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Johnson, GC, Purkey SG, Zilberman NV, Roemmich D.  2019.  Deep Argo quantifies bottom water warming rates in the southwest Pacific Basin. Geophysical Research Letters. 46:2662-2669.   10.1029/2018gl081685   AbstractWebsite

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 4.4years 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 degrees 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 degrees C/year from 5,000 to 5,600dbar, 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 4.4years. 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.

Purkey, SG, Johnson GC, Talley LD, Sloyan BM, Wijffels SE, Smethie W, Mecking S, Katsumata K.  2019.  Unabated bottom water warming and freshening in the South Pacific Ocean. Journal of Geophysical Research-Oceans. 124:1778-1794.   10.1029/2018jc014775   AbstractWebsite

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>4,000m) warming of 3.5 (1.4) m degrees C/year (m degrees C=10(-3)degrees C) is observed in the Ross Sea, directly downstream from bottom water formation sites, with warming rates of 2.5 (0.4) m degrees C/year to the east in the Amundsen-Bellingshausen Basin and 1.3 (0.2) m degrees 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 2,000m. The rates and patterns of deep and abyssal (deeper than 4,000m) 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.

Cazenave, A, Meyssignac B, Ablain M, Balmaseda M, Bamber J, Barletta V, Beckley B, Benveniste J, Berthier E, Blazquez A, Boyer T, Caceres D, Chambers D, Champollion N, Chao B, Chen JL, Cheng LJ, Church JA, Chuter S, Cogley JG, Dangendorf S, Desbruyeres D, Doll P, Domingues C, Falk U, Famiglietti J, Fenoglio-Marc L, Forsberg R, Galassi G, Gardner A, Groh A, Hamlington B, Hogg A, Horwath M, Humphrey V, Husson L, Ishii M, Jaeggi A, Jevrejeva S, Johnson G, Kolodziejczyk N, Kusche J, Lambeck K, Landerer F, Leclercq P, Legresy B, Leuliette E, Llovel W, Longuevergne L, Loomis BD, Luthcke SB, Marcos M, Marzeion B, Merchant C, Merrifield M, Milne G, Mitchum G, Mohajerani Y, Monier M, Monselesan D, Nerem S, Palanisamy H, Paul F, Perez B, Piecuch CG, Ponte RM, Purkey SG, Reager JT, Rietbroek R, Rignot E, Riva R, Roemmich DH, Sorensen LS, Sasgen I, Schrama EJO, Seneviratne SI, Shum CK, Spada G, Stammer D, van de Wal R, Velicogna I, von Schuckmann K, Wada Y, Wang YG, Watson C, Wiese D, Wijffels S, Westaway R, Woppelmann G, Wouters B, Grp WGSLB.  2018.  Global sea-level budget 1993-present. Earth System Science Data. 10:1551-1590.   10.5194/essd-10-1551-2018   AbstractWebsite

Global mean sea level is an integral of changes occurring in the climate system in response to unforced climate variability as well as natural and anthropogenic forcing factors. Its temporal evolution allows changes (e.g.,acceleration) to be detected in one or more components. Study of the sea-level budget provides constraints on missing or poorly known contributions, such as the unsurveyed deep ocean or the still uncertain land water component. In the context of the World Climate Research Programme Grand Challenge entitled "Regional Sea Level and Coastal Impacts", an international effort involving the sea-level community worldwide has been recently initiated with the objective of assessing the various datasets used to estimate components of the sea-level budget during the altimetry era (1993 to present). These datasets are based on the combination of a broad range of space-based and in situ observations, model estimates, and algorithms. Evaluating their quality, quantifying uncertainties and identifying sources of discrepancies between component estimates is extremely useful for various applications in climate research. This effort involves several tens of scientists from about 50 research teams/institutions worldwide (, last access: 22 August 2018). The results presented in this paper are a synthesis of the first assessment performed during 2017-2018. We present estimates of the altimetry-based global mean sea level (average rate of 3.1 +/- 0.3mm yr(-1) and acceleration of 0.1 mm yr(-2) over 1993-present), as well as of the different components of the sea-level budget (, last access: 22 August 2018). We further examine closure of the sea-level budget, comparing the observed global mean sea level with the sum of components. Ocean thermal expansion, glaciers, Greenland and Antarctica contribute 42%, 21%, 15% and 8% to the global mean sea level over the 1993-present period. We also study the sea-level budget over 2005-present, using GRACE-based ocean mass estimates instead of the sum of individual mass components. Our results demonstrate that the global mean sea level can be closed to within 0.3 mm yr(-1) (1 sigma). Substantial uncertainty remains for the land water storage component, as shown when examining individual mass contributions to sea level.

