Thermohaline Intrusions Research Articles

An inventory of salty and warm subsurface intrusions in the South Brazil Bight

Subsurface intrusions of salty and warm waters onto the shelf can potentially impact the thermodynamics and biogeochemistry of these relatively shallow water environments. Such thermohaline intrusions are usually associated with double diffusive processes and can enhance the vertical and lateral fluxes. In this work, we characterized subsurface intrusions of Tropical Waters (TW) at the South Brazil Bight (SBB) - a mid-latitude continental shelf, which were detected based on salinity vertical profiles from a historical dataset. We found the intrusions tend to follow the pycnocline and are relatively common year-round, occurring at 27% of the dataset, with a maximum salinity highly correlated with the maximum stratification. Intrusions have different along-shelf characteristics, varying with the shelf width and depth. North of São Sebastião Island (SSI) – at the central portion of the SBB –, intrusions were thicker, saltier, and shallower than the ones found at the south of SSI. These differences could be related to the variations of the Coastal Water (CW) and South Atlantic Central Water (SACW) volume along the shelf and throughout the year. We found that intrusions’ thickness presents an inverse correlation with stratification, resulting in thinner intrusions in more stratified environments. This inverse correlation suggests double-diffusively driven intrusions. We apply the Turner Angle method to investigate whether the environment is susceptible to double diffusive processes. This analysis suggests that thermohaline intrusions in the SBB might not be primarily double diffusive. However, the dominance of different double diffusion processes - diffusive convection (salt fingering) at the upper (lower) layer - suggests that these processes influence intrusion growth, steady-state, and erosion. Nonetheless, the mechanism responsible for the initial density perturbation remains unknown and further studies are needed to explore the relationship between the Brazil Current dynamics and thermohaline warm and salty subsurface intrusions.

A Glider View of the Spreading and Mixing Processes of Antarctic Intermediate Water in the Northeastern Subtropical Atlantic

The Antarctic Intermediate Water (AAIW), one of the most important global intermediate water masses, spreads over the world ocean. Its propagation limit at the Northeast Subtropical Atlantic is characterized by its encounter with the Mediterranean Water (MW), which presents dissimilar thermohaline properties. Previous studies of the AAIW in this region have been based on traditional oceanographic cruise observations, which were later complemented by observations using autonomous systems such as ARGO floats. However, these observations present limitations for the study of processes occurring at mesoscale and smaller scales. In this study, we used high-resolution observations made by cutting edge platforms such as underwater gliders. Specifically, a meridional glider section realized in spring 2016 between the islands of Madeira and Gran Canaria has been used. The temperature, salinity and dissolved oxygen minima have allowed the detection of the AAIW signal north of the Canarian archipelago and significantly westward from its main northward propagation pathway in this region. The results of this work have shown that the encounter of AAIW and MW generates thermohaline intrusions or interleaving layers. It is suggested that double diffusion processes may play a role in the development of these structures, which may be important for water masses mixing and, therefore, in determining the northward spreading boundary of AAIW. The use of the high-resolution glider observations combined with other data products is essential for the study of water masses and dynamics when relevant processes have a wide range of scales.

Overview of the New England shelf break acoustics (NESBA) experiment

A joint ocean acoustics experiment was conducting at the southern edge of the New England Shelf in spring 2021 to investigate the sound propagation and scattering effects of physical oceanographic (PO) processes, marine geological (GEO) features, and biology (BIO) in the shelfbreak area. The experiment consisted of a network of sound sources, hydrophone arrays, and physical oceanography, and biological survey equipment was established. The environmental processes of particular interest include shelfbreak fronts, thermohaline intrusions, shelf water streamers, and internal waves, along with seafloor depressions and slopes, submarine canyons, and variable seabed properties (GEO), as well as fish schooling and shoaling (BIO). With fixed propagation paths, acoustic fluctuations can be correlated with environmental variations within the observation network with a temporal and spatial scales ranging from minutes to weeks and up to 30 km. One of the primary experiment objectives was on adaptive sampling and tracking of acoustic sensitivity “hot spots” to improve the real-time joint ocean acoustics and circulation modeling that was conducted on a research vessel in the field. This talk will review the design concept of this ocean acoustic network and provide an overview the experiment results. [Work supported by the Office of Naval Research.]

