Sarah. G. Purkey

Small-scale turbulent mixing in the ocean interior is vital in governing ocean circulation and tracer distributions, and hence global climate. However, the planetary extent of this role and its dependence on the microphysics of mixing remain inadequately understood. Here, we emphasize the variety of spatio-temporal scales on which such interior turbulent mixing can shape the climate system. In addition to its well-established role in facilitating the equilibration of deep branches of ocean circulation on centennial-to-millennial timescales, interior turbulent mixing is a leading determinant of oceanic tracer budgets on timescales as short as sub-annual. We highlight the importance of the co-dependence of vertical (diapycnal) mixing and lateral (isopycnal) stirring in establishing the large-scale impacts of oceanic turbulence. We conclude with a summary of theoretical, observational and computational bottlenecks in the way of a sufficiently accurate representation of mixing in Earth System Models, and discuss emerging opportunities for making progress in these areas.

The upwelling of warm Circumpolar Deep Water is a key process in the global climate system, transporting heat, nutrients, and carbon poleward towards Antarctic ice shelves. Here we use physical and chemical seawater properties from repeat ship-based observations to classify Southern Ocean water masses and show changes in warm water abundance south of the Antarctic Circumpolar Current over the past two decades. We then train a random forest model ensemble to extend this classification to a monthly gridded Argo climatology beginning in 2004, enabling further decomposition of the spatial and temporal variability of the signal. Both analyses reveal an increase in upper-2000 m warm water thickness near the continent, consistent with a circumpolar-mean poleward redistribution of the upper Circumpolar Deep Water core of 1.26km yr ⁻¹ (95% CI: 0.53–1.98). Together, these shifts suggest enhanced heat flux towards the Antarctic shelf, with implications for basal ice shelf melting and sea-level rise.

Atmospheric rivers (ARs) are elongated structures in the atmosphere that transport water vapor from the tropics to mid‐ and high‐latitudes in highly episodic events. ARs are a major source of precipitation in the western United States and many other regions of the world and play a role in air‐sea exchanges of heat and freshwater in the subtropics. For the first time, an experiment was designed to observe (open) ocean changes during AR precipitation, using profiling floats in the northeast subtropical Pacific. Results show upper ocean freshening and cooling with AR precipitation (lasting several hours), especially when surface winds are low ( m/s). ARs contribute 70%–90 of the total winter precipitation in the subtropical northeast Pacific: a salt budget analysis (using ECCO) shows that precipitation over winter is the main driver of ocean freshening in the top 25–30 m and contributes by to the integrated changes in the top m.

The formation and downwelling of cold Antarctic Bottom Water (AABW) and the concurrent upwelling of warm Circumpolar Deep Water (CDW) around the Antarctic margin are key processes in the global climate system, driving the renewal of cold dense water to the deep ocean, while transporting heat, nutrients, and carbon upwards and poleward towards Antarctic ice shelves. Observational and modelling studies have documented that AABW has warmed, freshened, and contracted in recent decades. However, the response of CDW to anthropogenic forcing is less clear, with some model-based studies projecting a poleward migration of its southern boundary. In this study, we utilise the physical and chemical seawater properties measured from relatively sparse ship-based observations to robustly classify Southern Ocean water masses. We then train a random forest machine learning model to accurately apply the water mass classification to the abundant Argo temperature and salinity data spanning the Southern Ocean over the past two decades, allowing for local assessment of variability on monthly-to-decadal timescales. Our analysis reveals an increase (decrease) in CDW thickness near (away from) the continent, indicating a poleward migration of CDW upwelling. This trend is accompanied by a contraction in AABW layer thickness along the Antarctic margin. This spatial redistribution suggests an enhanced heat flux toward the Antarctic continental shelf, with potential implications for basal ice shelf melting and sea-level projections.

A key component of the ocean's global overturning circulation is the return of deep water from the Indian and Pacific Oceans to the Southern Ocean transporting heat, carbon, and nutrients poleward and toward the ocean surface. Indian Deep Water (IDW) and Pacific Deep Water (PDW) traverse the South Australian basin as they bring high‐carbon, low‐oxygen water to upwell in the Southern Ocean. Historically, limited sampling of this basin has made characterizing this circulation difficult. Here, we use 20 years of data from the Biogeochemical and Core Argo arrays to identify the upper portion of deep water above 2,000 m in this basin. We map time‐mean potential vorticity, oxygen, temperature, and salinity. We combine Argo profiles with trajectory data to calculate time‐mean geostrophic velocities. These property and velocity maps indicate three branches of flow on isopycnals between 1,400 and 2,000 dbar: a deep eastern boundary current (DEBC) carrying warm salty IDW along the continental slope, a central southeastward pathway carrying IDW through the center of the basin, and a westward pathway carrying fresher, cooler PDW into the basin. These pathways transport 0.3 ± 0.3 Sv, 3.6 ± 2.3 Sv, and 3.4 ± 0.9 Sv, respectively, between σ₁ = 32.2 and 32.3 kg/m³ where σ₁ is the potential density referenced to 1,000 dbar. Over the full deep water layer, we estimate the transport of the deep eastern boundary current as 2.2 ± 2.1 Sv, the southeast IDW as 27 ± 17 Sv, and the westward PDW as 18 ± 6 Sv.