Durack, PJ, Gleckler PJ, Purkey SG, Johnson GC, Lyman JM, Boyer TP.  2018.  Ocean warming: From the surface to the deep in observations and models. Oceanography. 31:41-51.   10.5670/oceanog.2018.227   AbstractWebsite

The ocean is the primary heat sink of the global climate system. Since 1971, it has been responsible for storing more than 90% of the excess heat added to the Earth system by anthropogenic greenhouse-gas emissions. Adding this heat to the ocean contributes substantially to sea level rise and affects vital marine ecosystems. Considering the global ocean's large role in ongoing climate variability and change, it is a good place to focus in order to understand what observed changes have occurred to date and, by using models, what future changes might arise under continued anthropogenic forcing of the climate system. While sparse measurement coverage leads to enhanced uncertainties with long-term historical estimates of change, modern measurements are beginning to provide the clearest picture yet of ongoing global ocean change. Observations show that the ocean is warming from the near-surface through to the abyss, a conclusion that is strengthened with each new analysis. In this assessment, we revisit observation- and model-based estimates of ocean warming from the industrial era to the present and show a consistent, full-depth pattern of change over the observed record that is likely to continue at an ever-increasing pace if effective actions to reduce greenhouse-gas emissions are not taken.

Desbruyeres, DG, Purkey SG, McDonagh EL, Johnson GC, King BA.  2016.  Deep and abyssal ocean warming from 35 years of repeat hydrography. Geophysical Research Letters. 43:10356-10365.   10.1002/2016gl070413   AbstractWebsite

Global and regional ocean warming deeper than 2000m is investigated using 35years of sustained repeat hydrographic survey data starting in 1981. The global long-term temperature trend below 2000m, representing the time period 1991-2010, is equivalent to a mean heat flux of 0.065 0.040Wm(-2) applied over the Earth's surface area. The strongest warming rates are found in the abyssal layer (4000-6000m), which contributes to one third of the total heat uptake with the largest contribution from the Southern and Pacific Oceans. A similar regional pattern is found in the deep layer (2000-4000m), which explains the remaining two thirds of the total heat uptake yet with larger uncertainties. The global average warming rate did not change within uncertainties pre-2000 versus post-2000, whereas ocean average warming rates decreased in the Pacific and Indian Oceans and increased in the Atlantic and Southern Oceans.

Johnson, GC, Lyman JM, Purkey SG.  2015.  Informing Deep Argo Array Design Using Argo and Full- Depth Hydrographic Section Data. Journal of Atmospheric and Oceanic Technology. 32:2187-2198.   10.1175/JTECH-D-15-0139.1   AbstractWebsite

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 degrees latitude x 5 degrees longitude x 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 degrees x 5 degrees 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 degrees C decade(-1) in the quiescent abyssal North Pacific to about 26 m degrees C decade(-1) below 2000 dbar along 50 degrees 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.

Purkey, SG, Johnson GC, Chambers DP.  2014.  Relative contributions of ocean mass and deep steric changes to sea level rise between 1993 and 2013. Journal of Geophysical Research-Oceans. 119:7509-7522.   10.1002/2014JC010180   AbstractWebsite

Regional and global trends of Sea Level Rise (SLR) owing to mass addition centered between 1996 and 2006 are assessed through a full-depth SLR budget using full-depth in situ ocean data and satellite altimetry. These rates are compared to regional and global trends in ocean mass addition estimated directly using data from the Gravity Recovery and Climate Experiment (GRACE) from 2003 to 2013. Despite the two independent methods covering different time periods with differing spatial and temporal resolution, they both capture the same large-scale mass addition trend patterns including higher rates of mass addition in the North Pacific, South Atlantic, and the Indo-Atlantic sector of the Southern Ocean, and lower mass addition trends in the Indian, North Atlantic, South Pacific, and the Pacific sector of the Southern Ocean. The global mean trend of ocean mass addition is 1.5 (0.4) mm yr(-1) for 1996-2006 from the residual method and the same for 2003-2013 from the GRACE method. Furthermore, the residual method is used to evaluate the error introduced into the mass budget if the deep steric contributions below 700, 1000, 2000, 3000, and 4000 m are neglected, revealing errors of 65%, 38%, 13%, 8%, and 4% respectively. The two methods no longer agree within error bars when only the steric contribution shallower than 1000 m is considered.Key PointsRegional and global mass addition estimates from sea level and steric data Regional and global mass addition estimates from GRACE compare well Full-depth steric measurements yield deep ocean contribution to sea level rise

Purkey, SG, Johnson GC.  2013.  Antarctic Bottom Water Warming and Freshening: Contributions to Sea Level Rise, Ocean Freshwater Budgets, and Global Heat Gain. Journal of Climate. 26:6105-6122.   10.1175/JCLI-D-12-00834.1   AbstractWebsite

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 degrees 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.