Turbulent Heat Fluxes in a Mediterranean Eddy Quantified Using Seismic and Hydrographic Observations

Mediterranean eddies (meddies) play an essential role in transferring heat, salinity and momentum into the Atlantic Ocean. The rate of heat (and salt) flux from the meddy and its ultimate lifetime are key proxies to understanding how meddies impact the redistribution of heat and salt in the ocean system. A Mediterranean eddy was observed in the Gulf of Caidz in 2007 using seismic and hydrographic data. The spatial distribution of turbulent dissipation rates around the meddy is estimated from the seismically derived internal wave spectra subrange using fine-scale parameterization. Turbulent dissipated rates are lowest (10−11 W/kg) within the core of the meddy but rise by nearly two orders of magnitude at the upper and lower boundaries, where signs of double diffusive convection are observed. Along the left flank of the meddy, thermohaline intrusions and interleaving of water masses are found in inverted temperature and salinity profiles, transporting heat laterally from the warm core to the Atlantic water with a flux of around 470 Wm−2. The meddy presented in this study is shown to decay in 2 years, primarily due to the heat loss associated with thermohaline intrusions. For the first time, heat fluxes around the meddy and its lifetime are quantified using seismic oceanography data, and the methods proposed here can be applied to more seismic datasets in the global oceans.

Correlation between the fluctuations of underwater sound propagation and shelfbreak oceanography

A network of sound sources, hydrophone arrays and physical oceanography and biological survey equipment was established at the southern edge of the New England Shelf in May 2021 to investigate how the oceanic processes at shelfbreak regions affect underwater sound propagation. The ocean processes of particular interest include shelfbreak fronts, shelf water streamers, thermohaline intrusions, and internal waves along with other significant marine geological features and biological factors, such as seafloor slopes, submarine canyons, variable seabed properties, and fish schooling and shoaling. With fixed propagation paths, acoustic fluctuations can be correlated with environmental variations between the network nodes. The temporal scale of the acoustic measurements ranges from minutes to weeks, and the spatial coverage is up to 20–30 km. Adaptive sampling and tracking of acoustic sensitivity “hot spots” was also conducted during the experiment to assist real-time joint ocean acoustics and circulation modeling. This presentation will review the design concept of this ocean acoustic network experiment and provide an overview the preliminary results. [Work supported by the Office of Naval Research.]

Thermohaline interleaving induced by horizontal temperature and salinity gradients from above

In this work we show that horizontal gradients of temperature and salinity with compensating effects on density can drive thermohaline intrusion in the fluid layer below. Specifically, different types of double diffusive convection generate differential vertical fluxes from the top boundary, which then sustain horizontal temperature and salinity gradients within the bulk. Interleaving layers develop in the bulk and slope downward towards the cold fresh side, which are of the diffusive type. New layers emerge near the bottom boundary and shift the existing layers upward due to the density difference induced by the divergence of the vertical fluxes through the top surface. Detailed analyses reveal that the present intrusion is consistent with those in the narrow fronts, and both layer thickness and current velocity follow the corresponding scaling laws. Such intrusion process provides an extra path to transfer heat and salinity horizontally towards the cold and fresh side, but transfer the density anomaly towards the warm and salty side. These findings extend the circumstances where thermohaline intrusions may be observed.

Thermohaline Suppression of Upper Circumpolar Deep Water Eddies in the Ross Gyre

AbstractTemperature and salinity measurements of a warm‐core eddy at the northern flank of the Ross Gyre are analyzed for dominant mixing mechanisms. The eddy is centered at the depths of the Circumpolar Deep Water and carries heat towards the gyre. Vertical and horizontal heat fluxes out of the eddy associated with internal wave turbulent mixing and thermohaline intrusions are estimated. Upward internal wave turbulent heat flux is W, whereas, the lateral intrusive heat flux is of the order of W. The horizontal flux due to intrusions is suggested to be the dominant mechanism for eddy decay and yields an eddy lifetime of about 6 months. The thermohaline intrusion‐eddy suppression mechanism is proposed and shown to be effective in suppressing the eddy field at the northern flank of the Ross Gyre. This effect has important implications for setting the basin‐wide heat budget and regulating sea‐ice cover.

