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Elaine L. McDonagh and Karen J. Heywood

Abstract

A warm core ring in the southeast Atlantic, previously thought to have come from the Brazil–Falklands (Malvinas) confluence, is traced back to the Agulhas retroflection. The path of this ring, sampled at 36°S, 4°E on 23 January 1993 during the World Ocean Circulation Experiment one-time hydrographic section A11, is resolved using a combination of satellite altimeter data from ERS-1 and TOPEX/POSEIDON and infrared radiometer data from ERS-1’s Along Track Scanning Radiometer (ATSR). The ERS-1 35-day repeat and the TOPEX/POSEIDON 10-day repeat altimeter-derived sea surface height anomalies are combined and used to trace the ring back to 40°S, 10°E at the beginning of the ERS-1 35-day repeat mission in April 1992 when it had a sea surface height anomaly in excess of 50 cm. This anomaly in this part of the ocean is too large to be associated with a ring formed anywhere other than the Agulhas retroflection. The ring is identified in coincident 35-day altimetry fields and in a monthly average of ATSR sea surface temperature data in August 1992. That this anomaly was observed in the ATSR data indicates that the ring has not overwintered; the age and speed since formation inferred from this precludes formation outside of the Agulhas retroflection region. Using ATSR data prior to April 1992 and altimetry data from ERS-1’s 3-day repeat, this ring is traced back to the Agulhas retroflection where it formed in October 1991.

The ring was unlike any previously sampled ring in this region, as it had a homogeneous core between 100 and 550 db, potential temperature 13°C, salinity 35.2 psu and potential density 26.55. This water lay on the same potential temperature–salinity (θ–S) curve as the water within a typical Agulhas ring sampled a few days later. The peak velocity measured within the ring by an acoustic Doppler current profiler at 200 m was 58 cm s−1. This velocity was similar to other Agulhas rings, but higher than the velocities measured in rings originating in the Brazil–Falklands (Malvinas) confluence region. The high oxygen and low nutrient concentrations (relative to typical Agulhas water) and θ–S characteristics of the core of the anomalous ring were consistent with a mode water that forms in the subantarctic zone of the southwest Indian Ocean, subantarctic mode water (SAMW). The type of SAMW detected in the anomalous ring has been observed forming by deep convection in the subantarctic zone at 60°E. At this longitude water is readily subducted from the subantarctic zone into a pronounced and compact anticyclonic gyre in the western south Indian Ocean. A particle tracing experiment in the mean velocity field of the Fine Resolution Antarctic Model showed that it would take eight years (within a factor of 2) for water to recirculate from near 40°S, 60°E into the Agulhas retroflection in this compact gyre. Once in the retroflection, the SAMW is available to be pinched off into rings.

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Elaine L. McDonagh and Brian A. King

Abstract

A box inverse of the World Ocean Circulation Experiment A10 (30°S) and A11 (nominally 45°S) sections in the South Atlantic Ocean was undertaken. The authors find a heat flux across A10 of 0.22 ± 0.08 PW, consistent with previous studies, and a heat flux of 0.43 ± 0.08 PW across A11. The A11 heat flux is lower than some previous analyses of this section but implies a plausible oceanic heat convergence (heat loss to the atmosphere) of 0.21 ± 0.10 PW. The difference is principally due to adding a cyclonic component to the circulation in the Cape Basin. As compared with the solution of other studies, the anticyclonic circulation in the surface and intermediate water of the subtropical gyre is weakened. The circulation of the deep water is cyclonic rather than anticyclonic; this is in better agreement with previously published circulation schemes based on examination of water properties. A southward freshwater flux of 0.7 Sv (1 Sv ≡ 106 m3 s−1) at A11, consistent with previous inverse studies, is still inconsistent with the net Atlantic evaporation inferred from integrated surface climatologies. Results suggest a small gain of freshwater (0.2 ± 0.1 Sv) between the sections.