Purkey, SG, Johnson GC.  2012.  Global Contraction of Antarctic Bottom Water between the 1980s and 2000s. Journal of Climate. 25:5830-5844.   10.1175/JCLI-D-11-00612.1   AbstractWebsite

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 (theta) surfaces using all available repeated hydrographic sections. The Southern Ocean is losing water below theta = 0 degrees C at a rate of -8.2 (+/- 2.6) 3 10(6) m(3) 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 theta 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 theta 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.

Purkey, SG, Johnson GC.  2010.  Warming of Global Abyssal and Deep Southern Ocean Waters between the 1990s and 2000s: Contributions to Global Heat and Sea Level Rise Budgets. Journal of Climate. 23:6336-6351.   10.1175/2010JCLI3682.1   AbstractWebsite

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

Johnson, GC, Purkey SG.  2009.  Deep Caribbean Sea warming. Deep-Sea Research Part I-Oceanographic Research Papers. 56:827-834.   10.1016/j.dsr.2008.12.011   AbstractWebsite

Data collected from hydrographic stations occupied within the Venezuelan and Columbian basins of the Caribbean Sea from 1922 through 2003 are analyzed to study the decadal variability of deep temperature in the region. The analysis focuses on waters below the 1815-m sill depth of the Anegada-Jungfern Passage. Relatively dense waters (compared to those in the deep Caribbean) from the North Atlantic spill over this sill to ventilate the deep Caribbean Sea. Deep warming at a rate of over 0.01 degrees C decade(-1) below this sill depth appears to have commenced in the 1970s after a period of relatively constant deep Caribbean Sea temperatures extending at least as far back as the 1920s. Conductivity-temperature-depth station data from World Ocean Circulation Experiment Section A22 along 66 degrees W taken in 1997 and again in 2003 provide an especially precise, albeit geographically limited, estimate of this warming over that 6-year period. They also suggest a small (0.001 PSS-78, about the size of expected measurement biases) deep freshening. The warming is about 10 times larger than the size of geothermal heating in the region, and is of the same magnitude as the average global upper-ocean heat uptake over a recent 50-year period. Together with the freshening, the warming contributes about 0.012 m decade(-1) of sea level rise in portions of the Caribbean Sea with bottom depths around 5000 m. Published by Elsevier Ltd.

Johnson, GC, Purkey SG, Bullister JL.  2008.  Warming and Freshening in the Abyssal Southeastern Indian Ocean. Journal of Climate. 21:5351-5363.   10.1175/2008JCLI2384.1   AbstractWebsite

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 degrees C and freshening of about 0.01 Practical Salinity Scale 1978 (PSS-78) in the Princess Elizabeth Trough, and warming of 0.1 degrees 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.

Johnson, GC, Purkey SG, Toole JM.  2008.  Reduced Antarctic meridional overturning circulation reaches the North Atlantic Ocean. Geophysical Research Letters. 35   Artn L2260110.1029/2008gl035619   AbstractWebsite

We analyze abyssal temperature data in the western North Atlantic Ocean from the 1980s-2000s, showing that reductions in Antarctic Bottom Water (AABW) signatures have reached even that basin. Trans-basin oceanographic sections occupied along 52 degrees W from 1983-2003 and 66 degrees W from 1985-2003 quantify abyssal warming resulting from deepening of the strong thermal boundary between AABW and North Atlantic Deep Water (NADW), hence a local AABW volume reduction. Repeat section data taken from 1981-2004 along 24 degrees N also show a reduced zonal gradient in abyssal temperatures, consistent with decreased northward transport of AABW. The reduction in the Antarctic limb of the MOC within the North Atlantic highlights the global reach of climate variability originating around Antarctica. Citation: Johnson, G. C., S. G. Purkey, and J. M. Toole (2008), Reduced Antarctic meridional overturning circulation reaches the North Atlantic Ocean, Geophys. Res. Lett., 35, L22601, doi: 10.1029/2008GL035619.