A distinct freshening trend has been observed in the eastern South Indian Ocean (ESIO) since the early 2000s. However, analysis of recent satellite and in-situ observations indicates that this trend reversed around 2011, initiating a decade-long salinification that persisted until late 2020. Further analysis of these observations reveals substantial sea surface salinity (SSS) variability, suggesting that the observed decadal trends are simply manifestations of the region’s inherent variability rather than persistent long-term changes. To investigate the underlying processes driving this SSS variability, the present study provides a quantitative assessment of the mixed layer salinity budget, based entirely on observational data. The results demonstrate that the SSS tendency in the ESIO is primarily governed by the interplay between surface freshwater flux and horizontal advection, with the Indonesian throughflow likely playing an important role. On interannual time scales, the SSS tendency is tightly linked to the El Niño and Southern Oscillation, highlighting the potential influence of Pacific variability through both atmospheric and oceanic pathways. In contrast, the decadal SSS tendency is relatively minor and not significantly distinguishable from the budget residual, reflecting strong influence of unresolved small-scale processes and uncertainties in the data.

<p>We introduce a new version of the multi-mission sea surface salinity (SSS) optimum interpolation analysis (OISSS) which combines observations from NASA’s AQUARIUS/SAC-D and SMAP (Soil Moisture Active-Passive) satellite missions into continuous and consistent SSS data record. The dataset covers the period from September 2011 to present. Measurements from ESA’s SMOS (Soil Moisture and Ocean Salinity) satellite are used to fill gaps in SMAP observations during June-July 2019 and August-September 2022, when the SMAP satellite was in a safe mode and did not deliver scientific data. The analysis is based on Optimum Interpolation (OI), utilizes Level-2 (swath) data, and uses satellite-specific bias-correction algorithms to correct the satellite retrievals for large-scale biases.  The dataset includes uncertainty estimates, both formal and empirical. We use this dataset as an example to discuss requirements for the multi-mission SSS data products.</p> <p>To demonstrate its utility, the new dataset is used to characterize spatial patterns of SSS variability in the global ocean and on different time scales. The spatial pattern of the regional SSS trends show that the subtropical North Pacific is becoming fresher while the subtropical South Indian Ocean is becoming saltier. This is seemingly a part of a longer term oscillation as the trends are reversed compared to the preceding decade (2005-2015) estimated from Argo data. In particular, abrupt changes occurred during 2015, related, presumably, to a strong El Nino event of 2015-2016. The annual cycle is a dominant signal globally and can nicely be described by two leading empirical orthogonal functions (EOFs) explaining more than 35% of the total SSS variance. Except for the Indian Ocean, the oscillations are out of phase in the Northern and Southern Hemispheres and describe poleward propagation away from the Equator driven, presumably, by Ekman dynamics. The intra-seasonal signal is strongest in the tropics, particularly in the quasi-zonal bands associated with the Inter-tropical convergence zone (ITCZ) and South Pacific convergence zone (SPCZ), but also near outflows of major rivers, including the Amazon, Congo, Mississippi, Plata, Ganges and Brahmaputra.  Another region of interest is the northern North Atlantic, where satellite observations during the last decade have provided an unprecedented resource to study the spatial distribution and temporal evolution of SSS, allowing to observe areas typically not available by in-situ components of the ocean observing system. Here, the multi-mission SSS dataset is examined in its accuracy and appropriateness for studying SSS variability in high latitudes and marginal seas.</p> <p> </p>

Abstract This study provides a quantitative assessment of steric changes associated with sea level rise in the upper (0–500 m) South Indian Ocean (SIO) during 1993–2017, using the latest ocean state estimate of Estimating the Circulation and Climate of the Ocean (ECCO) combined with in‐situ observations from Argo. Both the observations and ECCO estimate show a sea level rise in the low‐latitude (0°–30°S) SIO that is faster than its South Pacific and South Atlantic counterparts by a factor at least two. Much of this fast sea level rise is due to warming and freshening of the upper ocean, with no significant contribution from the deeper layers (>500 m). We emphasize the importance of halosteric effect, whose contribution to sea level rise is comparable with that of thermosteric effect in the eastern basin. On interannual time scale, up to 90% of the region's sea level variability can be explained by steric changes that in turn are dominated by upper ocean convergence.

