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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]

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]

Data reported from mid‐2014 to late 2018 by a regional pilot array of Deep Argo floats in the Southwest Pacific Basin are used to estimate regional temperature anomalies from a long‐term climatology as well as regional trends over the 4.4 years of float data as a function of pressure. The data show warm anomalies that increase with increasing pressure from effectively 0 near 2,000 dbar to over 10 (±1) m°C by 4,800 dbar, uncertainties estimated at 5–95%. The 4.4‐year trend estimate shows warming at an average rate of 3 (±1) m°C/year from 5,000 to 5,600 dbar, in the near‐homogeneous layer of cold, dense bottom water of Antarctic origin. These results suggest acceleration of previously reported long‐term warming trends in the abyssal waters in this region. They also demonstrate the ability of Deep Argo to quantify changes in the deep ocean in near real‐time over short periods with high accuracy. Plain Language Summary: The coldest waters that fill much of the deep ocean worldwide originate near Antarctica. Temperature data collected from oceanographic cruises around the world at roughly 10‐year intervals show that these near‐bottom waters have been warming on average since the 1990s, absorbing a substantial amount of heat. Data from an array of robotic profiling Deep Argo floats deployed in the Southwest Pacific Ocean starting in mid‐2014 reveal that near‐bottom waters there have continued to warm over the past 4.4 years. Furthermore, these new data suggest an acceleration of that warming rate. These data show that Deep Argo floats are capable of accurately measuring regional changes in the deep ocean. The ocean is the largest sink of heat on our warming planet. A global array of Deep Argo floats would provide data on how much Earth's climate system is warming and possibly improve predictions of future warming. Key Points: Deep and bottom waters in the Southwest Pacific Basin from 2014 to 2018 are increasingly warmer than climatology with increasing depthThe warm anomaly is statistically different from zero at pressures >2,800 dbar and reaches 11 (±1) m°C by 5,500 dbar (1 m°C = 0.001 °C)Deep Argo floats measure a regional mean warming rate of about 3 (±1) m°C/year from 5,000 to 5,600 dbar and from 2014 to 2018 [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: The Irminger Sea, between Greenland and Iceland, is known to be an important driver of variability in the global ocean circulation that regulates global climate. During the 2015 winter, the Irminger Sea experienced widespread cooling and buoyancy loss down to 1,000 m, resulting in deeper wintertime mixing than had been observed in the region for many years. This low buoyancy state and deep wintertime mixing persisted from 2015 to 2018, despite a return to average atmospheric wintertime conditions. Here we study how processes that typically provide summertime buoyancy gain to balance wintertime buoyancy loss contribute to the multi‐year recovery of the region. We characterize the recovery of the interior of the Irminger Sea as having two stages: initially slow recovery, followed by more rapid recovery. Our findings are consistent with earlier studies stating that the main source of buoyancy to the interior is eddies shed from the current flowing around the Irminger Sea. This study shows that the deep mixing that is important for global circulation and climate is influenced by changes in the current around the basin. Key Points: Widespread buoyancy loss across the Irminger interior and Irminger Current (IC) delayed the recovery of the interior from strong cooling in 2015Baroclinic instabilities shed from the IC are the dominant source of buoyancy restratifying the sub‐surface Irminger interiorIt is important to consider changes in the IC when considering drivers of variability in convection, ventilation, and the Atlantic Meridional Overturning Circulation [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]

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 N2. We find that below the θ=0.8°C isotherm, N2 is on average reduced by a scaling factor of s=0.88±0.06, a 12% reduction, per decade that intensifies with depth. At Θ=0.63°C, we observe the biggest change: s=0.71±0.07, or a 29% reduction per decade. Within the same period, the magnitude of vertical diffusive heat flux is also reduced by about 0.01Wm−2, 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. Plain Language Summary: The large‐scale circulation of the ocean is primarily driven by density differences. As dense, heavy water sinks, it fills the deep ocean basins and aids in pushing water around the globe, cycling around the world over many centuries. A key location where this happens is around Antarctica. The ice and cold winds cool the water, making it denser. This cooled water sinks, displacing the deep water and pushing it northwards. As Antarctica warms, this water carries the extra heat into the rest of the world, causing the deep ocean to rapidly warm. In the Southwest Pacific Basin, we find that this bottom intensified warming has caused a significant reduction in the stratification of the deepest layer over the past three decades. This change can disrupt the global ocean conveyor belt, impacting the transport of heat, carbon dioxide, nutrients, and other dissolved matter around the world. Key Points: A decadal stratification decrease is estimated from 25 years of repeat hydrography in the abyssal Southwest Pacific BasinThis change is significant below Θ = 0.75°C and intensifies with depth, reaching a 22%–36% decrease by Θ = 0.65°CVertical diffusive heat flux is also reduced during the same time period by ∼0.01 Wm−2/decade [ABSTRACT FROM AUTHOR]

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 N2. We find that below the θ=0.8°C isotherm, N2 is on average reduced by a scaling factor of s=0.88±0.06, a 12% reduction, per decade that intensifies with depth. At Θ=0.63°C, we observe the biggest change: s=0.71±0.07, or a 29% reduction per decade. Within the same period, the magnitude of vertical diffusive heat flux is also reduced by about 0.01Wm−2, 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. Plain Language Summary: The large‐scale circulation of the ocean is primarily driven by density differences. As dense, heavy water sinks, it fills the deep ocean basins and aids in pushing water around the globe, cycling around the world over many centuries. A key location where this happens is around Antarctica. The ice and cold winds cool the water, making it denser. This cooled water sinks, displacing the deep water and pushing it northwards. As Antarctica warms, this water carries the extra heat into the rest of the world, causing the deep ocean to rapidly warm. In the Southwest Pacific Basin, we find that this bottom intensified warming has caused a significant reduction in the stratification of the deepest layer over the past three decades. This change can disrupt the global ocean conveyor belt, impacting the transport of heat, carbon dioxide, nutrients, and other dissolved matter around the world. Key Points: A decadal stratification decrease is estimated from 25 years of repeat hydrography in the abyssal Southwest Pacific BasinThis change is significant below Θ = 0.75°C and intensifies with depth, reaching a 22%–36% decrease by Θ = 0.65°CVertical diffusive heat flux is also reduced during the same time period by ∼0.01 Wm−2/decade [ABSTRACT FROM AUTHOR]