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Damien Desbruyères, Elaine L. McDonagh, Brian A. King, and Virginie Thierry
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Damien Desbruyères, Elaine L. McDonagh, Brian A. King, and Virginie Thierry

Abstract

The early twenty-first century’s warming trend of the full-depth global ocean is calculated by combining the analysis of Argo (top 2000 m) and repeat hydrography into a blended full-depth observing system. The surface-to-bottom temperature change over the last decade of sustained observation is equivalent to a heat uptake of 0.71 ± 0.09 W m−2 applied over the surface of Earth, 90% of it being found above 2000-m depth. The authors decompose the temperature trend pointwise into changes in isopycnal depth (heave) and temperature changes along an isopycnal (spiciness) to describe the mechanisms controlling the variability. The heave component dominates the global heat content increase, with the largest trends found in the Southern Hemisphere’s extratropics (0–2000 m) highlighting a volumetric increase of subtropical mode waters. Significant heave-related warming is also found in the deep North Atlantic and Southern Oceans (2000–4000 m), reflecting a potential decrease in deep water mass renewal rates. The spiciness component shows its strongest contribution at intermediate levels (700–2000 m), with striking localized warming signals in regions of intense vertical mixing (North Atlantic and Southern Oceans). Finally, the agreement between the independent Argo and repeat hydrography temperature changes at 2000 m provides an overall good confidence in the blended heat content evaluation on global and ocean scales but also highlights basin-scale discrepancies between the two independent estimates. Those mismatches are largest in those basins with the largest heave signature (Southern Ocean) and reflect both the temporal and spatial sparseness of the hydrography sampling.

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Elaine L. McDonagh, Paula McLeod, Brian A. King, Harry L. Bryden, and Sinhué Torres Valdés

Abstract

In May and June 2005, a transatlantic hydrographic section along 36°N was occupied. A velocity field is calculated using inverse methods. The derived 36°N circulation has an overturning transport (maximum in the overturning streamfunction) of 16.6 Sv (1 Sv ≡ 106 m3 s−1) at 1070 m. The heat transport across the section, 1.14 ± 0.12 PW, is partitioned into overturning and horizontal heat transports of 0.75 and 0.39 PW, respectively. The horizontal heat flux is set by variability at the gyre rather than by mesoscale. The freshwater flux across the section is 1.55 ± 0.18 Sv southward based on a 0.8-Sv flow from the Pacific through the Bering Strait at a salinity of 32.5 psu. The oceanic divergence of freshwater implies a net input of freshwater to the ocean of 0.75 Sv over the North Atlantic and Arctic between 36°N and the Bering Strait. Most (85%) of the recently ventilated upper North Atlantic Deep Water (water originating in the Labrador Sea) transport across the section occurs in the deep western boundary current rather than being associated with an interior pathway to the west of the mid-Atlantic ridge.

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Jan D. Zika, Jonathan M. Gregory, Elaine L. McDonagh, Alice Marzocchi, and Louis Clément

Abstract

Over 90% of the buildup of additional heat in the Earth system over recent decades is contained in the ocean. Since 2006, new observational programs have revealed heterogeneous patterns of ocean heat content change. It is unclear how much of this heterogeneity is due to heat being added to and mixed within the ocean leading to material changes in water mass properties or is due to changes in circulation that redistribute existing water masses. Here we present a novel diagnosis of the “material” and “redistributed” contributions to regional heat content change between 2006 and 2017 that is based on a new “minimum transformation method” informed by both water mass transformation and optimal transportation theory. We show that material warming has large spatial coherence. The material change tends to be smaller than the redistributed change at any geographical location; however, it sums globally to the net warming of the ocean, whereas the redistributed component sums, by design, to zero. Material warming is robust over the time period of this analysis, whereas the redistributed signal only emerges from the variability in a few regions. In the North Atlantic Ocean, water mass changes indicate substantial material warming while redistribution cools the subpolar region as a result of a slowdown in the meridional overturning circulation. Warming in the Southern Ocean is explained by material warming and by anomalous southward heat transport of 118 ± 50 TW through redistribution. Our results suggest that near-term projections of ocean heat content change and therefore sea level change will hinge on understanding and predicting changes in ocean redistribution.

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Harry L. Bryden, William E. Johns, Brian A. King, Gerard McCarthy, Elaine L. McDonagh, Ben I. Moat, and David A. Smeed

Abstract

Northward ocean heat transport at 26°N in the Atlantic Ocean has been measured since 2004. The ocean heat transport is large—approximately 1.25 PW, and on interannual time scales it exhibits surprisingly large temporal variability. There has been a long-term reduction in ocean heat transport of 0.17 PW from 1.32 PW before 2009 to 1.15 PW after 2009 (2009–16) on an annual average basis associated with a 2.5-Sv (1 Sv ≡ 106 m3 s−1) drop in the Atlantic meridional overturning circulation (AMOC). The reduction in the AMOC has cooled and freshened the upper ocean north of 26°N over an area following the offshore edge of the Gulf Stream/North Atlantic Current from the Bahamas to Iceland. Cooling peaks south of Iceland where surface temperatures are as much as 2°C cooler in 2016 than they were in 2008. Heat uptake by the atmosphere appears to have been affected particularly along the path of the North Atlantic Current. For the reduction in ocean heat transport, changes in ocean heat content account for about one-quarter of the long-term reduction in ocean heat transport while reduced heat uptake by the atmosphere appears to account for the remainder of the change in ocean heat transport.

