Abstract A theoretically operable but unvalidated method has been presented to observe the strength of the upwelling induced by global tropical cyclones (TCs); here, it is validated using observations from the daily 2-km-resolution level-3 Surface Water and Ocean Topography (SWOTl3) satellite data. The method quantifies the upwelling strength as the change of the sea surface height (SSH) extreme within a cyclonic ocean eddy (COE) or anticyclonic ocean eddy (AOE) identified from low-resolution gridded altimetry datasets. Although the artificial smoothness segment (ASS) always exists in the time series of an observed eddy SSH extreme, the method further argued that the maximum change in the ASS theoretically equals to the upwelling strength. To evidence its reliability, the time series of swath-mean SSH and the eddy extremes of three COEs and an AOE are derived from four gridded datasets. In sharp contrast to the abrupt decreases in SWOTl3-observed data, all the time series from the gridded datasets show the slow decrease near the typhoon’s passage and thus directly verify the existing ASSs. Using the method, the quantified upwelling strengths near 17°N are very consistent with the SWOTl3 observations, confirming its reliability. The quantified upwelling strengths in the Kuroshio near 27°N seem to be enhanced with the increasing quality of the gridded datasets. However, even in the Kuroshio, the upwelling strength quantified from the best available gridded dataset in recent years should still be reliable. The validated method will provide an effective tool to quantify global TC-induced upwelling. Significance Statement The upwelling induced by a tropical cyclone plays a key role in the effects of global tropical cyclones on ocean. However, a feasible method to quantify the upwelling strength is still absent for global tropical cyclones. A theoretically operable but unvalidated method has been presented. Here, this method is validated to be reliable using observations from the daily 2-km-resolution level-3 Surface Water and Ocean Topography satellite data.

Abstract Antarctic Intermediate Water (AAIW), occupying a broad region at intermediate depths in the Southern Hemisphere oceans, plays a crucial role in global heat and freshwater redistribution. However, its origin at the sea surface remains a subject of ongoing debate. A recently defined distance metric, which distinguishes similar water parcels, is applied to Argo data to trace the surface origins of the AAIW cores in each ocean basin. By examining the spatial distribution of distances to the AAIW cores, we identify that the Pacific and Atlantic AAIW cores primarily originate from localized surface regions, specifically near the Subantarctic Front in the southeast Pacific and the Falkland Plateau in the southwest Atlantic. Further investigation indicates that through subduction in the deep mixed layer, low-salinity water penetrates to intermediate depths in these regions. North of the subduction sites, water masses primarily circulate within the subtropical gyre in the South Pacific, whereas to the south, they are advected eastward by the Antarctic Circumpolar Current into the Atlantic Ocean. In the Indian Ocean, the AAIW core, distinguished by its relatively low oxygen concentration, originates both from eastward inflow carried by the Antarctic Circumpolar Current from the southeast Pacific and local surface sources near 90°E. Our findings confirm the reliability of the distance metric and emphasize the importance of localized physical processes in the penetration of AAIW into the ocean interior. Significance Statement Antarctic Intermediate Water (AAIW), found in Southern Hemisphere intermediate depths, is essential for global heat and freshwater distribution. Using a newly defined distance metric, it is found that AAIW originates primarily from the Subantarctic Front in the southeast Pacific and the Falkland Plateau in the southwest Atlantic, where low-salinity water subducts into deeper layers. In contrast, the Indian Ocean lacks this direct path, with its AAIW formed by the inflow of AAIW from the Atlantic via the Antarctic Circumpolar Current. These findings improve our understanding of AAIW origins and pathways within the ocean.

Abstract The overturning circulation in the subpolar North Atlantic and Nordic Seas is the product of a net densification by air–sea buoyancy forcing and mixing that transforms light Atlantic Water into dense North Atlantic Deep Water. Recent studies have shown that the magnitude of the time-mean overturning circulation as measured at the OSNAP (Overturning in the Subpolar North Atlantic Programme) mooring array can be attributed to the water mass transformation by air–sea fluxes and mixing to the north of the array. However, connecting subpolar overturning variability on seasonal timescales to seasonal variations in water mass transformation is contentious. Here, we show in observations and a state estimate that seasonal variability in water mass transformation by air–sea buoyancy fluxes and mixing predominantly affects the storage ofwater inwater mass classes within the subpolar basins, rather than driving seasonal changes in the overturning circulation at the OSNAP mooring array. Furthermore, we show that the seasonal formation and export of dense water from the Irminger and Labrador Seas cannot fully explain overturning seasonality at OSNAP. Instead, the seasonal cycle of overturning as measured at the OSNAP mooring array is the result of seasonal imbalances in the inflows and outflows of dense water across the array, linked to seasonal changes in the density field imprinted on the circulation of the subpolar gyre. This implies that intra-annual to seasonal dense water transport variability at OSNAP cannot be interpreted as an overturning circulation associated with the formation and export of dense water.

