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Using 9 years of full‐depth profiles from 55 Deep Argo floats in the Southwest Pacific Basin collected between 2014 and 2023, we find consistent warm anomalies compared to a long‐term climatology below 2,000 m ranging between 11 ± 2 to 34 ± 2 m°C, most pronounced between 3,500 and 5,000 m. Over this period, a cooling trend is found between 2,000 and 4,000 m and a significant warming trend below 4,000 m with a maximum rate of 4.1 ± 0.31 m°C yr−1 near 5,000 m, with a possible acceleration over the second half of the period. The integrated Steric Sea Level expansion below 2,000 m was 7.9 ± 1 mm compared to the climatology with a trend of 1.3 ± 1.6 mm dec−1 over the Deep Argo era, contributing significantly to the local sea level budget. We assess the ability to close a full Sea Level Budget, further demonstrating the value of a full‐depth Argo array. Plain Language Summary: Cold, dense waters formed near polar regions in both hemispheres, sink to great depths and fill‐up the majority of the world's deep ocean. Compilation of sparse observations of temperature from global ship‐based surveys at roughly 10‐year intervals worldwide have shown that sequestration of excess atmospheric heat into the deep ocean has caused these waters to warm steadily since the 1990's into the Present. Not only does this warming have implications for changes in large scale ocean circulation, but is also associated with warming‐induced sea level rise. Using a new data set collected between 2014 and 2023 from 55 freely drifting robotic floats (Deep Argo) which gather crucial bimonthly temperature and salinity data between the surface ocean and the ocean floor, we find the greatest warming trend at a depth of 5,000 m of 4 ± 0.3 m°C yr−1 and an associated sea level rise rate below 2,000 m of 1.3 ± 1.6 mm dec−1. Deep Argo data being collected in ocean basins worldwide are crucial in providing high resolution data of the warming deep ocean and its implications on global sea level, ocean mixing and large‐scale ocean circulation. Key Points: Nine years of Deep Argo data in the S.W. Pacific reveals continued warming in the abyss while the mid‐depths cooledWaters below 4,000 m show an accelerated warming trend with a maximum overall warming rate of 4.1 ± 0.31 m°C yr−1 at 5,000 mDeep ocean steric expansion contributed 1.3 ± 1.6 mm dec−1 to total the local sea level [ABSTRACT FROM AUTHOR]

The abyssal southwest Pacific basin has warmed significantly between 1992 and 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 decadal warming 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. 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. Significance Statement: The deep overturning circulation of the ocean is driven by northward flow of cold waters formed in the Southern Ocean and diffusion-driven upwelling from oceanic turbulence, both of which are difficult to measure and quantify accurately. This study aims to reconcile different methods of measuring the overturning in a large ocean basin by 1) inferring mixing required to sustain the overturning using long-term observations of inflow and outflow heat fluxes from a time-dependent heat budget and 2) direct/indirect observations of mixing within the basin. While our results show consistency of the basin-averaged mixing estimates from several different methods, this study also highlights the need to make sustained oceanic observations of mixing and circulation to further reduce uncertainty in the estimates. [ABSTRACT FROM AUTHOR]

There are two varieties of Antarctic Bottom Water present in the Australian Antarctic Basin (AAB): locally produced Adélie Land Bottom Water (ALBW) and distantly produced Ross Sea Bottom Water (RSBW). Between 2014 and 2018, RSBW has rebounded from a multidecade freshening trend. The return of the salty RSBW to the AAB is revealed by six Deep Argo floats that have occupied the region from January of 2018 to March of 2020. The floats depict a zonal variation in temperature and salinity in the bottom waters of the AAB, driven by the inflow of RSBW. A simple Optimum Multiparameter Analysis based on potential temperature and salinity gives a sense of scale to the composition of the bottom waters, which are nearly 80% of the new, salty RSBW in the south‐east corner of the basin by 2019 and generally less than 40% to the west closer to the ALBW outflow region and the abyssal plain. Plain Language Summary: The dense waters that form along the Antarctic coast and fill the global deep oceans have been warming and freshening over the past three decades. This water is integral to the earth's natural system of temperature regulation. New robotic floats from the Deep Argo program are able to record temperature, salinity, and pressure down to the bottom of the ocean and are capturing variability in the quantity and properties of these dense waters near their formation sites better than ever before. Here we examine data from the first 2 years of float deployments in the Indian sector of the Southern Ocean to quantify the rebound in salinity of dense water from the Ross Sea and study how different varieties of this dense water spread and fill the Australian Antarctic Basin before ultimately flowing out into the Indian and Pacific Oceans. Key Points: A new Deep Argo pilot array improves spatial and temporal resolution of bottom water sampling in the Australian Antarctic BasinThe floats capture the return of high salinity Ross Sea Bottom Water to the region after a period of multi‐decadal fresheningA simple Optimum Multiparameter calculation quantifies the local zonal variation in the new water mass concentrations from 2018 to 2020 [ABSTRACT FROM AUTHOR]

