Mixed Layer Salinity Research Articles

Large Mixed Layer Salinity Variation in the Southern Tropical Indian Ocean Due To the Blending of Water Masses

AbstractThe southern tropical Indian Ocean (TIO) displays large mixed layer salinity (MLS) variation. Circulation in this region is governed by the Indian Ocean tropical gyre (IOTG), where the source water proportion and associated mixing remain unclear. Particles integrating into the IOTG and entering the central southern TIO originate from the Bay of Bengal, Malacca Strait, western Indian Ocean, and Indonesian Throughflow. Surprisingly, cross‐equatorial advection is particularly important, implying a significant connection between both the Bay of Bengal and the South China Sea via Malacca Strait into the southern TIO. The anomalous anticlockwise circulation weakens the IOTG during positive Indian Ocean Dipole (IOD). An opposite pattern is observed in the negative IOD. A particle experiment reveals that water masses are modulated by the anomalous circulation that drives the redistribution of MLS by changing the proportion of the different source waters. This represents a potential predictability for the southern TIO MLS variability.

Ocean component of the first operational version of Hurricane Analysis and Forecast System: Evaluation of HYbrid Coordinate Ocean Model and hurricane feedback forecasts

The first operational version of the coupled Hurricane Analysis and Forecast System (HAFSv1) launched in 2023 consists of the HYbrid Coordinate Ocean Model (HYCOM) and finite-volume cubed-sphere (FV3) dynamic atmosphere model. This system is a product of efforts involving improvements and updates over a 4-year period (2019–2022) through extensive collaborations between the Environmental Modeling Center at the US National Centers for Environmental Prediction (NCEP) and NOAA Atlantic Oceanography and Meteorology Laboratory. To provide two sets of numerical guidance, the initial operational capability of HAFSv1 was configured to two systems—HFSA and HFSB. In this study, we present in-depth analysis of the forecast skills of the upper ocean that was co-evolved by the HFSA and HFSB. We chose hurricane Laura (2020) as an example to demonstrate the interactions between the storm and oceanic mesoscale features. Comparisons performed with the available in situ observations from gliders as well as Argos and National Data Buoy Center moorings show that the HYCOM simulations have better agreement for weak winds than high winds (greater than Category 2). The skill metrics indicate that the model sea-surface temperature (SST) and mixed layer depth (MLD) have a relatively low correlation. The SST, MLD, mixed layer temperature (MLT), and ocean heat content (OHC) are negatively biased. For high winds, SST and MLT are more negative, while MLD is closer to the observations with improvements of about 8%–19%. The OHC discrepancy is proportional to predicted wind intensity. Contrarily, the mixed layer salinity (MLS) uncertainties are smaller and positive for higher winds, probably owing to the higher MLD. The less-negative bias of MLD for high winds implies that the wind-force mixing is less effective owing to the higher MLD and high buoyancy stability (approx. 1.5–1.7 times) than the observations. The heat budget analysis suggests that the maximum heat loss by hurricane Laura was O(< 3°C per day). The main contributor here is advection, followed by entrainment, which act against or with each other depending on the storm quadrant. We also found relatively large unaccountable heat residuals for the in-storm period, and the residuals notably led the heat tendency, meaning that further improvements of the subscale simulations are warranted. In summary, HYCOM simulations showed no systematic differences forced by either HFSA or HFSB.