Abstract Ocean turbulence is highly intermittent, making statistical models crucial for interpreting sparse microstructure data and quantifying mixing rates. Classical theory predicts a log-normal distribution of oceanic turbulent dissipation and recent work suggested that a log-skew-normal distribution better describes microstructure data. In this work, it is shown—using turbulent dissipation data from the Brazil Basin and North Atlantic tracer release experiments—that the log-skew-normal fit systematically overestimates the largest mixing events near the ocean bottom. Instead, the data are well described by a mixture of two log-normal distributions: one with a small mode representing a population of relatively weak mixing events, and another with a larger mode representing a distinct population of more vigorous mixing events. The mixture model is consistent with a partitioned two-regime view of oceanic dissipation related to two dynamically different stratified turbulent processes. Several plausible (and largely consistent) hypotheses regarding the physical origins of two different pathways of mixing are discussed.

Abstract In the equatorial Indian Ocean, observations and the GLORYS dataset reveal significant intraseasonal variability of meridional velocity in the upper 500 m and near the bottom, primarily in the 10–30-day period band. These oscillations are associated with 10–20-day (biweekly) and 20–30-day (monthly) Yanai waves. The strongest biweekly energy occurs in the upper 100 m between 45°E and 95°E, where vertical mode-2 Yanai waves dominate and are primarily driven by meridional wind stress. Monthly Yanai-wave energy is enhanced in two source regions: (1) the offshore western boundary (45°E–55°E) in the upper 200 m, where mode-2 Yanai waves are excited by convergence of eddy kinetic energy; and (2) the surface layer between 55°E and 73°E, where meridional wind stress inputs energy, forcing predominantly mode-2 Yanai waves on a 20–30-day timescale. In addition, a subsurface trapping region for monthly Yanai waves lies between 55°E and 95°E at 50–500 m, collocated with a subsurface maximum of 20–30-day meridional kinetic energy and dominated by higher-order baroclinic modes (3–5). Yanai wave energy excited in the upper layer propagates eastward and downward, leading to bottom-enhanced oscillations near sloping seafloor regions, notably west of the Maldives (60°E–73°E) and near the Ninety East Ridge (85°E–95°E). Meridional wind stress is the primary energy source for Yanai waves in the surface layer, while barotropic instability and vertical pressure work drive Yanai waves in the deep ocean.

Abstract The dispersal of meltwater discharged from the Laurentian Channel (LC) is investigated from numerical experiments with an eddy-resolving model representing the western North Atlantic during the last ice age. Meltwater dispersal is simulated over a full summer, when glacial ablation rates were presumably the highest. In our experiments, meltwater forms a buoyant plume, which flows to the southwest along the continental slope owing to the Coriolis force. Four mechanisms of offshore export are identified. 1) Meltwater is carried seaward by Ekman currents driven by upwelling-favorable winds along the slope. 2) Part of it is entrained away from the slope by meander crests and warm-core rings of the Gulf Stream (GS) between the LC and Cape Hatteras. 3) The other part is generally diverted offshore by the GS near Cape Hatteras, where the GS leaves the slope. 4) Meltwater can be trapped in a GS meander trough that pinches off and produces a cold-core ring, leading to its penetration into the subtropical gyre. In turn, the buoyant plume has relatively small but noticeable effects on the GS. In the western, weakly meandering segment of the GS, the vertical shear in horizontal velocity is generally reduced due to the presence of melt (light) water along the inshore flank of the GS. Our results are discussed in light of (i) a two-layer theory of a surface density front subjected to background flow and wind stress and (ii) sediment records from the Laurentian Fan and the Sargasso Sea.

Abstract The current conceptual theory of the ocean meridional overturning circulation (MOC) describes an interaction between a re-entrant southern channel and a northern sinking region, coupled through a basin with uniform upwelling through flat isopycnals. Near the surface and bottom of the real ocean basin, sloping isopycnals induce flows that these models are unable to resolve by construction. Here, a model that relaxes the flat assumption made on the basin density is proposed, where the basin overturning is related to the meridional gradient of density through a local thermal wind balance. The simple 2D model is tested against idealized 3D simulations from a general circulation model and is found to reproduce the zonally averaged dynamics. Leveraging the low numerical cost of the model, the effects of vertical mixing and surface buoyancy forcing on the MOC are explored. Results show that the strength and depth of the middepth cell generally increase with vertical mixing, consistent with previous studies. For a fixed vertical mixing profile, the depth of the middepth cell primarily decreases with the buoyancy contrast across the entire meridional domain, while its strength primarily increases with the shared range of densities between the basin and channel.