Sea surface salinity (SSS) observations from Aquarius, Soil Moisture and Ocean Salinity (SMOS), and Soil Moisture Active Passive (SMAP) satellite missions are compared to characterize the time and length scales of SSS variability globally. Overall, there is general agreement between the global patterns of the time and length scales of SSS variability estimated from the three satellite missions. The temporal scales of SSS variability vary from more than 90 days in the tropics to ~15 days in the Southern Ocean. The very short temporal scales (close to the Nyquist period) in some parts of the ocean are probably due to the high level of noise in the satellite data or the high noise-to-signal ratio. The longest temporal scales are observed along the South Pacific Convergence Zone (SPCZ) and in the central and western tropical Pacific. These areas are also related to the strongest ENSO-related signal in SSS. The processes governing the SSS variability and distribution are also non-stationary, such that the scales determined over different observation periods may differ. Dominant spatial scales of SSS variability are generally the longest (up to 150 km) in the tropics and the shortest (<60 km) in the subpolar regions. The distribution of the dominant spatial scales is not simply latitudinal but exhibits a more complex spatial pattern. In the tropics, there is slight east-west and inter-hemispheric asymmetry observed in the Pacific but absent in the other two oceans. The analysis also reveals that the length scales of SSS variability are highly anisotropic in the tropics (the zonal scales are generally shorter than the meridional ones) and become more isotropic towards higher latitudes. Regional differences in the estimates of the scales from the three satellite SSS datasets may arise due to differences in the observation duration, spatial resolution and/or different level of noise.

Seasonal cycle is the largest source of variability for sea surface salinity (SSS) and has a significant influence on the upper-ocean stratification and water-mass formation. The advent of the Argo...

We analyze the interannual variability and trends of the eddy heat transport (EHT) in the surface mixed layer of the Atlantic Ocean from covariances of sea surface temperature (SST) and geostrophic velocities from satellite observations between 1993 and 2018. The EHT is largest along the path of the Gulf Stream in the Northern Hemisphere and the vicinity of the Agulhas Retroflection and Argentine basin in the Southern Hemisphere. On average, meridional EHT in the mixed layer leads to a divergence of heat away from the subtropics in both hemispheres toward the equator and higher latitudes. Depending on latitude, the divergence of the EHT accounts for around 1%-5% of the atmospheric net surface heat flux, but reaches as much as 20% for certain regions. The EHT can be linked to eddy kinetic energy (EKE) and meridional SST gradients using mixing length hypothesis, but the variability of EKE and SST gradients are not enough to capture local EHT variability. For different latitudes, the EHT shows different behavior over time, with a strong increase in northward EHT in the Gulf Stream region, and a decadal oscillation of poleward EHT in the tropical and subtropical regions. This oscillation on larger scales is highly correlated with spatially averaged EKE and the large-scale SST gradients. A comparison with climate indices indicates a relation to Atlantic climate variability, especially the meridional modes of the Tropical Atlantic Variability and the North Atlantic Oscillation (NAO).

Observations of sea surface salinity (SSS) from NASA's Soil Moisture Active-Passive (SMAP) and ESA's Soil Moisture and Ocean Salinity (SMOS) satellite missions are used to characterize and quantify the contribution of mesoscale eddies to the ocean transport of salt. Given large errors in satellite retrievals and, consequently, SSS maps, we evaluate two products from the two missions and also use two different methods to assess the eddy transport of salt. Comparing the two missions, we find that the estimates of the eddy transport of salt agree very well, particularly in the tropics and subtropics. The transport is divergent in the subtropical gyres (eddies pump salt out of the gyres) and convergent in the tropics. The estimates from the two satellites start to differ regionally at higher latitudes, particularly in the Southern Ocean and along the Antarctic Circumpolar Current (ACC), resulting, presumably, from a considerable increase in the level of noise in satellite retrievals (because of poor sensitivity of the satellite radiometer to SSS in cold water), or they can be due to insufficient spatial resolution. Overall, our study demonstrates that the possibility of characterizing and quantifying the eddy transport of salt in the ocean surface mixed layer can rely on the use of satellite observations of SSS. Yet, new technologies are required to improve the resolution capabilities of future satellite missions in order to observe mesoscale and sub-mesoscale variability, improve the signal-to-noise ratio, and extend these capabilities to the polar oceans.

The interannual variability of the meridional eddy heat transport in the North Atlantic is investigated based on the combined use of satellite observations of sea level anomaly (SLA) and sea surface temperature (SST) over the 25 year period from 1993 to 2017. The focus is on 47 degrees N, a latitude close to the boundary between the subpolar and the subtropical gyres. The vertical structure of the eddy heat transport is reconstructed in 10 degrees longitude x 5 degrees latitude boxes from a composite analysis of Argo profile data and observations of mesoscale eddies in altimetric SLA. The transport is surface intensified with almost 95% happening above the pycnocline. It can vary immensely both zonally and vertically, in some cases reversing sign below the pycnocline. The basin-wide integrated eddy heat transport accounts for around 6% of the total meridional heat transport. During the observed period, the eddy heat transport increased on average by 0.6 TW/yr, accelerating to 1.3 TW/yr after 2005. The observed trends and variations in the eddy heat transport can be related to the large-scale temperature gradients in the region and in response to the North Atlantic Oscillation. They provide a considerable contribution to the variability of the total oceanic heat transport, suggesting a potentially important forcing mechanism in the ocean component of the climate system. Key Points Eddy heat transport across 47 degrees N in the subpolar North Atlantic is estimated from satellite altimetry, SST, and Argo profile data Eddy heat transport across 47 degrees N accounts for 6% of the total observed meridional heat transport Time variability and trends in the eddy heat transport account up to one third of the mean