A Case Study of the Thermohaline Fine-scale Structure of the Sub-Arctic Front

ABSTRACT This paper analyzes the characteristics of the thermohaline fine-scale structure of the sub-Arctic front (SAF) in the Kuroshio–Oyashio confluence region using temperature, salinity, and current data obtained from surveys in June 2016. Thermohaline intrusion is found to be prevalent in the sea area of the SAF, occurring predominantly in frontal zones with depths less than 200 m and in North Pacific intermediate water (NPIW) layers with depths greater than 350 m. The intrusion in the SAF zones with depths less than 200 m has a higher intensity but smaller dimensions in the NPIW. According to our analysis, the thermohaline intrusion in the SAF sea zone is mainly attributable to the joint effect of double diffusion and flow instability. At depths of 0–350 m, the thermohaline intrusion is vulnerable to the influences of salt fingering, diffusive convection, and symmetric instability. At depths deeper than 350 m, diffusive convection is significant. In addition, shear instability might be a contributor. Considering that the effect of turbulent mixing is primarily governed by symmetric instability in the SAF sea areas, the flow instability characterized by a dominant contribution from symmetric instabilities might exert a more significant influence on thermohaline intrusion. The more distinct thermohaline staircases at layers deeper than 350 m, which are mainly produced by diffusive convection, have larger vertical scales than the staircases in other layers.

A view of the Brazil-Malvinas confluence, March 2015

The encountering of the subtropical Brazil Current (BC) and the subantarctic Malvinas Current (MC) along the western margin of the Argentine Basin forms the Brazil-Malvinas Confluence (BMC), one of the most intense open-ocean fronts in the world ocean and a site for the formation of intermediate water masses. Here, we provide a comprehensive description of the BMC based on physical and biogeochemical data – hydrographic stations, profiling floats and subsurface drifters – gathered in March 2015. We use these data in order to characterize the impinging and outflowing currents and to describe the cross- and along-frontal thermohaline structure. In addition, we compare the in-situ measurements with both climatological data and the Mercator Ocean eddy-resolving reanalysis. The hydrographic sections illustrate the contrasting properties between the two western boundary currents: warm, salty, nutrient- and oxygen-poor oligotrophic subtropical waters carried southward by the BC and the cold, fresh, oxygen- and nutrient-rich subantarctic waters carried northward by the MC. The frontal system is also characterized by the presence of thermohaline intrusions, with the cross-frontal gradients and along-front velocities sharpening as the colliding currents shape the frontal system. We also observe brackish waters spreading on top of the frontal jet as a result of both the confluence dynamics and off-shelf advection favored by north-easterly winds. These low-salinity waters are positively correlated with surface ageostrophic speeds over the frontal jet. The cruise data illustrates the high regional and mesoscale variability as compared with climatological conditions, and further document the submesoscale subsurface complexity, which is not properly captured by available operational models.

Spring cold water intrusions as the beginningof the cold intermediate layer formation in the Baltic sea

Thermohaline intrusions are a typical feature of the Baltic Sea water body. We report observations of vivid colder/saltier intrusion activity in intermediate layers and upper pycnocline of the Gdansk Bay (the south-eastern part of the Baltic Sea) in early spring (March–April, 2013). Extremely low water temperature (down to 1.4–2 °C) and specific water salinity (SÃ7.5–7.8) of the intrusions point at a possible area of the intrusion waters formation: within the upper mixed layer in the Arkona/Bornholm basins at the end of March. Data on water temperature and salinity available from the International Council for the Exploration of the Sea (ICES) data repository, data of the Arkona Becken automated station, meteorological information, and remote sensing data for March–April 2013 are used to confirm the conclusions. It is confirmed once again that the transfer from the two-layered winter water stratification to the three-layered summer one in the Baltic Sea is a consequence of the sea-scale exchange process. The latter is manifested as simultaneous advection in upper and intermediate layers of waters with lower and higher salinity, correspondingly. The reported extremely cold intrusions indicate the beginning of the formation of the Cold Intermediate Layer (CIL), which thus contains waters of the Arkona/Bornholm basins. The main environmental factors driving the observed vivid intrusions in April 2013 are easterly winds and negative buoyancy fluxes due to seasonal solar heating of waters with the temperature below that of the density maximum. Substantial prolongation of the period of seasonal vertical mixing might be the most important process related to the transition of water temperature over the temperature of the density maximum (Tmd) in the Baltic Sea. With the climate warming, the cooling of surface waters below the Tmd becomes rare, limiting deep ventilation of Baltic waters in spring.