Global overturning circulation partitions the deep ocean into regions, each with different physicochemical characteristics, but the extent to which these water masses represent distinct ecosystems remains unknown. In this work, we integrate extensive genomic information with hydrography and water mass age to delineate microbial taxonomic and functional boundaries across the South Pacific. Prokaryotic richness steeply increases with depth in the surface ocean, which forms a so-called phylocline, below which, richness is consistently high, dipping slightly in highly aged water. Reconstructed genomes self-organize into six spatially distinct taxonomic cohorts and 10 functionally distinct biomes that are primarily structured by wind-driven circulation at the surface and density-driven circulation at depth. Overall, water physicochemistry, modulated at depth by water age, drives microbial diversity patterns and functional potential in the pelagic ocean.

Cross‐shelf exchange at Greenland's continental margins transports warm waters toward the glacier margins and freshwater offshore into the convective basins of the North Atlantic and Nordic Seas. Several studies have suggested that the exchange is enhanced by the presence of deep, glacial troughs, but observations from Greenland's troughs are scarce. This work presents data from a ship‐based survey at Narsaq Trough, a wide, branched trough in southwest Greenland, during the summer of 2022. We use Conductivity‐Temperature‐Depth‐Oxygen profiles, water samples for nutrient analysis, and underway current profiles to compare the water mass properties and distribution inside and outside the trough, describe the flow‐field in and around the trough, and estimate mixing in the trough. Narsaq Trough is found to provide a pathway for warm, salty Atlantic Water to intrude onto the continental shelf where these waters are mixed with the overlying cold, fresh Polar Water. As a result, waters in the trough are fresher, oxygen‐enriched, macronutrient‐depleted, and at times colder, relative to the unmodified Atlantic Water offshore. This trough‐modified water has the potential to freshen and oxygenate the flow on the shelf‐break and/or reduce the thermal forcing of waters in the adjacent fjord, limiting ice melt.

A key component of the ocean’s global overturning circulation is the return of deep water from the Indian and Pacific Oceans to the Southern Ocean, transporting heat, carbon and nutrients poleward and towards the ocean surface. Indian Deep Water (IDW) and Pacific Deep Water (PDW) traverse the South Australian basin as they bring high carbon, low oxygen water to upwell in the Southern Ocean. Historically, limited sampling of this basin has made characterizing this circulation difficult. Here, we use data from the Biogeochemical and Core Argo arrays to identify the upper portion of the deep water in this basin. We map potential vorticity, oxygen, temperature, and salinity. We combine profiles with trajectory data to calculate mean geostrophic velocities on isopycnals. From these property and velocity maps, we identify three branches of flow within the σ1=32.2 to 32.3 kg/m3 isopycnal layer, which captures the deep water within the Argo range: a deep eastern boundary current (DEBC) carrying warm salty IDW along the continental slope, a central southeast pathway carrying IDW through the center of the basin, and a westward pathway carrying fresher, cooler PDW into the basin. We measure these pathways with a transport of 0.29 ± 0.28 Sv, 3.6±2.3 Sv, and 3.4±0.93 Sv respectively in the neutral density layer γN = 27.77 kg/m3 -27.87 kg/m3; we estimate the full transport of the DEBC as 2.2±2.1 Sv, the southeast IDW as 27±17 Sv, and the westward PDW as 18±5.8 Sv. These three pathways characterize the properties across the South Australian Basin.

Global overturning circulation partitions the deep ocean into regions with unique physicochemical characteristics, but the extent to which these water masses represent distinct ecosystems remains unknown. Here, we integrate extensive genomic information with hydrography and water mass age to delineate microbial taxonomic and functional boundaries across the South Pacific. Prokaryotic richness steeply increases with depth in the surface ocean, forming a "phylocline", below which richness is consistently high, dipping slightly in highly aged water. Reconstructed genomes self-organize into six spatially-distinct taxonomic cohorts and ten functionally-distinct biomes that are primarily structured by wind-driven circulation at the surface and density-driven circulation at depth. Overall, water physicochemistry, modulated at depth by water age, drives microbial diversity and functional potential in the pelagic ocean.

Deep and abyssal layer decadal temperature trends from the mid‐1980s to the mid‐2010s are mapped globally using Deep Argo and historical ship‐based Conductivity‐Temperature‐Depth (CTD) instrument data. Abyssal warming trends are widespread, with the strongest warming observed around Antarctic Bottom Water (AABW) formation regions. The warming strength follows deep western boundary currents transporting abyssal waters north and decreases with distance from Antarctica. Abyssal cooling trends are found in the North Atlantic and eastern South Atlantic, regions primarily ventilated by North Atlantic Deep Water (NADW). Deep warming trends are prominent in the Southern Ocean south of about 50°S, the Greenland‐Iceland‐Norwegian (GIN) Seas and the western subpolar North Atlantic, with cooling in the eastern subpolar North Atlantic and the subtropical and tropical western North Atlantic. Globally integrated decadal heat content trends of 21.6 (±6.5) TW in the deep and 12.9 (±1.8) TW in the abyssal layer are more certain than previous estimates.