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Elaine L. McDonagh, Harry L. Bryden, Brian A. King, Richard J. Sanders, Stuart A. Cunningham, and Robert Marsh

Abstract

A significant change in properties of the thermocline is observed across the whole Indian Ocean 32°S section between 1987 and 2002. This change represents a reversal of the pre-1987 freshening and decreasing oxygen concentrations of the upper thermocline that had been interpreted as a fingerprint of anthropogenic climate change. The thermocline at the western end of the section (40°–70°E) is occupied by a single variety of mode water with a potential temperature of around 13°C. The thermocline at the eastern end of the 32°S section is occupied by mode waters with a range of properties cooling from ∼11°C at 80°E to ∼9°C near the Australian coast. The change in θS properties between 1987 and 2002 is zonally coherent east of 80°E, with a maximum change on isopycnals at 11.6°C. Ages derived from helium–tritium data imply that the mode waters at all longitudes take about the same time to reach 32°S from their respective ventilation sites. Dissolved oxygen concentration changes imply that all of the mode water reached the section ∼20% faster in 2002 than in 1987.

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Sally E. Close, Alberto C. Naveira Garabato, Elaine L. McDonagh, Brian A. King, Martin Biuw, and Lars Boehme

Abstract

The evolution of the physical properties of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the Drake Passage region is examined on time scales down to intraseasonal, within the 1969–2009 period. Both SAMW and AAIW experience substantial interannual to interdecadal variability, significantly linked to the action of the Amundsen Sea low (ASL) in their formation areas. Observations suggest that the interdecadal freshening tendency evident in SAMW over the past three decades has recently abated, while AAIW has warmed significantly since the early 2000s. The two water masses have also experienced a substantial lightening since the start of the record. Examination of the mechanisms underpinning water mass property variability shows that SAMW characteristics are controlled predominantly by a combination of air–sea turbulent heat fluxes, cross-frontal Ekman transport of Antarctic surface waters, and the evaporation–precipitation balance in the Subantarctic zone of the southeast Pacific and Drake Passage, while AAIW properties reflect air–sea turbulent heat fluxes and sea ice formation in the Bellingshausen Sea. The recent interdecadal evolution of the ASL is consistent with both the dominance of the processes described here and the response of SAMW and AAIW on that time scale.

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Elaine L. McDonagh, Brian A. King, Harry L. Bryden, Peggy Courtois, Zoltan Szuts, Molly Baringer, Stuart A. Cunningham, Chris Atkinson, and Gerard McCarthy

Abstract

The first continuous estimates of freshwater flux across 26.5°N are calculated using observations from the RAPID–MOCHA–Western Boundary Time Series (WBTS) and Argo floats every 10 days between April 2004 and October 2012. The mean plus or minus the standard deviation of the freshwater flux (F W) is −1.17 ± 0.20 Sv (1 Sv ≡ 106 m3 s−1; negative flux is southward), implying a freshwater divergence of −0.37 ± 0.20 Sv between the Bering Strait and 26.5°N. This is in the sense of an input of 0.37 Sv of freshwater into the ocean, consistent with a region where precipitation dominates over evaporation. The sign and the variability of the freshwater divergence are dominated by the overturning component (−0.78 ± 0.21 Sv). The horizontal component of the freshwater divergence is smaller, associated with little variability and positive (0.35 ± 0.04 Sv). A linear relationship, describing 91% of the variance, exists between the strength of the meridional overturning circulation (MOC) and the freshwater flux (−0.37 − 0.047 Sv of F W per Sverdrups of MOC). The time series of the residual to this relationship shows a small (0.02 Sv in 8.5 yr) but detectable decrease in the freshwater flux (i.e., an increase in the southward freshwater flux) for a given MOC strength. Historical analyses of observations at 24.5°N are consistent with a more negative freshwater divergence from −0.03 to −0.37 Sv since 1974. This change is associated with an increased southward freshwater flux at this latitude due to an increase in the Florida Straits salinity (and therefore the northward salinity flux).

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