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Abstract 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. 

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 Global climate change has impacted ocean biogeochemistry and physical dynamics, causing increases in acidity and temperature, among other phenomena. These changes can lead to deleterious effects on marine ecosystems and communities that rely on these ecosystems for their livelihoods. To better quantify these changes, an array of floats fitted with biogeochemical sensors (BGC‐Argo) is being deployed throughout the ocean. This paper presents an algorithm for deriving a deployment strategy that maximizes the information captured by each float. The process involves using a model solution as a proxy for the true ocean state and carrying out an iterative process to identify optimal float deployment locations for constraining the model variance. As an example, we use the algorithm to optimize the array for observing ocean surface dissolved carbon dioxide concentrations (pCO₂) in a region of strong air–sea gas exchange currently being targeted for BGC‐Argo float deployment. We conclude that 54% of the pCO₂variability in the analysis region could be sampled by an array of 50 Argo floats deployed in specified locations. This implies a relatively coarse average spacing, though we find the optimal spacing is nonuniform, with a denser sampling being required in the eastern equatorial Pacific. We also show that this method could be applied to determine the optimal float deployment along ship tracks, matching the logistics of real float deployment. We envision this software package to be a helpful resource in ocean observational design anywhere in the global oceans. 

Watermass transformation in the Irminger Sea, a key region for the Atlantic Meridional Overturning Circulation, is influenced by atmospheric and oceanic variability. Strong wintertime atmospheric forcing in 2015 resulted in enhanced convection and the densification of the Irminger Sea. Deep convection persisted until 2018, even though winters following 2015 were mild. We show that this behavior can be attributed to an initially slow convergence of buoyancy, followed by more rapid convergence of buoyancy. This two‐stage recovery, in turn, is consistent with restratification driven by baroclinic instability of the Irminger Current (IC), that flows around the basin. The initial, slow restratification resulted from the weak horizontal density gradients created by the widespread 2015 atmospheric heat loss. Faster restratification occurred once the IC recovered. This mechanism explains the delayed recovery of the Irminger Sea following a single extreme winter and has implications for the ventilation and overturning that occurs in the basin. 

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. 

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.