Using newly available satellite observations of sea surface salinity (SSS), we provide, for the first time, a detailed and synoptic view of the spatiotemporal variability of SSS in the South China Sea (SCS). The results depict the SCS as a very dynamic region exhibiting variability over a broad range of time scales, from intraseasonal to interannual, with the seasonal cycle dominating (similar to 47% of the total SSS variance). The seasonal distribution of SSS has considerable latitudinal variations: the strongest variance across the southern SCS (similar to 5-12 degrees N), weaker in the northern part of the sea (north of similar to 18 degrees N), with the weakest seasonal SSS variability in between. The factors controlling patterns of seasonal SSS distribution are closely related to both the external freshwater forcing and ocean processes over the entire SCS monsoon system. The most active interannual SSS variability is found in the northeastern and eastern parts of the SCS, as well as the adjacent western Pacific. A significant basin-wide salinification began in summer of 2015, peaked in spring of 2016 with the averaged amplitude of up to 0.5 PSU, and maintained until the fall of 2016. Such persistent salinification during 2015-2016 following a strong El Nino event can be largely modulated by El Nino-related atmospheric and oceanic dynamics. The intraseasonal variability was found to be surprisingly weak throughout the SCS (the standard deviation <0.2 PSU), except for a few regions near the coast where it is likely related to the intraseasonal variability in the monsoon rainfall and subsequent variations in river runoff.

Sea surface salinity (SSS) observations from NASA’s satellite missions, Aquarius/SAC-D and Soil Moisture Active Passive (SMAP), are used to describe spatial patterns of the seasonal cycle, as well as intraseasonal and interannual variability, in the eastern tropical Pacific, the location of the second Salinity Processes in the Upper-ocean Regional Study (SPURS-2) field experiment. The results indicate that the distribution of SSS variance is highly inhomogeneous in both space and time. The seasonal signal is largest in the core of the Eastern Pacific Fresh Pool and in the Gulf of Panama. The interannual signal is highest in a relatively narrow zonal band along approximately 5°N, while the intraseasonal signal appears to be a dominant mode of variability in the zonally stretched near-equatorial region. Located right in the middle of a hotspot of high SSS variance, the SPURS-2 site appears to be at the crossroads of many different processes that shape the distribution of SSS in the eastern tropical Pacific and beyond.

Advances in L-band microwave satellite radiometry in the past decade, pioneered by ESA’s SMOS and NASA’s Aquarius and SMAP missions, have demonstrated an unprecedented capability to observe global sea surface salinity (SSS) from space. Measurements from these missions are the only means to probe the very-near surface salinity (top cm), providing a unique monitoring capability for the interfacial exchanges of water between the atmosphere and the upper-ocean, and delivering a wealth of information on various salinity processes in the ocean, linkages with the water cycle and climate, and constraints for ocean prediction models. The satellite SSS data are complimentary to the existing in situ systems such as Argo that provide accurate depiction of large-scale salinity variability in the open ocean but under-sample mesoscale variability, coastal oceans and marginal seas, and energetic regions such as boundary currents and fronts. In particular, salinity remote sensing has proven valuable to systematically monitor the open oceans as well as coastal regions up to approximately 40 km from the coasts . This is critical to addressing societally relevant topics, such as land-sea linkages, coastal-open ocean exchanges, research in the carbon cycle, near-surface mixing, and air-sea exchange of gas and mass. In this paper, we provide a community perspective on the major achievements of satellite SSS for the aforementioned topics, the unique capability of satellite salinity observing system and its complementarity with other platforms, uncertainty characteristics of satellite SSS, and measurement versus sampling errors in relation to in situ salinity measurements. We also discuss the need for technological innovations to improve the accuracy, resolution, and coverage of satellite SSS, and the way forward to both continue and enhance salinity remote sensing as part of the integrated Earth Observing System in order to address societal needs.

Aquarius was the first NASA satellite to observe the sea surface salinity (SSS) over the global ocean. The mission successfully collected data from 25 August 2011 to 7 June 2015. The Aquarius project released its final version (Version-5) of the SSS data product in December 2017. The purpose of this paper is to summarize the validation results from the Aquarius Validation Data System (AVDS) and other statistical methods, and to provide a general view of the Aquarius SSS quality to the users. The results demonstrate that Aquarius has met the mission target measurement accuracy requirement of 0.2 psu on monthly averages on 150 km scale. From the triple point analysis using Aquarius, in situ field and Hybrid Coordinate Ocean Model (HYCOM) products, the root mean square errors of Aquarius Level-2 and Level-3 data are estimated to be 0.17 psu and 0.13 psu, respectively. It is important that caution should be exercised when using Aquarius salinity data in areas with high radio frequency interference (RFI) and heavy rainfall, close to the coast lines where leakage of land signals may significantly affect the quality of the SSS data, and at high-latitude oceans where the L-band radiometer has poor sensitivity to SSS.

