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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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Kevin I. C. Oliver and Karen J. Heywood

Abstract

The major exchanges of volume, heat, and freshwater between the Arctic Ocean and the World Ocean occur through the Nordic seas. Here is presented the northernmost estimate for the oceanic transport of these properties that is derived from a set of hydrographic and direct current measurements, using a lowered acoustic Doppler current profiler, between the Greenland and European coasts. By applying box inverse methods to a synoptic section from the summer of 1999, a heat transport of 0.20 ± 0.08 PW toward the Arctic and a freshwater transport of 0.10 ± 0.05 Sv (1 Sv ≡ 106 m3 s−1) away from the Arctic are calculated, with a likely additional freshwater transport on the order of 0.05 Sv near the Greenland coast. Uncertainties associated with how representative the section is of the seasonal mean are included in the error analysis. Large depth-independent components in the currents throughout the section, including the Atlantic inflow, are observed. The increase (decrease) in the heat transport resulting from an increase (decrease) in the transport of this inflow is 0.033 PW Sv−1, and this is the dominant source of uncertainty in the solution. Therefore, determining only depth-dependent transports is unlikely to be sufficient when measuring heat transport in the region. The overturning components of the section heat and freshwater transport are 0.15 ± 0.07 PW and 0.04 ± 0.02 Sv, respectively. From the horizontal transport of layers within the section, a densification of 4.3 ± 2.5 Sv of waters north of the section is predicted, to densities greater than the boundary between inflow and outflow waters between the Atlantic Ocean and the Nordic seas.

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Jennifer A. Graham, David P. Stevens, and Karen J. Heywood

Abstract

The global impact of changes in Antarctic Intermediate Water (AAIW) properties is demonstrated using idealized perturbation experiments in a coupled climate model. Properties of AAIW were altered between 10° and 20°S in the Atlantic, Pacific, and Indian Oceans separately. Potential temperature was changed by ±1°C, along with density-compensating changes in salinity. For each of the experiments, sea surface temperature responds to changes in AAIW when anomalies surface at higher latitudes (>30°). Anomalous sea-to-air heat fluxes leave density anomalies in the ocean, resulting in nonlinear responses to opposite-sign perturbations. In the Southern Ocean, these affect the meridional density gradient, leading to changes in Antarctic Circumpolar Current transport. The response to cooler, fresher AAIW is both greater in magnitude and significant over a larger area than that for warmer, saltier AAIW. The North Atlantic is particularly sensitive to cool, fresh perturbations, with density anomalies causing reductions in the meridional overturning circulation of up to 1 Sv (1 Sv ≡ 106 m3 s−1). Resultant changes in meridional ocean heat transport, along with surfacing anomalies, cause basinwide changes in the surface ocean and overlying atmosphere on multidecadal time scales.

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Alberto C. Naveira Garabato, David P. Stevens, and Karen J. Heywood

Abstract

An inverse box model of the Scotia Sea is constructed using hydrographic, tracer, and velocity data collected along the rim of the basin during the Antarctic Large-Scale Box Analysis and the Role of the Scotia Sea (ALBATROSS) cruise. The model provides an estimate of the time-mean three-dimensional circulation as the Antarctic Circumpolar Current (ACC) crosses the region. It concurrently solves for geostrophic and wind-driven Ekman transports across the boundaries of the basin, air–sea-driven diapycnal fluxes, and “interior” diapycnal fluxes below the ocean surface. An increase is diagnosed in the ACC volume transport from 143 ± 13 Sv (Sv ≡ 106 m3 s−1) at Drake Passage to 149 ± 16 Sv on leaving the Scotia Sea, supplied by the import of 5.9 ± 1.7 Sv of Weddell Sea Deep Water (WSDW) over the South Scotia Ridge. There is a lateral redistribution of the transport, primarily in response to a topographically induced branching of the 70–80 Sv polar front (PF) jet and an increase in the transport associated with the subantarctic front (SAF) from 31 ± 7 to 48 ± 4 Sv. A vertical rearrangement of the transport also occurs, with differences O(2 Sv) in the transports of intermediate and deep water masses. These volume transport changes are accompanied by a net reduction (increase) in the heat (freshwater) flux associated with the ACC by 0.02 ± 0.020 PW (0.020 ± 0.017 Sv), the main cause of which is the cooling and freshening of the Circumpolar Deep Water (CDW) layer in the Scotia Sea. The model suggests that the Scotia Sea hosts intense diapycnal mixing in the ocean interior extending 1500–2000 m above the rough topography of the basin. Despite these model results, no evidence is found for a significant diapycnal link between the upper and lower classes of CDW (and hence between the “shallow” and “deep” cells of the Southern Ocean meridional overturning circulation). On the contrary, the boundary between Upper and Lower CDW separates two distinct regimes of diapycnal mixing involving volume fluxes of 1–3 Sv. Whereas in the denser waters topographic mixing is important, in lighter layers air–sea-driven diapycnal volume fluxes are dominant and diapycnal transfers of heat and freshwater are mainly effected by upper-ocean mixing processes. The model indicates that the ventilation of the deep ACC in the Scotia Sea is driven primarily by isopycnal exchanges with the northern Weddell Sea and to a lesser extent by diapycnal mixing with WSDW. The model reveals the existence of a mesoscale eddy-driven overturning circulation across the ACC core involving an isopycnal poleward transport of 8 ± 1 Sv of CDW and an equatorward transport of intermediate water of the same magnitude. This circulation induces a cross-ACC poleward heat flux of 0.022 ± 0.009 PW and an equatorward freshwater flux of 0.02 ± 0.01 Sv. Adequately scaled, the former compares favorably to measurements of the cross-stream eddy heat flux by moored current meters and floats in the ACC and to budget estimates of the circumpolar cross-ACC heat flux.