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, 〈w′2〉 $\sqrt{\langle {w}^{\prime 2}\rangle }$, 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, λz, 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. Both 〈w′2〉 $\sqrt{\langle {w}^{\prime 2}\rangle }$ and λz exhibit regional variations among and within some deep ocean basins, with generally larger 〈w′2〉 $\sqrt{\langle {w}^{\prime 2}\rangle }$ and shorter λz in regions of rougher bathymetry or stronger deep currents. These correlations are both expected, since larger 〈w′2〉 $\sqrt{\langle {w}^{\prime 2}\rangle }$ and shorter λz should be found near internal wave generation regions. Plain Language Summary: Ocean density increases with increasing depth, supporting internal waves below the ocean surface. These internal waves are generated near the surface by varying wind forcing such as passing storms and near the bottom by interactions of currents (including tidal) with rough bathymetry (such as seamounts and ridges). They can travel for long distances in both the vertical and the horizontal. When they break, they play important roles in mixing temperature, salinity, and other water properties. Deep Argo is an observing system designed to measure temperature and salinity profiles from the surface to the bottom of the ocean. One model of Deep Argo float serendipitously observes internal wave signals as variations in descent rate data, which it collects primarily for navigation purposes, from the surface to the seafloor. These observations reveal patterns in the magnitudes of these internal wave signals, with stronger internal wave activity near continental rises and mid‐ocean ridges and lower levels over smoother abyssal plains. Also, regions with strong deep flows, such as the Samoan Passage through which bottom water is funneled into the North Pacific, or the region south of the Campbell Plateau through which the Antarctic Circumpolar Current flows, exhibit stronger deep internal wave signatures. Key Points: Descent rate data used for navigation by one deep float model serendipitously reveal vertical velocity signatures of internal wavesDeep internal wave vertical velocities average root mean square variances are ∼0.0045 m s−1 and dominant vertical wavelengths are ∼757 mVertical velocity variances and dominant vertical wavelengths exhibit geographical patterns, varying among and within deep ocean basins [ABSTRACT FROM AUTHOR]

Deployment of Deep Argo regional pilot arrays is underway as a step toward a global array of 1250 surface-to-bottom profiling floats embedded in the upper-ocean (2000 m) Argo Program. Of the 80 active Deep Argo floats as of July 2019, 55 are Deep Sounding Oceanographic Lagrangian Observer (SOLO) 6000-m instruments, and the rest are composed of three additional models profiling to either 4000 or 6000 m. Early success of the Deep SOLO is owed partly to its evolution from the Core Argo SOLO-II. Here, Deep SOLO design choices are described, including the spherical glass pressure housing, the hydraulics system, and the passive bottom detection system. Operation of Deep SOLO is flexible, with the mission parameters being adjustable from shore via Iridium communications. Long lifetime is a key element in sustaining a global array, and Deep SOLO combines a long battery life of over 200 cycles to 6000 m with robust operation and a low failure rate. The scientific value of Deep SOLO is illustrated, including examples of its ability (i) to observe large-scale spatial and temporal variability in deep ocean temperature and salinity, (ii) to sample newly formed water masses year-round and within a few meters of the sea floor, and (iii) to explore the poorly known abyssal velocity field and deep circulation of the World Ocean. Deep SOLO's full-depth range and its potential for global coverage are critical attributes for complementing the Core Argo Program and achieving these objectives. [ABSTRACT FROM AUTHOR]