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Abstract 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. 

Abstract The abyssal Southwest Pacific Basin has warmed significantly between 1992-2017, consistent with warming along the bottom limb of the meridional overturning circulation seen throughout the global oceans. Here we present a framework for assessing the abyssal heat budget that includes the time-dependent unsteady effects of decadal warming and direct and indirect estimates of diapycnal mixing from microscale temperature measurements and finescale parameterizations. The unsteady terms estimated from the decadalwarming rate are shown to be within a factor of 3 of the steady state terms in the abyssal heat budget for the coldest portion of the water column and therefore, cannot be ignored. We show that a reduction in the lateral heat flux for the coldest temperature classes compensated by an increase in warmer waters advected into the basin has important implications for the heat balance and diffusive heat fluxes in the basin. Finally, vertical diffusive heat fluxes are estimated in different ways: using the newly available CTD-mounted microscale temperature measurements, a finescale strain parameterization, and a vertical kinetic energy parameterization from data along the P06 transect along 32.5°S. The unsteady-state abyssal heat budget for the basin shows closure within error estimates, demonstrating that (i) unsteady terms have become consequential for the heat balance in the isotherms closest to the ocean bottom and (ii) direct and indirect estimates from full depth GO-SHIP hydrographic transects averaged over similarly large spatial and temporal scales can capture the basin-averaged abyssal mixing needed to close the deep overturning circulation. 

Abstract Despite technological advances over the last several decades, ship-based hydrography remains the only method for obtaining high-quality, high spatial and vertical resolution measurements of physical, chemical, and biological parameters over the full water column essential for physical, chemical, and biological oceanography and climate science. The Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP) coordinates a network of globally sustained hydrographic sections. These data provide a unique data set that spans four decades, comprised of more than 40 cross-ocean transects. The section data are, however, difficult to use owing to inhomogeneous format. The purpose of this new temperature, salinity, and dissolved oxygen data product is to combine, reformat and grid these data measured by Conductivity-Temperature-Depth-Oxygen (CTDO) profilers in order to facilitate their use by a wider audience. The product is machine readable and readily accessible by many existing visualisation and analysis software packages. The data processing can be repeated with modifications to suit various applications such as analysis of deep ocean, validation of numerical simulation, and calibration of autonomous platforms. 

Abstract Oceanic transient tracers, such as chlorofluorocarbons (CFCs) and sulfur‐hexafluoride (SF₆), trace the propagation of intermediate‐to‐abyssal water masses in the ocean interior. Their temporal and spatial sparsity, however, has limited their utility in quantifying the global ocean circulation and its decadal variability. TheTime‐Correction Method(TCM) presented here is a new approach to leverage the available CFCs and SF₆observations to solve for the Green's functions (GFs) describing the steady‐state transport from the surface to the ocean interior. From the GFs, we reconstruct global tracer concentrations (and associated uncertainties) in the ocean interior at annual resolution (1940–2021). The spatial resolution includes 50 neutral density levels that span the water column along World Ocean Circulation Experiment/Global Ocean Ship‐Based Hydrographic Investigations Program lines. The reconstructed tracer concentrations return a global view of CFCs and SF₆spreading into new regions of the interior ocean, such as the deep north‐western Pacific. For example, they capture the southward spreading and equatorial recirculation of distinct North Atlantic Deep Water components, and the spreading of CFC‐rich Antarctic Bottom Water out of the Southern Ocean and into the North Pacific, East Indian, and West Atlantic. The reconstructed tracer concentrations fit the data in most locations (∼75%), indicating that a steady‐state circulation holds for the most part. Discrepancies between the reconstructed and observed concentrations offer insight into ventilation rate changes on decadal timescales. As an example, we infer decadal changes in Subantartic Mode Water (SAMW) and find an increase in SAMW ventilation from 1992 to 2014, highlighting the skill of the TCM in leveraging the sparse tracer observations. 

Abstract Serendipitous measurements of deep internal wave signatures are evident in oscillatory variations around the background descent rates reported by one model of Deep Argo float. For the 10,045 profiles analyzed here, the average root‐mean‐square of vertical velocity variances,, from 1,000 m to the seafloor, is 0.0045 m s⁻¹, with a 5%–95% range of 0.0028–0.0067 m s⁻¹. Dominant vertical wavelengths,λ_z, estimated from the integrals of lagged autocorrelation sequences have an average value of 757 m, with a 5%–95% range of 493–1,108 m. Bothandλ_zexhibit regional variations among and within some deep ocean basins, with generally largerand shorterλ_zin regions of rougher bathymetry or stronger deep currents. These correlations are both expected, since largerand shorterλ_zshould be found near internal wave generation regions. 

Abstract As the abyssal oceans warm, stratification is also expected to change in response. This change may impact mixing and vertical transport by altering the buoyancy flux, internal wave generation, and turbulent dissipation. In this study, repeated surveys of three hydrographic sections in the Southwest Pacific Basin between the 1990s and 2010s are used to estimate the change in buoyancy frequency. We find that below the°C isotherm,is on average reduced by a scaling factor of, a 12% reduction, per decade that intensifies with depth. At°C, we observe the biggest change:, or a 29% reduction per decade. Within the same period, the magnitude of vertical diffusive heat flux is also reduced by about, although this estimate is sensitive to the choice of estimated diffusivity. Finally, implications of these results for the heat budget and global ocean circulation are qualitatively discussed.