Upper Ocean Responses to Tropical Cyclone Mekunu (2018) in the Arabian Sea

Based on Argo observations and a coupled atmosphere–ocean–wave model, the upper ocean responses to the tropical cyclone (TC) Mekunu (2018) were investigated, and the role of a pre-existing cold eddy in modulating the temperature response to TC Mekunu was quantified by employing numerical experiments. With TC Mekunu’s passage, the mixed layer depth (MLD) on both sides of its track significantly deepened. Moreover, two cold patches (<26 °C) occurred, where the maximum cooling of the mixed layer temperature (MLT) reached 6.62 °C and 6.44 °C. Both the MLD and MLT changes exhibited a notable rightward bias. However, the changes in the mixed layer salinity (MLS) were more complex. At the early stage, the MLS on both sides of the track increased by approximately 0.5 psu. When TC Mekunu made landfall, the MLS change around the track was asymmetric. Significantly, a cold eddy pre-existed where the second cold patch emerged, and this eddy was intensified after TC Mekunu’s passage, with an average sea surface height reduction of approximately 2.7 cm. By employing the stand-alone ocean model, the numerical experimental results demonstrated that the pre-existing cold eddy enhanced TC-induced MLT cooling by an average of approximately 0.41 °C due to steeper temperature stratification at the base of mixed layer. Moreover, heat budget analysis indicated that the pre-existing cold eddy also enhanced subsurface temperature cooling mainly through zonal advection.

The Mixed Layer Salinity Balance in the Western Arctic Ocean

AbstractIn this study, we explore the mixed layer salinity (MLS) balance in the western Arctic Ocean based on the Arctic Subpolar gyre sTate Estimate (ASTE) results. The key components of the MLS budgets and their variabilities in response to the Beaufort Gyre (BG) spin‐up are identified. Seasonally, the surface forcing (brine rejection plus freshwater input) is the most important dominant contributor to the MLS balance. On the other hand, the entrainment dominates the interannual variability of MLS tendency inside the BG, while the advection dominates that in the Beaufort Sea. The sensitivity test of increased river discharge revealed a greater role of the advection term, along with weakened contributions from the surface forcing and entrainment, in determining the interannual variability of MLS balance. In contrast, the seasonal variabilities remained largely unchanged. The Lagrangian particle tracking reveals that the majority of BG freshwater within the mixed layer exits through the Canadian Archipelago prior to the BG spin‐up (2002–2006) and during its relaxation (2012–2017). We found a reduction in mixed layer freshwater sources from the external BG that could feed the gyre during its spin‐up (2007–2011), with the major contributions coming from the Beaufort Sea and the BG region itself through Ekman convergence. The mixed layer freshwater pathways are similar in the two versions of ASTE, but with noticeable proportion changes with the increasing river discharge.

Evaluation of the ocean component on different coupled hurricane forecasting models using upper-ocean metrics relevant to air-sea heat fluxes during Hurricane Dorian (2019)

In August 2019 Hurricane Dorian traveled through the Caribbean Sea and Tropical Atlantic before devastating the Bahamas. The operational hurricane forecasting models under-predicted the intensity evolution of Dorian prior to the storm reaching its maximum strength. Research studies have shown that a more realistic upper-ocean characterization in coupled atmosphere-ocean models used to forecast hurricanes has the potential to lead to more accurate hurricane intensity forecasts. In this work, we evaluated four ocean products: the ocean component from one NOAA operational hurricane forecasting model that used ocean initial conditions from climatology, the ocean components from two NOAA experimental models using ocean initial conditions from a data-assimilative operational ocean model, and one US Navy data-assimilative operational ocean model for reference. The upper-ocean metrics used to evaluate the models include mixed layer temperature, mixed layer salinity, ocean heat content and depth-averaged temperature in the top 100 m. The observations used are temperature and salinity profiles from an array of six autonomous underwater gliders deployed in the Caribbean region during the 2019 hurricane season. We found that, even though the four models have good skill in predicting temperature and salinity over the whole observed water column, skill significantly deteriorates for the upper-ocean metrics. In particular, the models failed to capture the barrier layer that was present during the passage of Hurricane Dorian through the glider array. We also found that even small differences in the mixed layer temperature along the storm track on the hurricane models evaluated, led to noticeable differences in the total enthalpy fluxes delivered from the ocean to the atmosphere throughout the storm’s synoptic history. These findings highlight the need to improve the upper-ocean initial conditions and representation in coupled atmosphere-ocean models as part of the larger efforts to improve the various modeling aspects that control the hurricane intensity forecast.

Intrusion of Arabian Sea high salinity water and monsoon-associated processes modulate planktic foraminiferal abundance and carbon burial in the southwestern Bay of Bengal.