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Chris W. Hughes, Mike P. Meredith, and Karen J. Heywood

Abstract

It is proposed that, for periods between about 10 and 220 days, the variability in Antarctic circumpolar transport is dominated by a barotropic mode that follows f/H contours almost everywhere. Theoretical arguments are given that suggest the possible importance of this mode and show that bottom pressure to the south of the current should be a good monitor of its transport. The relevance of these arguments to eddy-resolving models is confirmed by data from the Fine Resolution Antarctic Model and the Parallel Ocean Climate Model. The models also show that it may be impossible to distinguish the large-scale barotropic variability from local baroclinic processes, given only local measurements, although this is not generally a problem to the south of the Antarctic Circumpolar Current. Comparison of bottom pressures measured in Drake Passage and subsurface pressure on the Antarctic coast, with wind stresses derived from meteorological analyses, gives results consistent with the models, showing that wind stress to the south of Drake Passage can explain most of the observed coherence between wind stresses and circumpolar transport. There is an exception to this in a narrow band of periods near 20 days for which winds farther north seem important. It is suggested that this may be due to a sensitivity of the “almost free” mode to winds at particular locations, where the current crosses f/H contours.

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Louise C. Biddle, Karen J. Heywood, Jan Kaiser, and Adrian Jenkins

Abstract

Pine Island Ice Shelf, in the Amundsen Sea, is losing mass because of warm ocean waters melting the ice from below. Tracing meltwater pathways from ice shelves is important for identifying the regions most affected by the increased input of this water type. Here, optimum multiparameter analysis is used to deduce glacial meltwater fractions from water mass characteristics (temperature, salinity, and dissolved oxygen concentrations), collected during a ship-based campaign in the eastern Amundsen Sea in February–March 2014. Using a one-dimensional ocean model, processes such as variability in the characteristics of the source water masses on shelf and biological productivity/respiration are shown to affect the calculated apparent meltwater fractions. These processes can result in a false meltwater signature, creating misleading apparent glacial meltwater pathways. An alternative glacial meltwater calculation is suggested, using a pseudo–Circumpolar Deep Water endpoint and using an artificial increase in uncertainty of the dissolved oxygen measurements. The pseudo–Circumpolar Deep Water characteristics are affected by the under ice shelf bathymetry. The glacial meltwater fractions reveal a pathway for 2014 meltwater leading to the west of Pine Island Ice Shelf, along the coastline.

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Benjamin G. M. Webber, Karen J. Heywood, David P. Stevens, and Karen M. Assmann

Abstract

The ice shelves around the Amundsen Sea are rapidly melting as a result of the circulation of relatively warm ocean water into their cavities. However, little is known about the processes that determine the variability of this circulation. Here we use an ocean circulation model to diagnose the relative importance of horizontal and vertical (overturning) circulation within Pine Island Trough, leading to Pine Island and Thwaites ice shelves. We show that melt rates and southward Circumpolar Deep Water (CDW) transports covary over large parts of the continental shelf at interannual to decadal time scales. The dominant external forcing mechanism for this variability is Ekman pumping and suction on the continental shelf and at the shelf break, in agreement with previous studies. At the continental shelf break, the southward transport of CDW and heat is predominantly barotropic. Farther south within Pine Island Trough, northward and southward barotropic heat transports largely cancel, and the majority of the net southward temperature transport is facilitated by baroclinic and overturning circulations. The overturning circulation is related to water mass transformation and buoyancy gain on the shelf that is primarily facilitated by freshwater input from basal melting.