Abstract Good knowledge of mesoscale eddy properties and their spatial and temporal distribution in the world ocean is important for an accurate estimate of their role in heat, salt, and/or momentum transports. Composites of satellite sea level anomaly, used to evaluate internal eddy structure, commonly produce, at larger radii, a series of rings of alternating sign. We suggest that these secondary rings are not a part of the dynamical structure of the “central” eddy but are a rectified signature of the surrounding eddies. We show that statistical distribution of eddy polarities is not random but that an eddy of given polarity tends to be surrounded by eddies of opposite polarity. This observed spatial eddy polarity distribution, which is hypothesized to be a result of eddy self‐organization, is not only responsible for the secondary rings observed in the composite estimates but also causes a reduction of around 20% of the eddy composite amplitude and an increment of around 10% in the maximum rotational velocity, while it has negligible effect on the estimated eddy scale.

The mean vertical structure and transport properties of mesoscale eddies are investigated in the North Atlantic subtropical gyre by combining historical records of Argo temperature/salinity profiles and satellite sea level anomaly data in the framework of the eddy tracking technique. The study area is characterized by a low eddy kinetic energy and sea surface salinity maximum. Although eddies have a relatively weak signal at surface (amplitudes around 3-7 cm), the eddy composites reveal a clear deep signal that penetrates down to at least 1200 m depth. The analysis also reveals that the vertical structure of the eddy composites is strongly affected by the background stratification. The horizontal patterns of temperature/salinity anomalies can be reconstructed by a linear combination of a monopole, related to the elevation/depression of the isopycnals in the eddy core, and a dipole, associated with the horizontal advection of the background gradient by the eddy rotation. A common feature of all the eddy composites reconstructed is the phase coherence between the eddy temperature/salinity and velocity anomalies in the upper similar to 300 m layer, resulting in the transient eddy transports of heat and salt. As an application, a box model of the near-surface layer is used to estimate the role of mesoscale eddies in maintaining a quasi-steady state distribution of salinity in the North Atlantic subtropical salinity maximum. The results show that mesoscale eddies are able to provide between 4 and 21% of the salt flux out of the area required to compensate for the local excess of evaporation over precipitation.

A persistent signature of coherent mesoscale eddies in sea surface salinity (SSS) is revealed by analyzing the relationship between satellite SSS and sea surface height (SSH) variability in an eddy-following reference frame. Our analysis focuses on mid-ocean eddies in two representative regions, the southern Indian Ocean and the North Atlantic subtropical gyre. The resulting composite averages reveal a clear signature of mesoscale eddies in satellite SSS with typical SSS anomalies of 0.03-0.05 psu. The spatial structure of eddy-induced SSS perturbations can be characterized as a superposition of a dipole structure, arising from horizontal advection of the background SSS gradient by eddy velocity field, and a monopole structure related to the eddy core. The observed relationships between SSS and SSH anomalies are used to provide a regional assessment of the role of mesoscale eddies in the ocean freshwater transport in the North Atlantic subtropical gyre.

The straits in Indonesia allow for low-latitude exchange of water between the Pacific and Indian Oceans. Collectively known as the Indonesian Throughflow (ITF), this exchange is thought to occur primarily via the Makassar Strait and downstream via Lombok Strait, Ombai Strait, and Timor Passage. The Sunda Strait, between the islands of Sumatra and Java, is a very narrow ( approximate to 10km) and shallow ( approximate to 20m) gap, but it connects the Java Sea directly to the Indian Ocean. Flow through this strait is presumed to be small, given the size of the passage; however, recent observations from the Aquarius satellite indicate periods of significant freshwater transport, suggesting the Sunda Strait may play a more important role in Pacific to Indian Ocean exchange. The nature of this exchange is short-duration (several days) bursts of freshwater injected into the eastern Indian Ocean superimposed on a mean seasonal cycle. The mean volume transport is small averaging about 0.1 Sv toward the Indian Ocean, but the freshwater transport is nonnegligible (estimated at 5.8 mSv). Transport through the strait is hydraulically controlled and directly correlates to the along-strait pressure difference. The episodic low-salinity plumes observed by Aquarius do not, however, appear to be forced by this same mechanism but are instead controlled by convergence of flow at the exit of the Strait in the Indian Ocean. Numerical model results show the fate of this freshwater plume varies with season and is either advected to the northwest along the coast of Sumatra or southerly into the ITF pathway.