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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]

Abyssal ocean warming contributed substantially to anthropogenic ocean heat uptake and global sea level rise between 1990 and 2010. In the 2010s, several hydrographic sections crossing the South Pacific Ocean were occupied for a third or fourth time since the 1990s, allowing for an assessment of the decadal variability in the local abyssal ocean properties among the 1990s, 2000s, and 2010s. These observations from three decades reveal steady to accelerated bottom water warming since the 1990s. Strong abyssal (z > 4,000 m) warming of 3.5 (±1.4) m°C/year (m°C = 10−3 °C) is observed in the Ross Sea, directly downstream from bottom water formation sites, with warming rates of 2.5 (±0.4) m°C/year to the east in the Amundsen‐Bellingshausen Basin and 1.3 (±0.2) m°C/year to the north in the Southwest Pacific Basin, all associated with a bottom‐intensified descent of the deepest isotherms. Warming is consistently found across all sections and their occupations within each basin, demonstrating that the abyssal warming is monotonic, basin‐wide, and multidecadal. In addition, bottom water freshening was strongest in the Ross Sea, with smaller amplitude in the Amundsen‐Bellingshausen Basin in the 2000s, but is discernible in portions of the Southwest Pacific Basin by the 2010s. These results indicate that bottom water freshening, stemming from strong freshening of Ross Shelf Waters, is being advected along deep isopycnals and mixed into deep basins, albeit on longer timescales than the dynamically driven, wave‐propagated warming signal. We quantify the contribution of the warming to local sea level and heat budgets. Plain Language Summary: Over 90% of the excess energy gained by Earth's climate system has been absorbed by the oceans, with about 10% found deeper than 2,000 m. The rates and patterns of deep and abyssal (deeper than 4,000 m) ocean warming, while vital for understanding how this heat sink might behave in the future, are poorly known owing to limited data. Here we use highly accurate data collected by ships along oceanic transects with decadal revisits to quantify how much heat and freshwater has entered the South Pacific Ocean between the 1990s and 2010s. We find widespread warming throughout the deep basins there and evidence that the warming rate has accelerated in the 2010s relative to the 1990s. The warming is strongest near Antarctica where the abyssal ocean is ventilated by surface waters that sink to the sea floor and hence become bottom water, but abyssal warming is observed everywhere. In addition, we observe an infusion of freshwater propagating along the pathway of the bottom water as it moves northward from Antarctica. We quantify the deep ocean warming contributions to heat uptake as well as sea level rise through thermal expansion. Key Points: Bottom waters in the South Pacific Ocean have warmed steadily since the 1990s, based on data from multiple repeat oceanographic transectsMaximum warming of 0.04 degrees C/decade is found within the deep Ross Sea, with the signal weakening to the northAntarctic Bottom Water freshening, limited to around Antarctica in previous decades, has reached the Southwest Pacific Basin in the 2010s [ABSTRACT FROM AUTHOR]

Freshening and warming of Antarctic Bottom Water (AABW) between the 1980s and 2000s are quantified, assessing the relative contributions of water-mass changes and isotherm heave. The analysis uses highly accurate, full-depth, ship-based, conductivity-temperature-depth measurements taken along repeated oceanographic sections around the Southern Ocean. Fresher varieties of AABW are present within the South Pacific and south Indian Oceans in the 2000s compared to the 1990s, with the strongest freshening in the newest waters adjacent to the Antarctic continental slope and rise indicating a recent shift in the salinity of AABW produced in this region. Bottom waters in the Weddell Sea exhibit significantly less water-mass freshening than those in the other two southern basins. However, a decrease in the volume of the coldest, deepest waters is observed throughout the entire Southern Ocean. This isotherm heave causes a salinification and warming on isobaths from the bottom up to the shallow potential temperature maximum. The water-mass freshening of AABW in the Indian and Pacific Ocean sectors is equivalent to a freshwater flux of 73 ± 26 Gt yr−1, roughly half of the estimated recent mass loss of the West Antarctic Ice Sheet. Isotherm heave integrated below 2000 m and south of 30°S equates to a net heat uptake of 34 ± 14 TW of excess energy entering the deep ocean from deep volume loss of AABW and 0.37 ± 0.15 mm yr−1 of sea level rise from associated thermal expansion. [ABSTRACT FROM AUTHOR]

Abyssal global and deep Southern Ocean temperature trends are quantified between the 1990s and 2000s to assess the role of recent warming of these regions in global heat and sea level budgets. The authors 1) compute warming rates with uncertainties along 28 full-depth, high-quality hydrographic sections that have been occupied two or more times between 1980 and 2010; 2) divide the global ocean into 32 basins, defined by the topography and climatological ocean bottom temperatures; and then 3) estimate temperature trends in the 24 sampled basins. The three southernmost basins show a strong statistically significant abyssal warming trend, with that warming signal weakening to the north in the central Pacific, western Atlantic, and eastern Indian Oceans. Eastern Atlantic and western Indian Ocean basins show statistically insignificant abyssal cooling trends. Excepting the Arctic Ocean and Nordic seas, the rate of abyssal (below 4000 m) global ocean heat content change in the 1990s and 2000s is equivalent to a heat flux of 0.027 (±±0.009) W m−−2 applied over the entire surface of the earth. Deep (1000--4000 m) warming south of the Subantarctic Front of the Antarctic Circumpolar Current adds 0.068 (±±0.062) W m−−2. The abyssal warming produces a 0.053 (±±0.017) mm yr−−1 increase in global average sea level and the deep warming south of the Subantarctic Front adds another 0.093 (±±0.081) mm yr−−1. Thus, warming in these regions, ventilated primarily by Antarctic Bottom Water, accounts for a statistically significant fraction of the present global energy and sea level budgets. [ABSTRACT FROM AUTHOR]