Hypersaline overflows from marginal seas contribute to global ocean circulation and climate variability. The Persian/Arabian Gulf (P/AG) contributes dense, saline, oxygenated deep outflow into the Arabian Sea's Oxygen Minimum Zone (OMZ). Here, we present high-resolution hydrographic mapping of the United Arab Emirates-Oman margin in the northwestern Arabian Sea, integrating conductivity-temperature-depth profiles, multibeam bathymetry and remotely-operated vehicle imagery. The P/AG outflow forms a thin (10–30 m) bottom-attached layer that, on crossing an abrupt bathymetric step, undergoes a hydraulic transition and thickens to 200 m over the rough, irregular seafloor of the unstable shelf, where dilution and loss of bottom contact occur. Near-bottom salinity, oxygen, and pH decrease systematically downstream in association with enhanced mixing. Distinct benthic assemblages coincide with attached versus detached flow sectors, consistent with differing near-bed hydrodynamic regimes. These observations indicate that the bathymetric step/hydraulic transition is responsible for the near-field plume transformation followed by the lower effect of bathymetric roughness. Because outflow density depends on Gulf warming and evaporation, modest climatic shifts may alter outflow dynamics, its detachment depth, pathway geometry, and intermediate-depth ventilation of the Arabian Sea OMZ.
The Arctic Mediterranean is rapidly changing, a statement that is often made, but the follow-up statements: changing from what and towards what are often omitted. Hence its role in the global climate system, particularly regarding the Atlantic Meridional Overturning Circulations remains poorly constrained. This review of the oceanography of the Arctic Mediterranean Sea develops an unified perspective of how interacting components of the system evolve in space and time, and the processes that determine their evolution. To set the stage a succinct overview of the geographic setting and early explorations is given. We then follow the pathways of the principal water masses to describe inflows, inter-basin circulations, water mass transformations, and outflows of heat and salt to the bordering subpolar gyre and the global ocean. The fundamental connection to the global ocean, the Atlantic water, is traced along its route into and through the Arctic Mediterranean. Its transformations, driven by cooling, and by the freezing and melting of sea ice, lead to the creation of both denser and less dense waters that form and maintain the water column structures within the Arctic Mediterranean; the so-called double estuary. A second advective component to the Arctic, the low salinity water carried by the Pacific inflow, is concentrated into the Amerasian Basin and acts there to further isolate the denser waters derived from the Atlantic inflow. Hence, the waters return to the North Atlantic either as dense overflows or buoyant outflows. The water masses within diverse regions of the Arctic Mediterranean have changed over the past few decades, and this influences their exchanges with the world ocean east and west of Greenland. That the water mass transformations in the Arctic Mediterranean take place beyond a ridge system that allows for the build-up of significant density differences that, through entrainment, can increase the impact of the Arctic Mediterranean on global overturning circulation.
Data of seawater stable isotopologs in the quality-controlled database from the isotopic platform facility at LOCEAN (hereafter “CISE-LOCEAN”) provide an unprecedented sampling of the surface Atlantic Ocean and the southwest Indian Ocean. This investigation of δ18O, δ2H and deuterium excess (d-excess) in surface sea water for 20 subregions indicates a large variability of the regional regression lines with salinity for δ18O and δ2H. Weakest slopes are found near the equator, and increase towards higher latitudes, but also to a lesser extent in the western Atlantic compared with the eastern Atlantic in the tropics and subtropics. These regressions often explain a large share of the variability in the data (40 % to 70 %), except for d-excess at high latitudes and δ18O, δ2H near the equator. The data are sufficient to extract a rather small seasonal variability in these relationships and an average seasonal cycle in some regions of the North Atlantic subpolar gyre, with signals probably related to changes in the input of sea ice melt and depleted water of Arctic origin. A significant correlation between observed seasonal cycles in δ18O, δ2H, and d-excess and that of salinity is found in the central equatorial Atlantic. These seasonal cycles change between the vicinity of the equator and the region to its north, which is associated with the seasonal shift of the Inter-tropical Convergence Zone and with the seasonality of the equatorial upwelling. Elsewhere, the seasonal signal usually does not emerge from the noise. An independent over 7-years long time series of nearly monthly surface sampling at the BATS site near Bermuda (31°50′ N, 64°10′ W) also indicates a rather well-defined seasonal cycle of surface salinity, δ18O, δ2H and d-excess. Its interannual variability in δ18O does not appear closely related to the one in salinity.
In recent years, more frequent marine heatwaves (MHWs) in the South China Sea (SCS) have affected surface chlorophyll a (Chl a). Traditional linear statistical methods have limited capacity to identify the contributions of multiple highly interrelated physical drivers in complex oceanic systems. To address this limitation, we applied the Synergistic–Unique–Redundant Decomposition (SURD) method to investigate MHW-related Chl a anomalies from 1998 to 2024. Analyses reveal a marked regional contrast during summer MHWs, with positive Chl a anomalies in the northern SCS and negative anomalies in the western SCS. Although summer thermal extremes in both regions were linked to similar anomalous atmospheric circulations associated with the western North Pacific subtropical high, the local physical mechanisms controlling Chl a responses differed. In the South Vietnam upwelling region, weakened wind forcing reduced Ekman pumping and vertical transport, limiting surface cooling and nutrient replenishment to the euphotic zone, thereby contributing to negative Chl a anomalies. In the Pearl River Estuary, enhanced incident shortwave radiation promoted rapid surface warming, while weakened wind-driven export favoured the nearshore retention of low-salinity plume water. The SURD results suggest that retained plume water likely strengthened surface-layer stratification and maintained relatively nutrient-rich surface conditions, potentially allowing plume retention to act synergistically with cumulative heat stress and favouring positive Chl a anomalies. However, the associated nutrient pathway remains inferred rather than directly demonstrated. These results suggest that besides thermal forcing, Chl a responses to MHWs in marginal seas are also shaped by regional physical pathways.
The global ocean absorbs a significant portion of anthropogenic carbon dioxide (CO2) emissions. Tracking the fate of absorbed CO2 and its impacts requires an internally consistent global observational dataset spanning decades. In the Global Ocean Data Analysis Project (GLODAP), data from disparate research cruises are compared in a secondary quality-control process, adjusted for consistency where necessary and compiled into a data product. Differences between cruises are quantified with a crossover analysis, comparing data at depth (where natural variability is minimal) from nearby sampling stations, and an inversion algorithm calculates a set of adjustments that would minimise these differences globally. The adjustments are reviewed by an expert committee and a subset applied to produce the final data product. The previous major version (GLODAPv2) used a weighted least squares (WLSQ) approach for the inversion. However, several issues became apparent when applied to the GLODAPv3 dataset, primarily significant regional biases in calculated adjustments. To address these issues, a new inversion algorithm called furthest-first (FF) has been developed for use in GLODAPv3, which has been implemented in a freely available, open-source Python package (xover). Here, we describe the FF approach and test it on simulated datasets and by comparing it to WLSQ, finding that it produces accurate adjustments. We also show how the FF approach can be adapted (i) to avoid the regional biases that appear in WLSQ, (ii) to find an optimal set of adjustments accounting for the fact that some cruises will ultimately not be adjusted, and (iii) to approximately preserve selected trends in the cruise-by-cruise differences. Points (i) and (ii) above are addressed by a two-step variant of the approach denoted FF2, which was applied in GLODAPv3. While FF represents a marked improvement on WLSQ for the GLODAP application, no algorithm can completely replace the role of expert judgement in the inversion process, for example when selecting convergence criteria, which cruises are permitted to be adjusted, and which trends should be preserved.