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Adrian J. Matthews, Dariusz B. Baranowski, Karen J. Heywood, Piotr J. Flatau, and Sunke Schmidtko

Abstract

A surface diurnal warm layer is diagnosed from Seaglider observations and develops on half of the days in the Cooperative Indian Ocean Experiment on Intraseasonal Variability/Dynamics of the Madden–Julian Oscillation (CINDY/DYNAMO) Indian Ocean experiment. The diurnal warm layer occurs on days of high solar radiation flux (>80 W m−2) and low wind speed (<6 m s−1) and preferentially in the inactive stage of the Madden–Julian oscillation. Its diurnal harmonic has an exponential vertical structure with a depth scale of 4–5 m (dependent on chlorophyll concentration), consistent with forcing by absorption of solar radiation. The effective sea surface temperature (SST) anomaly due to the diurnal warm layer often reaches 0.8°C in the afternoon, with a daily mean of 0.2°C, rectifying the diurnal cycle onto longer time scales. This SST anomaly drives an anomalous flux of 4 W m−2 that cools the ocean. Alternatively, in a climate model where this process is unresolved, this represents an erroneous flux that warms the ocean. A simple model predicts a diurnal warm layer to occur on 30%–50% of days across the tropical warm pool. On the remaining days, with low solar radiation and high wind speeds, a residual diurnal cycle is observed by the Seaglider, with a diurnal harmonic of temperature that decreases linearly with depth. As wind speed increases, this already weak temperature gradient decreases further, tending toward isothermal conditions.

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Benjamin G. M. Webber, David P. Stevens, Adrian J. Matthews, and Karen J. Heywood

Abstract

The authors show that a simple three-dimensional ocean model linearized about a resting basic state can accurately simulate the dynamical ocean response to wind forcing by the Madden–Julian oscillation (MJO). This includes the propagation of equatorial waves in the Indian Ocean, from the generation of oceanic equatorial Kelvin waves to the arrival of downwelling oceanic equatorial Rossby waves in the western Indian Ocean, where they have been shown to trigger MJO convective activity. Simulations with idealized wind forcing suggest that the latitudinal width of this forcing plays a crucial role in determining the potential for such feedbacks. Forcing the model with composite MJO winds accurately captures the global ocean response, demonstrating that the observed ocean dynamical response to the MJO can be interpreted as a linear response to surface wind forcing. The model is then applied to study “primary” Madden–Julian events, which are not immediately preceded by any MJO activity or by any apparent atmospheric triggers, but have been shown to coincide with the arrival of downwelling oceanic equatorial Rossby waves. Case study simulations show how this oceanic equatorial Rossby wave activity is partly forced by reflection of an oceanic equatorial Kelvin wave triggered by a westerly wind burst 140 days previously, and partly directly forced by easterly wind stress anomalies around 40 days prior to the event. This suggests predictability for primary Madden–Julian events on times scales of up to five months, following the reemergence of oceanic anomalies forced by winds almost half a year earlier.

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Michael P. Meredith, Alberto C. Naveira Garabato, David P. Stevens, Karen J. Heywood, and Richard J. Sanders

Abstract

Two meridional hydrographic transects (in 1995 and 1999) across the eastern Scotia Sea are used to investigate variability in the deep and bottom waters between the South Scotia Ridge and South Georgia. There is a significant warming of the warm deep water (WDW) south of the southern boundary of the Antarctic Circumpolar Current (ACC); waters are approximately 0.1°–0.2°C warmer in 1999 than 1995. This is due mainly to raised WDW potential temperatures in the Weddell Sea being fed through to the Scotia Sea as the WDW flows northeastward in the Weddell Gyre. There is a warming of the Weddell Sea Deep Water (WSDW) of approximately 0.05°C across the whole extent of the section, and an accompanying change in salinity that maintains the potential temperature–salinity relationship. This is caused by variability in the properties of the water overflowing the South Scotia Ridge, rather than enhanced outflow of the bottom layer of the Scotia Sea or movements of the ACC fronts, and may be related to changes in the intensity of the Weddell Gyre circulation. Consideration of other works suggests that the colder WSDW of 1995 is likely to be the anomalous case, rather than the warmer WSDW of 1999. The 1999 section reveals an inflow of Lower WSDW from east of the South Sandwich Arc via the Georgia Passage; this is constrained to the south of the southern boundary, and is not apparent in the 1995 measurements. Meanders in the southern boundary at Georgia Passage are likely to play a role in controlling the inflow of Lower WSDW, although changes in the peak density of the WSDW flowing across the South Scotia Ridge may be important also, with a denser inflow from the south acting to preclude an inflow of similar density from the northeast.

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