Abstract The Cape Basin off the western coast of South Africa is abundant in mesoscale and submesoscale variability. Here, the Agulhas retroflection sheds eddies and filaments that bring warm, salty Indian Ocean water into contact with the cooler, fresher South Atlantic Ocean. Thermohaline interleaving is prevalent in the Cape Basin, presumably due to strong lateral water-mass gradients and eddy stirring. These intrusions are an important pathway to water-mass transformation because they result in an increased surface area over which diapycnal mixing can work to homogenize water property contrasts. This work presents the first observational study that tracks interleaving features at high vertical and horizontal resolution over distances of O (100) km in the Cape Basin using the Wire Flyer towed profiling vehicle and rapid profiling floats. Interleaving features are present throughout mode, thermocline, and intermediate waters ( σ = 26.0–27.5 kg m −3 ), but their scales and slopes vary significantly. Wire Flyer sections highlight strong differences in interleaving characteristics and generation mechanisms over distances of tens of kilometers. Several transects exhibit signatures of internal waves, which appear to modulate the interleaving structure. Profiling floats provide an alternative, semi-Lagrangian sampling perspective and show the persistence of interleaving over time scales of 1–20 days. Sections from both the Wire Flyer and profiling floats reveal that the interleaving strength can change abruptly over short distances of O (10) km. This work demonstrates the variety of physical processes existing at different length and time scales that contribute to the formation and structure of interleaving in the Cape Basin. Significance Statement This study reveals novel observations of interleaving in the unique and highly energetic environment of the Cape Basin. The Wire Flyer’s unparalleled capability to sample across strong currents and resolve interleaving characteristics in thermocline waters at unprecedented horizontal and vertical scales provides an objective snapshot of the ocean’s fine-scale structure at the periphery of mesoscale eddies. These observations should prompt a renewed discussion about the mechanisms driving the formation of interleaving, as well as future work on the complex interactions between the physical processes that contribute to interleaving, which is integral to the forward cascade of temperature and salinity variance in the ocean.

Abstract In the flat-bottomed open ocean, the seawater density distribution yields alone the existence of geostrophic baroclinic currents and steric sea-level spatial changes. However, at basin margins, the bathymetry exerts a considerable control on both these quantities. Indeed, the steric sea level vanishes at the coast where depth is zero. Also, continental slopes are vorticity barriers hindering convergence (divergence) of baroclinic transport toward the coast and accumulation (removal) of water there. In the limit of no temporal development, how the coastal sea level is impacted by open-ocean density changes is hence nontrivial and must involve ageostrophic mechanisms. Here, we focus on bottom friction as one such process, provide derivations extending the arrested topographic wave theories to a fully baroclinic ocean, and discuss an application for an eastern boundary margin (representative of the Rockall Slope Current region, for example). We demonstrate that open-ocean density changes yield important joint effect of baroclinicity and relief (JEBAR) along-slope currents, which generate cross-slope Ekman currents due to friction with the seabed. The latter are associated with divergence and convergence leading to sea-level changes above the slope, mediated to the coast via coastally trapped waves. Through this process, not only the coastal sea level is modified but also the along-slope currents are slowed down to well-known asymptotical solutions. Hence, our results link modern theoretical developments in sea-level research and past analytical studies of slope currents. Our effort describes the fundamental notions, and we anticipate it paves the way for more sophisticated works.

Abstract Upper thermohaline properties play a critical role in mediating the transfer of momentum, heat, and biogeochemical tracers, thereby influencing the global carbon cycle and climate system. Thermohaline compensation – where temperature and salinity exert opposing effects on seawater density–is more prevalent in the mixed layer and modulates frontal dynamics. In this study, observations from 12 underwater gliders reveal that thermohaline compensation within salinity fronts becomes more pronounced at submesoscales during a tropical cyclone in the northern South China Sea. Comparative analyses from idealized numerical experiments demonstrate that surface cooling enhances submesoscale activity and thermohaline compensation at salinity fronts, with the compensated ratio reaching approximately 20%. Through intense submesoscale ageostrophic motions and restratification, surface cooling generates temperature perturbations to rapidly form temperature fronts aligned with salinity gradients, thereby enhancing submesoscale thermohaline compensation more effectively. Surface diabatic effects are incorporated into the frontogenesis function by accounting for surface cooling and submesoscale restratification. This process becomes more pronounced and efficient as submesoscale restratification shoals the mixed layer. Atmospheric cooling over submesoscale salinity fronts, together with the accompanying submesoscale compensation, constitutes a sink of oceanic eddy potential energy (EPE). These findings advance our understanding of thermohaline compensation mechanisms and the modulation of submesoscale frontal dynamics by atmospheric forcing, offering further insights into air-sea interactions in dynamically active, salinity-dominated ocean regimes.