The unicellular calcareous planktic foraminifera sequester a significant portion of the carbon dioxide dissolved in the ocean, thus burying the carbon in sediments for millions of years. The global warming and associated processes are likely to affect the planktic foraminiferal abundance and diversity. Therefore, their baseline distribution has to be documented and correlated with ambient parameters to assess its fate under different climate change scenarios. Here, we report an exceptionally high abundance of planktic foraminifera and thus large carbon burial in the southwestern Bay of Bengal. The very high absolute abundance of planktic foraminifera in the Cauvery River basin is attributed to biannual productivity, warmer and saline waters. Globigerinita glutinata is the highest abundant species followed by Globigerinoides ruber and Globigerina bulloides. Globigerina bulloides is abundant on the shelf, where the upwelling is more frequent. The relative abundance of Globorotalia menardii is positively correlated with thermocline salinity and negatively correlated with thermocline temperature. Similarly, Neogloboquadrina dutertrei and Globoquadrina conglomerata are negatively correlated with mixed layer as well as thermocline temperature and mixed layer salinity. Both these species are positively correlated with thermocline salinity. Globigerina falconensis is more abundant in the southernmost transect influenced by intense winter monsoon precipitation. We report that G. ruber prefers high saline and warmer waters with the highest abundance in the southernmost transect. From the foraminiferal distribution, it is evident that the temperature and salinity of the mixed layer as well as thermocline, food availability, and monsoon-associated processes affect the planktic foraminiferal abundance and thus carbon burial in the southwestern Bay of Bengal. The changes in influx of southeastern Arabian Sea water will affect the planktic foraminiferal population and subsequent carbon burial in the southwestern Bay of Bengal.

Mechanism and impact of zonally contrasting seasonal variations in sea-surface salinity in the North Pacific and North Atlantic oceans

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

The Mixed Layer Salinity Budget in the Northern South China Sea: A Modeling Study

The seasonal variation in mixed layer salinity (MLS) plays a crucial role in global ocean circulation and hydrological cycle. The salinity budget of the mixed layer is important to understand the mechanism of the variation, but in the South China Sea (SCS), the details in the budget are missing due to insufficient observations. Here, we employed an eddy-resolving (horizontal grid resolution ~10 km) SCS circulation model to quantify the key physical processes in the seasonal cycling of MLS in the northern South China Sea (NSCS). Built on the success of the realistic numerical simulation for 2008–2018, the model reproduced the primary features of the observed seasonal MLS, wherein fresher waters are present in the region during the summer monsoon and salty waters appear along the slope during the winter monsoon. According to the salinity budget that was calculated during model execution, the term for air–sea freshwater flux and meridional advection represent the primary freshwater input in the summer and winter, respectively, while vertical processes including vertical mixing and entrainment form the major balancing terms in the budget. In different regions of the NSCS, vertical mixing can play a dominant role in the vertical processes, but the associated seasonality is different for regions of strong internal wave influence and regions of strong horizontal advection influence. In the winter, the intrusion and spreading of western Pacific water over the NSCS could modify the MLS structure and cause larger vertical entrainment than mixing in regions where the effect of mixing decreases with the slackening of the seasonal internal wave activities. Overall, the analysis of the ML salinity budget reveals that vertical mixing, together with vertical entrainment, is vital to maintaining the seasonal variation in MLS of the NSCS.

Joint observation–model mixed-layer heat and salt budgets in the eastern tropical Atlantic

Abstract. In this study, we use a joint observation–model approach to investigate the mixed-layer heat and salt annual mean as well as seasonal budgets in the eastern tropical Atlantic. The regional PREFCLIM (PREFACE Climatology) observational climatology provides the budget terms with a relatively low spatial and temporal resolution compared to the online NEMO (Nucleus for European Modeling of the Ocean; Madec, G., 2014) model, and this is later resampled as in PREFCLIM climatology. In addition, advection terms are recomputed offline from the model as PREFCLIM gridded advection computation. In the Senegal, Angola, and Benguela regions, the seasonal cycle of mixed-layer temperature is mainly governed by surface heat fluxes; however, it is essentially driven by vertical heat diffusion in the equatorial region. The seasonal cycle of mixed-layer salinity is largely controlled by freshwater flux in the Senegal and Benguela regions; however, it follows the variability of zonal and meridional salt advection in the equatorial and Angola regions, respectively. Our results show that the time-averaged spatial distribution of NEMO offline heat and salt advection terms compares much better to PREFCLIM horizontal advection terms than the online heat and salt advection terms. However, the seasonal cycle of horizontal advection in selected regions shows that NEMO offline terms do not always compare well with PREFCLIM, sometimes less than online terms. Despite this difference, these results suggest the important role of small-scale variability in mixed-layer heat and salt budgets.

