in local summer and winter), while over the Southern Hemisphere Smax, the annual harmonic is dominant (peaking in local winter). Regardless, the surface layer salinity for all six Smax reaches a maximum in local fall and minimum in local spring. This study uses a multidecade integration of an eddy-resolving ocean circulation model to compute salinity budgets for each of the six Smax. The NI Smax budget is dominated by eddy advection related to the evolution of the seasonal monsoon. The five open-ocean Smax budgets reveal a common annual cycle of vertical diffusive fluxes that peak in winter. These Smax have regions on their eastward and poleward edges in which the vertical salinity gradient is destabilizing. These destabilizing gradients, in conjunction with wintertime surface cooling, generate a gradually deepening wintertime mixed layer. The vertical salinity gradient sharpens at the base of the mixed layer, making the water column susceptible to salt finger convection and enhancing vertical diffusive salinity fluxes out of the Smax into the ocean interior. This process is also observed in Argo float profiles and is related to the formation regions of subtropical mode waters.

Satellite and In Situ Salinity: Understanding Near-Surface Stratification and Subfootprint Variability

Abstract Remote sensing of salinity using satellite-mounted microwave radiometers provides new perspectives for studying ocean dynamics and the global hydrological cycle. Calibration and validation of these measurements is challenging because satellite and in situ methods measure salinity differently. Microwave radiometers measure the salinity in the top few centimeters of the ocean, whereas most in situ observations are reported below a depth of a few meters. Additionally, satellites measure salinity as a spatial average over an area of about 100 × 100 km2. In contrast, in situ sensors provide pointwise measurements at the location of the sensor. Thus, the presence of vertical gradients in, and horizontal variability of, sea surface salinity complicates comparison of satellite and in situ measurements. This paper synthesizes present knowledge of the magnitude and the processes that contribute to the formation and evolution of vertical and horizontal variability in near-surface salinity. Rainfall, freshwater plumes, and evaporation can generate vertical gradients of salinity, and in some cases these gradients can be large enough to affect validation of satellite measurements. Similarly, mesoscale to submesoscale processes can lead to horizontal variability that can also affect comparisons of satellite data to in situ data. Comparisons between satellite and in situ salinity measurements must take into account both vertical stratification and horizontal variability.