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

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

On the Advection of Upwelled Water on the Western Yucatan Shelf

Upwelling events over the Yucatan Shelf are an important physical phenomenon to the region. They typically happen during spring and summer and had been studied for some time with a primary focus on the development on the eastern side of the shelf and later transport to the central part of the Peninsula. There has been very little effort looking at the impact of upwelling on the western shelf, on the Campeche side. Using a combination of observations and modeling from 2018, we show evidence for the first time, of the presence of upwelled water on the western side. Particle tracking, integrated back-in-time, was used to identify the origin of the upwelled water. Our results show that Caribbean Subtropical Underwater was brought from the northeast shelf, over 500 km away from the study area, by advection. This water took over a month (40 days) to arrive at the study region, traveling along-shelf with an average velocity of 14.5 cm/s. In the nearshore waters off the Campeche Coast, Caribbean Tropical Water was underlain by upwelling Caribbean Subtropical Underwater. Monthly averaged sea surface temperature (SST) anomalies from a 39-year time series suggest that upwelled water off Campeche is a regular phenomenon during summer, while the recurrence of westward advection is supported by climatological Lagrangian Coherent Structures. More studies are needed to explore the frequency of occurrence and impact of these events on the western shelf.

Interannual variability in the subduction of the South Atlantic subtropical underwater

The South Atlantic subtropical underwater (STUW) is a high-salinity water mass formed by subduction within the subtropical gyre. It is a major component of the subtropical cell and affects stratification in the downstream direction due to its high salinity characteristics. Understanding the interannual variability in STUW subduction is essential for quantifying the impact of subtropical variability on the tropical Atlantic. Using the output from the ocean state estimate of the Consortium for Estimating the Circulation and Climate of the Ocean (ECCO), this study investigates the interannual variability in STUW subduction from 1992 to 2016. We find that heat fluxes, wind stress, and wind stress curl cause interannual variability in the subduction rate. Heat fluxes over the subduction area modulate the sea surface buoyancy and regulate the mixed layer depth (MLD) during its deepening and shoaling phases. Additionally, the wind stress curl and zonal wind stress can modulate the size of the subduction area by regulating the probability of particles entrained into the mixed layer within 1 year of tracing. This analysis evaluates the influence of subtropical wind patterns on the South Atlantic subsurface high-salinity water mass, highlighting the impact of heat and wind on the interannual changes in the oceanic component of the hydrological cycle.

Tracking the marine migration routes of South Pacific silver eels

It is still a mystery how catadromous eels find their way through the seemingly featureless open ocean to their spawning areas. Three catadromous Pacific eels (2 Anguilla marmorata, 1 A. megastoma) from the Archipelago of Vanuatu were tagged with pop-up satellite archival transmitters, and their migration tracks towards their presumed spawning area approximately 870 km northeast of the point of release were reconstructed in order to evaluate their movements in relation to oceanographic conditions. We used the timing of diel vertical migrations to derive the eels’ positions. The 2 A. marmorata exhibited steep-angled turns resulting in a zig-zag migration path along the east-west axis, while the A. megastoma took a relatively straight course towards the presumed spawning area. They migrated with a speed over ground of 21-23 km d-1. In this region, the eastward flow of the South Equatorial Counter Current (SECC, ~5°-10°S) separates the westward flowing South Equatorial Current (SEC; ~0°-5°S and 10°-18°S) into 2 branches. During shallower nighttime migration depths around 150 m, eels crossed a variable flow field through the southern branch of the westward SEC with westward propagating mesoscale eddies and the eastward SECC, but stayed south of the stronger northern branch of the SEC, possibly increasing retention time of larvae within this area. The eels headed towards a tongue of high-salinity Subtropical Underwater (STUW). The eels did not move beyond a salinity front of 35.9-36.0 at a depth of 100-200 m, which may have provided cues for orientation towards the spawning area.

Vertical Structure of the Water Column at the Virgin Islands Shelf Break and Trough

The steep US Virgin Islands Shelf Break (VISB) and the Virgin Islands Trough (VIT) at the Northeastern Caribbean Sea comprise a dynamic region of the Atlantic Ocean. In situ oceanographic data collected in the region during April 2017 were used to examine the spatial variability in temperature, density, salinity, and relative Chlorophyll-a. Analysis of data from the upper 300 m of the water column, that include deep and shallow water stations in the shelf break region, shows strong stratification of the water column. Stations shallower than 800 m along the shelf break are more variable in temperature, density, and salinity than those that are deeper than 800 m along the trough. For shallow stations, the mixed layer depth deepens along-shelf from West to East while at the deep stations the opposite occurs. Salinity maxima exhibit more variability in depth and range of values in the shallow stations compared to deep stations. Six different types of water masses that contribute to the strong stratification in the region were identified in our study: Caribbean Surface Water, Subtropical Underwater, Sargasso Sea Water, Tropical Atlantic Central Water, Antarctic Intermediate Water, and North Atlantic Deep Water. The upper level Caribbean Surface Water, Subtropical Underwater, and Sargasso Sea Water are present in shallow stations, indicating potential meridional intrusions from the VIT to the VISB which may not be resolved by current ocean circulation models and are not captured in satellite data. The analysis presented here indicates that competing physical processes may be controlling the vertical structure of the water column in the region and merit further examination.

Hydrography of the Gulf of Mexico Using Autonomous Floats

ABSTRACTFourteen autonomous profiling floats, equipped with CTDs, were deployed in the deep eastern and western basins of the Gulf of Mexico over a four-year interval (July 2011–August 2015), producing a total of 706 casts. This is the first time since the early 1970s that there has been a comprehensive survey of water masses in the deep basins of the Gulf, with better vertical resolution than available from older ship-based surveys. Seven floats had 14-day cycles with parking depths of 1500 m, and the other half from the U.S. Argo program had varying cycle times. Maps of characteristic water masses, including Subtropical Underwater, Antarctic Intermediate Water (AAIW), and North Atlantic Deep Water, showed gradients from east to west, consistent with their sources being within the Loop Current (LC) and the Yucatan Channel waters. Altimeter SSH was used to characterize profiles being in LC or LC eddy water or in cold eddies. The two-layer nature of the deep Gulf shows isotherms being deeper in the warm anticyclonic LC and LC eddies and shallower in the cold cyclones. Mixed layer depths have an average seasonal signal that shows maximum depths (~60 m) in January and a minimum in June–July (~20 m). Basin-mean steric heights from 0–50-m dynamic heights and altimeter SSH show a seasonal range of ~12 cm, with significant interannual variability. The translation of LC eddies across the western basin produces a region of low homogeneous potential vorticity centered over the deepest part of the western basin.