Turbulent Mixing in a Loop Current Eddy From Glider‐Based Microstructure Observations

AbstractTurbulence parameters have been estimated from microstructure velocity and temperature measurements in an anticyclonic Loop Current Eddy in the Gulf of Mexico for the very first time. Measurements were performed during a mission in October 2016, using a G2 Webb Research Slocum ocean glider instrumented with a microstructure sensors suite. Well‐defined turbulent kinetic energy dissipation rate patterns in the eddy and the surface mixed layer were clearly identified from both the velocity and temperature microstructure data. Higher dissipation rates were observed at the flanks and beneath the eddy core (up to 10−7 W/kg), while the core interior appeared to be very quiescent (~10−11 W/kg). Double diffusive thermohaline intrusions were detected on the side of the eddy, while the region below the eddy core was more conducive for salt fingering. Heat and salt fluxes associated with these processes suggest that a life span of such an eddy is ~1.5 years.

Evolution and Structure of the Kuroshio Extension Front in Spring 2019

Satellite data products and high-resolution in situ observations were combined to investigate the evolution and structure of the Kuroshio Extension Front in Spring 2019. The former reveals the variation of the front is influenced by the northward movement of the Kuroshio Extension through transporting warm and saline water to a cold and brackish water region. The latter indicates steep upward slopes of the isopycnals, tilting northward in the frontal zone, as well as several ~300 m thick blobs of North Pacific Intermediate Water between 26.25 and 26.75 kg/m3, where conspicuous thermohaline intrusions occur. Further analysis indicates these thermohaline intrusions prefer to alternate salt fingering and diffusive convection interfaces, and are affected by strong shears.

Temperature Spatiotemporal Correlation Scales in the Brazil‐Malvinas Confluence from High‐Resolution In Situ and Remote Sensing Data

AbstractOcean frontal systems may act both as barriers and mixers between different water masses, the latter thanks to very energetic structures with relatively short temporal and spatial scales. Here, we explore the high‐frequency temperature variability in the Brazil‐Malvinas Confluence through the joint analysis of novel high‐resolution SeaSoar measurements and sea surface temperature imagery. Surface spatiotemporal correlation scales range between 1.5 and 6 days and between 20 and 50 km, with the shortest scales along the shelf‐break path of the Brazil Current and over the confluence and the longest ones along the Malvinas Current. The spatial scales display minima along the front, at the surface because of the presence of brackish shelf waters and at the subsurface due to both mesoscale and submesoscale thermohaline intrusions. The smallest cross‐frontal vertical correlations, in the 5‐ to 10‐m range, are associated with submesoscale processes. Overall, temperature variability is enhanced at depth in the frontal system.

Origin and Attenuation of Mesoscale Structure in Circumpolar Deep Water Intrusions to an Antarctic Shelf

AbstractCross-isobath transport of Upper Circumpolar Deep Water (UCDW) provides a major source of heat to the Antarctic continental shelves. Adaptive sampling with a Slocum glider revealed that the UCDW regularly intrudes onto the western Antarctic Peninsula shelf within mesoscale eddies, and a linear stability analysis of the shelf-break current upstream confirmed eddy length scales and vertical structure are consistent with the baroclinic instability of the current. The properties of the most unstable mode are insensitive to current orientation but sensitive to bottom slope in accordance with modified Eady theory. Once on the shelf, the eddies’ core properties mix with ambient shelf water to form modified CDW (mCDW). Concurrent shipboard CTD and ADCP data are used to diagnose the responsible mixing processes and highlight the importance of thermohaline intrusions. The genesis mechanism of the interleaving layers cannot be confirmed, however a simple analytic model suggests the upper limit contribution of advection by internal waves cannot account for the observed temperature variance unless the cross-eddy temperature gradient is large. Data-adaptive sampling of an eddy with the glider revealed it lost heat across two isopycnals and a fixed radius at a rate of 7 × 109 J s−1 over 3.9 days. This rate is corroborated by a diffusion model initialized with the eddy’s initial hydrographic properties and informed by the heat fluxes parameterized from the shipboard data. The results suggest eddies predominately lose heat laterally and downward, which preserves subsurface heat for melting of marine-terminating glaciers.