Leading-Mode Connections of the Interannual Variability in Upper-Ocean Salinity in the Tropical Indian Ocean

Abstract Leading modes of interannual variability in upper-ocean salinity in the tropical Indian Ocean (TIO) and their connections were studied based on 17 years (2002–18) of oceanic historical and reanalysis data. Empirical orthogonal function (EOF) analysis depicted the dominant roles of the first two leading modes in salinity variability in the TIO over a wide range of interannual time scales. Among the rich oscillations of the leading EOF modes, a coherent near-biennial band was identified with basinwide loading of sea surface salinity anomalies (SSSa) (EOF1) leading/lagging the northeast–southwest dipolar mode of SSSa (EOF2) by around 4 months across the TIO, with southwestward migration of SSSa center. The spatial loadings of the SSSa leading modes in the TIO were strongly shaped by sea surface temperature–related freshwater fluxes and wind-driven regional ocean circulation on a near-biennial time scale. Composite analysis of the mixed layer salinity budget reflected characteristic features of basin-scale ocean–atmosphere coupling, both temporally and regionally during the life cycle of the near-biennial fluctuation in anomalous salinity in the TIO. Consistent with the intrinsic oscillation paradigm in the observed Indian Ocean dipole (IOD) variation, the dynamic and thermodynamic feedbacks associated with switches from the positive to negative IOD modes provided the phase-connection mechanisms for the SSSa leading-mode displacement over the TIO. Significance Statement This study investigates the leading modes of interannual variability in upper-ocean salinity in the tropical Indian Ocean (TIO). The intrinsic oscillation and associated dynamic and thermodynamic feedbacks over the TIO drive the basinwide connections of upper-ocean salinity variability. Our results show that a coherent near-biennial band is identifiable within the leading modes of sea surface salinity anomalies (SSSa), in which the wind-induced horizontal advections and evaporation-minus-precipitation anomalies associated with the switches from positive to negative Indian Ocean dipole modes mainly provide the phase-transition mechanism of SSSa. This research illustrates substantial evidence for the displacement of basin-scale sea surface temperature anomalies modulating the structures of SSSa and inducing the dynamical connections of leading modes of SSSa on the near-biennial time scale.

Asymmetric Response of Sea Surface Salinity to Extreme Positive and Negative Indian Ocean Dipole in the Southern Tropical Indian Ocean

AbstractBased on the Argo and multi‐satellite observations, this study investigates the asymmetric response of sea surface salinity (SSS) to two extreme positive and negative Indian Ocean Dipole (IOD) events in the southern tropical Indian Ocean (TIO). During positive IOD events, the easterly wind anomalies trigger zonal freshwater advection along the equator, decreasing SSS, while cold sea surface temperature (SST) depress the convective precipitation in the southeastern TIO (SETIO), increasing the SSS. Contrast SSS anomalies along the equator and in the SETIO show opposite phases during positive and negative IOD events. These SSS anomalies have enhanced in recent years, reaching the strongest in the extreme negative IOD in 2016 and positive IOD in 2019. The case study shows that positive SSS anomalies occurred in the southern TIO during both positive and negative IOD events in 2016 and 2019, suggesting significant asymmetry in SSS. Analysis of the mixed layer salinity budget shows that the local precipitation and the entrainment associated with the upwelling Rossby waves played an essential role in the SSS increase in 2016, while the decreased precipitation and horizontal advection caused by the westward South Equatorial Current contributed to the SSS increase in 2019. Further analyses suggest that SSS response is more sensitive to thermocline shoaling than deepening, and SSS response to precipitation is greater at shallower mixed layer than at deeper ones, which is the main reason for the asymmetric response of the SSS to negative and positive IOD events.

