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Sarah G. Purkey and Gregory C. Johnson

Abstract

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.

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Sarah G. Purkey and Gregory C. Johnson

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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.

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Sarah G. Purkey and Gregory C. Johnson

Abstract

A statistically significant reduction in Antarctic Bottom Water (AABW) volume is quantified between the 1980s and 2000s within the Southern Ocean and along the bottom-most, southern branches of the meridional overturning circulation (MOC). AABW has warmed globally during that time, contributing roughly 10% of the recent total ocean heat uptake. This warming implies a global-scale contraction of AABW. Rates of change in AABW-related circulation are estimated in most of the world’s deep-ocean basins by finding average rates of volume loss or gain below cold, deep potential temperature (θ) surfaces using all available repeated hydrographic sections. The Southern Ocean is losing water below θ = 0°C at a rate of −8.2 (±2.6) × 106 m3 s−1. This bottom water contraction causes a descent of potential isotherms throughout much of the water column until a near-surface recovery, apparently through a southward surge of Circumpolar Deep Water from the north. To the north, smaller losses of bottom waters are seen along three of the four main northward outflow routes of AABW. Volume and heat budgets below deep, cold θ surfaces within the Brazil and Pacific basins are not in steady state. The observed changes in volume and heat of the coldest waters within these basins could be accounted for by small decreases to the volume transport or small increases to θ of their inflows, or fractional increases in deep mixing. The budget calculations and global contraction pattern are consistent with a global-scale slowdown of the bottom, southern limb of the MOC.

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Gregory C. Johnson, Sarah G. Purkey, and John L. Bullister

Abstract

Warming and freshening of abyssal waters in the eastern Indian Ocean between 1994/95 and 2007 are quantified using data from two closely sampled high-quality occupations of a hydrographic section extending from Antarctica northward to the equator. These changes are limited to abyssal waters in the Princess Elizabeth Trough and the Australian–Antarctic Basin, with little abyssal change evident north of the Southeast Indian Ridge. As in previous studies, significant cooling and freshening is observed in the bottom potential temperature–salinity relations in these two southern basins. In addition, analysis on pressure surfaces shows abyssal warming of about 0.05°C and freshening of about 0.01 Practical Salinity Scale 1978 (PSS-78) in the Princess Elizabeth Trough, and warming of 0.1°C with freshening of about 0.005 in the abyssal Australian–Antarctic Basin. These 12-yr differences are statistically significant from zero at 95% confidence intervals over the bottom few to several hundred decibars of the water column in both deep basins. Both warming and freshening reduce the density of seawater, contributing to the vertical expansion of the water column. The changes below 3000 dbar in these basins suggest local contributions approaching 1 and 4 cm of sea level rise, respectively. Transient tracer data from the 2007 occupation qualitatively suggest that the abyssal waters in the two southern basins exhibiting changes have significant components that have been exposed to the ocean surface within the last few decades, whereas north of the Southeast Indian Ridge, where changes are not found, the component of abyssal waters that have undergone such ventilation is much reduced.

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Gregory C. Johnson, John M. Lyman, and Sarah G. Purkey

Abstract

Data from full-depth closely sampled hydrographic sections and Argo floats are analyzed to inform the design of a future Deep Argo array. Here standard errors of local decadal temperature trends and global decadal trends of ocean heat content and thermosteric sea level anomalies integrated from 2000 to 6000 dbar are estimated for a hypothetical 5° latitude × 5° longitude × 15-day cycle Deep Argo array. These estimates are made using temperature variances from closely spaced full-depth CTD profiles taken during hydrographic sections. The temperature data along each section are high passed laterally at a 500-km scale, and the resulting variances are averaged in 5° × 5° bins to assess temperature noise levels as a function of pressure and geographic location. A mean global decorrelation time scale of 62 days is estimated using temperature time series at 1800 dbar from Argo floats. The hypothetical Deep Argo array would be capable of resolving, at one standard error, local trends from <1 m °C decade−1 in the quiescent abyssal North Pacific to about 26 m °C decade−1 below 2000 dbar along 50°S in the energetic Southern Ocean. Larger decadal temperature trends have been reported previously in these regions using repeat hydrographic section data, but those very sparse data required substantial spatial averaging to obtain statistically significant results. Furthermore, the array would provide decadal global ocean heat content trend estimates from 2000 to 6000 dbar with a standard error of ±3 TW, compared to a trend standard error of ±17 TW from a previous analysis of repeat hydrographic data.

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Dean Roemmich, Jeffrey T. Sherman, Russ E. Davis, Kyle Grindley, Michael McClune, Charles J. Parker, David N. Black, Nathalie Zilberman, Sarah G. Purkey, Philip J. H. Sutton, and John Gilson

Abstract

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.

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Molly Baringer, Mariana B. Bif, Tim Boyer, Seth M. Bushinsky, Brendan R. Carter, Ivona Cetinić, Don P. Chambers, Lijing Cheng, Sanai Chiba, Minhan Dai, Catia M. Domingues, Shenfu Dong, Andrea J. Fassbender, Richard A. Feely, Eleanor Frajka-Williams, Bryan A. Franz, John Gilson, Gustavo Goni, Benjamin D. Hamlington, Zeng-Zhen Hu, Boyin Huang, Masayoshi Ishii, Svetlana Jevrejeva, William E. Johns, Gregory C. Johnson, Kenneth S. Johnson, John Kennedy, Marion Kersalé, Rachel E. Killick, Peter Landschützer, Matthias Lankhorst, Tong Lee, Eric Leuliette, Feili Li, Eric Lindstrom, Ricardo Locarnini, Susan Lozier, John M. Lyman, John J. Marra, Christopher S. Meinen, Mark A. Merrifield, Gary T. Mitchum, Ben Moat, Didier Monselesan, R. Steven Nerem, Renellys C. Perez, Sarah G. Purkey, Darren Rayner, James Reagan, Nicholas Rome, Alejandra Sanchez-Franks, Claudia Schmid, Joel P. Scott, Uwe Send, David A. Siegel, David A. Smeed, Sabrina Speich, Paul W. Stackhouse Jr., William Sweet, Yuichiro Takeshita, Philip R. Thompson, Joaquin A. Triñanes, Martin Visbeck, Denis L. Volkov, Rik Wanninkhof, Robert A. Weller, Toby K. Westberry, Matthew J. Widlansky, Susan E. Wijffels, Anne C. Wilber, Lisan Yu, Weidong Yu, and Huai-Min Zhang
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