Double‐Diffusive Layering in the Canada Basin: An Explanation of Along‐Layer Temperature and Salinity Gradients

AbstractDouble‐diffusive mixing gives rise to layered structures in the Arctic Ocean: layers within a double‐diffusive staircase overlying deeper layers associated with thermohaline intrusions. These layers exhibit well‐defined lateral temperature and salinity gradients. Gradients in salinity along individual layers change sign with depth, while along‐layer gradients in temperature remain the same sign with depth. A theoretical formalism is put forward to explain these features in terms of vertical divergences of double‐diffusive fluxes; temperature and salinity gradients along layers are set by the depth‐dependent ratio of double‐diffusive heat to salt fluxes. Examination of fine structure in temperature and salinity profiles reveals how the net flux ratio depends upon whether the layer is part of an evolving thermohaline intrusion or a staircase. The physical framework in context with observations of varying along‐layer gradients in temperature and salinity provides evidence for thermohaline intrusions evolving to a staircase and describes the parameters that dictate this process. Results bring new understanding to heat and salt transport in the Arctic Ocean as well as the physics of double‐diffusive layering in the world's oceans.

Inverse Modeling the Brazil‐Malvinas Confluence

AbstractThe Brazil‐Malvinas Confluence arises from the frontal encountering of the subtropical Brazil Current and subantarctic Malvinas Current. It displays a complex regional circulation that is accompanied by mesoscale features and thermohaline intrusions. Here we combine altimetry and cruise data to describe the circulation pattern in the upper 2,000 m at two spatial scales encircling the frontal system. The major regional features appear south of the confluence latitude at 39–40°S: (a) a relatively weak Malvinas Current near 41°S, 56°W (28.3 ± 1.4 Sv), followed by its cyclonic retroflection; (b) an intense subtropical anticyclone (59.3 ± 10.7 Sv) that replaces the Brazil Current overshoot; and (c) a very intense subantarctic inflow (78.9 ± 13.7 Sv) near 53°W that is maintained through both an upstream (near 42°S) earlier diversion of the Malvinas Current and the cyclonic recirculation of the flow exiting east along the confluence. North of the confluence, the Brazil Current provides a net input of 30.8 ± 12.0 Sv (29.1 ± 8.3 Sv along the slope). The southern inflow splits nearly equal between barotropic and baroclinic contributions while the entire northern flow is essentially baroclinic. These northern and southern inputs add to an eastward along‐front transport of 109.7 ± 15.1 Sv, with significant contribution of highly oxygenated, relatively fresh Subantarctic Mode and Antarctic Intermediate Waters (58.7 ± 5.6 Sv). The regional circulation experiences substantial temporal variability, with southern waters flowing into the Brazil‐Malvinas Confluence through along‐slope and interior pathways and partly recirculating within the subtropical South Atlantic gyre.

Microstructure Observations of Turbulent Heat Fluxes in a Warm-Core Canada Basin Eddy

AbstractAn intrahalocline eddy was observed on the Chukchi slope in September of 2015 using both towed CTD and microstructure temperature and shear sections. The core of the eddy was 6°C, significantly warmer than the surrounding −1°C water and far exceeding typical temperatures of warm-core Arctic eddies. Microstructure sections indicated that outside of the eddy the rate of dissipation of turbulent kinetic energy ε was quite low . However, at the edges of the eddy core, ε was elevated to . Three different processes were associated with elevated ε. Double-diffusive steps were found at the eddy’s top edge and were associated with an upward heat flux of 5 W m−2. At the bottom edge of the eddy, shear-driven mixing played a modest role, generating a heat flux of approximately 0.5 W m−2 downward. Along the sides of the eddy, density-compensated thermohaline intrusions transported heat laterally out of the eddy, with a horizontal heat flux of 2000 W m−2. Integrating these fluxes over an idealized approximation of the eddy’s shape, we estimate that the net heat transport due to thermohaline intrusions along the eddy flanks was 2 GW, while the double-diffusive flux above the eddy was 0.4 GW. Shear-driven mixing at the bottom of the eddy accounted for only 0.04 GW. If these processes continued indefinitely at the same rate, the estimated life-span would be 1–2 years. Such eddies may be an important mechanism for the transport of Pacific-origin heat, freshwater, and nutrients into the Canada Basin.