The Barrier Layer Effect on the Heat and Freshwater Balance from Moored Observations in the Eastern Pacific Fresh Pool

Abstract Formation and evolution of barrier layers (BLs) and associated temperature inversions (TIs) were investigated using a 1-yr time series of oceanic and air–sea surface observations from three moorings deployed in the eastern Pacific fresh pool. BL thickness and TI amplitude showed a seasonality with maxima in boreal summer and autumn when BLs were persistently present. Mixed layer salinity (MLS) and mixed layer temperature (MLT) budgets were constructed to investigate the formation mechanism of BLs and TIs. The MLS budget showed that BLs were initially formed in response to horizontal advection of freshwater in boreal summer and then primarily maintained by precipitation. The MLT budget revealed that penetration of shortwave radiation through the mixed layer base is the dominant contributor to TI formation through subsurface warming. Geostrophic advection is a secondary contributor to TI formation through surface cooling. When the BL exists, the cooling effect from entrainment and the warming effect from detrainment are both significantly reduced. In addition, when the BL is associated with the presence of a TI, entrainment works to warm the mixed layer. The presence of BLs makes the shallower mixed layer more sensitive to surface heat and freshwater fluxes, acting to enhance the formation of TIs that increase the subsurface warming via shortwave penetration.

Mixed Layer Salinity Balance in the Eastern Tropical Indian Ocean

AbstractThe mixed layer salinity (MLS) balance in the eastern tropical Indian Ocean is investigated using observations and output from the Estimating the Circulation and Climate of the Ocean model. The results suggest that the annual mean MLS is determined by the balance between the surface freshwater forcing and ocean advection. The seasonal MLS variation in the eastern equatorial Indian Ocean is driven strongly by salt advection associated with the Wyrtki Jet in spring and fall despite weak seasonal cycles of precipitation and evaporation over this region. Off the south Java coast, the MLS is the lowest in summer, due to the advection of freshwater southward associated with the meridional currents, whereas it reaches the maximum in fall, due to strong evaporation. During the negative Indian Ocean Dipole (IOD), the anomalous eastward currents transported high‐salinity water to the east, generating positive MLS anomalies in the eastern tropical Indian Ocean. The regional evaporation minus precipitation anomalies were negative, which reduced the MLS. The MLS balance during the positive IOD is found to be nearly a mirror reflection of that during the negative IOD.

The Daily‐Resolved Southern Ocean Mixed Layer: Regional Contrasts Assessed Using Glider Observations

AbstractWater mass transformation in the Southern Ocean is vital for driving the large‐scale overturning circulation, which transports heat from the surface to the ocean interior. Using profiling gliders, this study investigates the role of summertime buoyancy forcing and wind‐driven processes on the intraseasonal (1–10 days) mixed layer thermohaline variability in three Southern Ocean regions southwest of Africa important for water mass transformation—the Subantarctic Zone (SAZ), Polar Frontal Zone (PFZ), and Marginal Ice Zone (MIZ). At intraseasonal time scales, heat flux was shown as the main driver of buoyancy gain in all regions. In the SAZ and MIZ, shallow mixed layers and strong stratification enhanced mixed layer buoyancy gain by trapping incoming heat, while buoyancy loss resulted primarily from the entrainment of cold, salty water from below. In the PFZ, rapid mixing linked to Southern Ocean storms set persistently deep mixed layers and suppressed mixed layer intraseasonal thermohaline variability. In the polar regions, lateral stirring of meltwater from seasonal sea‐ice melt dominated daily mixed layer salinity variability. We propose that these meltwater fronts are advected to the PFZ during late summer, indicating the potential for seasonal sea‐ice freshwater to impact a region where the upwelling limb of overturning circulation reaches the surface. This study reveals a regional dependence of how the mixed layer thermohaline properties respond to small spatiotemporal processes, emphasizing the importance of surface forcing occurring between 1 and 10 days on the mixed layer water mass transformation in the Southern Ocean.

The Influence of Preexisting Stratification and Tropical Rain Modes on the Mixed Layer Salinity Response to Rainfall

AbstractThe freshwater input from rain to the surface ocean is a key component of the global water cycle. Frequent rainfall in the inter‐tropical convergence zone creates regions of strong surface stratification and low salinity, which vary seasonally. We evaluate how variations in rain type and preexisting upper ocean stratification influence the timing and duration of the salinity response to rainfall using the General Ocean Turbulence Model. A series of model simulations was run by prescribing three typical background stratification conditions and idealized rain and wind forcing that was consistent with observed convective, stratiform, and mixed convective and stratiform rainfall. Background stratification was assessed using underway CTD observations and rain forcing was identified from mooring observations collected in the eastern tropical Pacific during the second Salinity Processes in the Upper Ocean Regional Study. Model results show that strong stratification, whether preexisting or from convective rainfall, inhibits downward mixing of freshwater and allows near‐surface salinity anomalies to persist following rain. In contrast, when stratiform rain precedes convective rain, salinity anomalies are quickly mixed downward and longer lasting deeper in the mixed layer. This implies that accurately quantifying the salinity structure following rain should consider preexisting stratification and the type of rainfall. Furthermore, patterns of rainfall and stratification likely affect the bias between salinity observations at the surface and deeper in the mixed layer. Because satellite rain data do not correctly represent the small scales of rain forcing, the small‐scale surface salinity response to rain cannot be predicted from satellite data.

The Mixed Layer Salinity Budget in the Central Equatorial Indian Ocean

AbstractThe oceanic surface mixed layer salinity (MLS) budget of the central and eastern equatorial Indian Ocean during boreal fall and winter is studied using in situ and remote sensing measurements. Budgets on roughly 100 km scale were constructed using data from two DYNAmics of the Madden–Julian Oscillation and two Research Moored Array for African‐Asian‐Australian Monsoon Analysis and Prediction moorings near 79°E during September 2011 to January 2012. The horizontal advective salinity flux plays a significant role in the seasonal variation of equatorial MLS. In boreal fall, the equatorial and 1.5°S MLS increases due to horizontal advection and turbulent mixing, despite the freshening surface flux associated with MJOs. In boreal winter, with larger sub‐monthly variation and uncertainties, the decreasing of equatorial MLS is accounted by freshening zonal advection and surface flux, abated by salty meridional advection; the 1.5°S MLS is explained by the combination of freshening meridional advection and surface flux, and salty zonal advection. Budgets between 2011 and 2015 are investigated using data products from Tropical Rainfall Measuring Mission, Aquarius, Ocean Surface Current Analyses Real‐time, Objectively Analyzed air‐sea Fluxes, and Argo mixed layers over a wider region. The eastward development of the equatorial salinity tongue in the central to eastern Indian Ocean in boreal fall and the westward retreat in boreal winter are largely determined by the equatorial zonal current. The meridional migration of ITCZ rainfall plays a secondary role. In order to improve model prediction skills of MLS changes in the equatorial Indian Ocean, both zonal and meridional salinity advective fluxes, at a spatial scale of 1° longitude and latitude and a timescale less than days, need to be properly simulated.

Eddy Induced Trapping and Homogenization of Freshwater in the Bay of Bengal

AbstractFreshwater from rivers influences Indian summer monsoon rainfall and tropical cyclones by stratifying the upper layer and warming the subsurface ocean in the Bay of Bengal (BoB). Here, we use in situ and satellite data with reanalysis to showcase how river water experiences a significant increase in salinity on sub‐seasonal timescales. This involves the trapping and homogenization of freshwater by a cyclonic eddy in the Bay. Using a specific example from 2015, river water is shown to enter an eddy along its attracting manifolds within a period of 2 weeks. This leads to the formation of a highly stratified subsurface layer within the eddy. When freshest, the eddy has the largest sea‐level anomaly, spins fastest, and supports strong lateral gradients in salinity. Subsequently, observations reveal a progressive increase in salinity inside the eddy within a month. In particular, salty water spirals in, and freshwater is pulled out across the eddy boundary. Lagrangian experiments elucidate this process, whereby horizontal chaotic mixing provides a mechanism for the rapid increase in surface salinity. A salinity budget also suggests that horizontal advection explains much of the change in mixed layer salinity. Further, the adjustment of this freshwater eddy triggers submesoscale dynamics which appear to be an integral part of the process of salinity homogenization. This pathway is distinct from vertical diffusive mixing and is likely to be important for the evolution of salinity in the BoB.