The Relationship between Double-Diffusive Intrusions and Staircases in the Arctic Ocean

AbstractThe origin of double-diffusive staircases in the Arctic Ocean is investigated for the particular background setting in which both temperature and salinity increase with depth. Motivated by observations that show the coexistence of thermohaline intrusions and double-diffusive staircases, a linear stability analysis is performed on the governing equations to determine the conditions under which staircases form. It is shown that a double-diffusive staircase can result from interleaving motions if the observed bulk vertical density ratio is below a critical vertical density ratio estimated for particular lateral and vertical background temperature and salinity gradients. Vertical background temperature and salinity gradients dominate over horizontal gradients in determining whether staircases form, with the linear theory indicating that perturbations to stronger vertical temperature gradients are more likely to give rise to a staircase. Examination of Arctic Ocean temperature and salinity measurements indicates that observations are consistent with the theory for reasonable estimates of eddy diffusivity and viscosity.

Physical Properties of Persian Gulf Outflow Thermohaline Intrusion in the Oman Sea

Various CTD data obtained in the Oman Sea are analyzed to explain structural features of intrusive layering. Special attention is compensated to thermohaline intrusions observed in layers (depth ranges of 150 m to 450 m, 150 m to 350 m, 100 m to 350 m and 150 m and 400 m in the winter, spring, summer and autumn, respectively). The temperature and salinity profiles in thermohaline intrusion have sawtooth structure in all stations, while they have step structure in density field. Based on interpretations, detailed estimates of thickness are about 10 to 20 meters. The T-S diagrams show the positions of the outflow intrusion with different thicknesses and depths for all seasons in the Oman Sea. Vertical profiles of temperature and salinity show two boundaries in the upper and lower layers. They are prone to double diffusive convection. Salt fingering and diffusive convection can be seen in both the upper and lower boundaries, and salt fingering is stronger at the lower boundary. Diffusive convection also is visible from the surface to the mid-depth of the plume outflow, and the diffusive intrusion is more severe at the upper boundary than the surface and deep. The intensity of double diffusion in the bottom border is greater than the upper boundary. At the deeper parts, the stratification is completely stable. Variations of the positions of greatest salinities in different diagrams are due to changing water masses for different locations and depths and paths of intrusive flow.

Thermohaline intrusions in the thermocline of the western tropical Pacific Ocean

The existing high-resolution hydrographic data in the western tropical Pacific Ocean are used to explore the spatial distribution and primary characteristics of thermohaline intrusions in the thermocline. Statistics show that the vertical scales of intrusions are 20-40 m in the upper thermocline (22.0-26.0 sigma(0)) and 40-80 m in the lower thermocline (26.0-27.2 sigma(0)). In the upper thermocline, the most intensive intrusions exist at the equatorial front (EF) where north/south Pacific water masses converge, and the westward spreading of the north Pacific tropical water (NPTW) in the Philippines Sea also produces patches of intrusions surrounding its high-salinity tongue. In the lower thermocline, intrusions are also strong at the tropical front (TF) which is the boundary between the north Pacific subtropical/tropical waters. At the bottom of the thermocline (at about 27.0 sigma(0)), intrusions mainly exist near the western boundary, which are produced by intermediate water convergence through the advection of subthermocline western boundary flows. Most strikingly a "C"-shape distribution of intrusions at around 26.4 sigma(0) is revealed, covering the vicinity of the EF, the TF, and the Mindanao Current (MC), i.e., the western boundary pathway of the north Pacific subtropical cell (STC). Synoptic section analysis reveals that intrusions are more prominent on the warm/salty flank of the fronts, implying more cross-front tongues of cold/fresh water. Among the intrusions, those at the EF are of best lateral coherence which implies a unique driving mechanism involving near-inertial velocity perturbations near the equator.