Multi-decadal changes in the South China Sea mixed layer salinity

A recently assembled South China Sea Physical Oceanographic Dataset provides the first observational evidence for mixed layer salinity changes in the South China Sea from 1960 to 2015. During this period, the mixed layer waters freshened by 0.22 psu. The mixed layer salinity variability is found to be in sync with the Pacific Decadal Oscillation; it freshened in the 1960s, started to salinify in 1974, freshened again from 1993, and then salinified once again from 2012, with linear trends of − 0.019, 0.020, and − 0.024 psu/year, respectively. A box-average salinity budget analysis shows that the surface forcing, horizontal advection, and vertical entrainment terms together can, to a large degree, explain the observed trend in mixed layer salinity. The mixed layer freshening is driven by weakened surface fresh water loss and saline water transport, while salinification is associated with enhanced surface freshwater loss and salt transport through the Luzon Strait. The long-term mixed layer salinity changes affect the stratification, inducing a thinner mixed layer and stronger barrier layer during freshening periods that favor stronger regional ocean–atmosphere interaction.

Interannual variability of parameters of the Arctic Ocean surface layer and halocline

As the area of the sea ice cover has considerably reduced in the recent years, the Arctic Ocean surface layer has become more exposed to the effect of atmospheric processes. In order to evaluate the influence of the new conditions on the surface layer state, a large array of winter oceanographic data was used to calculate the Arctic Ocean mixed layer thickness and salinity along with the thickness and salinity of the halocline layer for the 1950-2013 period. Analysis of the interannual variability of the surface layer parameters listed has shown that the mixed layer thickness has increased in recent years, from 30-35 m in 1950-70s to 40-44m in 1980-2000s. The halocline thickness, conversely, has decreased from 65m in 1950-90s to 50 in the 2000s. At the same time, mixed layer salinity and the salinity of the halocline layer has become lower in the 2000s by about 0,5 % compared with 1950-90s. Three periods with distinctive configurations of the fields of the surface layer parameters have been identified. These quasi-20-year periods correspond to different climatic stages with different regimes of the atmospheric circulation. However, this is certainly not the only cause of the changes that occur in the surface layer condition. To reveal other causes of these changes, statistical methods were used to analyze the time series of the Arctic Ocean surface layer parameters. Linear regression equations allowed us to find out that the variability of the mixed layer thickness and salinity is almost equally determined by changes of the atmospheric circulation and fluctuations of the water exchange with the neighboring oceans along with river runoff and ice processes. At the same time, the thickness and salinity of the halocline layer strongly correlate with mixed layer parameters and, as there is no direct contact with the atmosphere, their variability is determined mostly by the mixed layer condition and the underlying Atlantic waters.

Decadal Variability in Salinity of the Indian Ocean Subtropical Underwater During the Argo Period

AbstractSalinity of the Indian Ocean Subtropical Underwater (IOSTUW) showed robust decadal variability during the Argo period, with a freshening trend between 2005 and 2013 and a salinification trend between 2014 and 2019. The salinity variability of the IOSTUW originated from both the sea surface salinity maximum region in the subtropical South Indian Ocean and the southeastern tropical Indian Ocean. The mixed layer salinity variabilities in these two regions are dominated by horizontal advection, which is mainly influenced by anomalous local surface currents and salinity anomalies from the western tropical Pacific and Maritime Continent. Both of these two factors have a close relationship with the El Niño‐Southern Oscillation (ENSO) signal. As a result, the mixed layer salinity in those two regions changed dramatically during the strong and long‐lasting ENSO events and thus led to the decadal variability in salinity of the IOSTUW.