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Louise C. Sime, David P. Stevens, Karen J. Heywood, and Kevin I. C. Oliver

Abstract

A decomposition of meridional overturning circulation (MOC) cells into geostrophic vertical shears, Ekman, and bottom pressure–dependent (or external mode) circulation components is presented. The decomposition requires the following information: 1) a density profile wherever bathymetry changes to construct the vertical shears component, 2) the zonal-mean zonal wind stress for the Ekman component, and 3) the mean depth-independent velocity information over each isobath to construct the external mode. The decomposition is applied to the third-generation Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3) to determine the meridional variability of these individual components within the Atlantic Ocean. The external mode component is shown to be extremely important where western boundary currents impinge on topography, and also in the area of the overflows. The Sverdrup balance explains the shape of the external mode MOC component to first order, but the time variability of the external mode exhibits only a very weak dependence on the wind stress curl. Thus, the Sverdrup balance cannot be used to determine the external mode changes when examining temporal change in the MOC. The vertical shears component allows the time-mean and the time-variable upper North Atlantic MOC cell to be deduced at 25°S and 50°N. A stronger dependency on the external mode and Ekman components between 8° and 35°N and in the regions of the overflows means that hydrographic sections need to be supplemented by bottom pressure and wind stress information at these latitudes. At the decadal time scale, variability in Ekman transport is less important than that in geostrophic shears. In the Southern Hemisphere the vertical shears component is dominant at all time scales, suggesting that hydrographic sections alone may be suitable for deducing change in the MOC at these latitudes.

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

Abstract

The confluence between the Brazil Current and the Malvinas Current [the Brazil–Malvinas Confluence (BMC)] in the Argentine Basin is characterized by a complicated thermohaline structure favoring the exchanges of mass, heat, and salt between the subtropical gyre and the Antarctic Circumpolar Current (ACC). Analysis of thermohaline properties of hydrographic sections in the BMC reveals strong interactions between the ACC and subtropical fronts. In the Subantarctic Front, Subantarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW), and Circumpolar Deep Water (CDW) warm (become saltier) by 0.4° (0.08), 0.3° (0.02), and 0.6°C (0.1), respectively. In the subtropical gyre, AAIW and North Atlantic Deep Water have cooled (freshened) by 0.4° (0.07) and 0.7°C (0.11), respectively.

To quantify those ACC–subtropical gyre interactions, a box inverse model surrounding the confluence is built. The model diagnoses a subduction of 16 ± 4 Sv (1 Sv ≡ 106 m3 s−1) of newly formed SAMW and AAIW under the subtropical gyre corresponding to about half of the total subduction rate of the South Atlantic found in previous studies. Cross-frontal heat (0.06 PW) and salt (2.4 × 1012 kg s−1) gains by the ACC in the BMC contribute to the meridional poleward heat and salt fluxes across the ACC. These estimates correspond to perhaps half of the total cross-ACC poleward heat flux. The authors’ results highlight the BMC as a key region in the subtropical–ACC exchanges.

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Paul A. Dodd, Martin R. Price, Karen J. Heywood, and Miles Pebody

Abstract

A compact water sampler rated to full ocean depth has been deployed from an autonomous underwater vehicle (AUV) to enable oceanographic tracer measurements. Techniques developed to allow the instrument to collect up to 49 samples of sufficient purity for tracer measurement without the need for extensive flushing have increased its sampling frequency, allowing a 200-mL seawater sample to be collected in 10 min. This is achieved by flushing the instrument and sample containers before deployment with a fluid of known properties that can be detected after recovery using salinity analysis. A deployment in which water samples were collected for oxygen isotope ratio analysis is presented as an example. Factors limiting the reliability of the instrument when deployed from an AUV are identified and procedures are developed to address critical problems.

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Céline Heuzé, Karen J. Heywood, David P. Stevens, and Jeff K. Ridley

Abstract

Changes in bottom temperature, salinity, and density in the global ocean by 2100 for CMIP5 climate models are investigated for the climate change scenarios RCP4.5 and RCP8.5. The mean of 24 models shows a decrease in density in all deep basins, except the North Atlantic, which becomes denser. The individual model responses to climate change forcing are more complex: regarding temperature, the 24 models predict a warming of the bottom layer of the global ocean; in salinity, there is less agreement regarding the sign of the change, especially in the Southern Ocean. The magnitude and equatorward extent of these changes also vary strongly among models. The changes in properties can be linked with changes in the mean transport of key water masses. The Atlantic meridional overturning circulation weakens in most models and is directly linked to changes in bottom density in the North Atlantic. These changes are the result of the intrusion of modified Antarctic Bottom Water, made possible by the decrease in North Atlantic Deep Water formation. In the Indian, Pacific, and South Atlantic Oceans, changes in bottom density are congruent with the weakening in Antarctic Bottom Water transport through these basins. The authors argue that the greater the 1986–2005 meridional transports, the more changes have propagated equatorward by 2100. However, strong decreases in density over 100 yr of climate change cause a weakening of the transports. The speed at which these property changes reach the deep basins is critical for a correct assessment of the heat storage capacity of the oceans as well as for predictions of future sea level rise.

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

Abstract

Hydrographic and lowered acoustic Doppler current profiler data along a line from the Falkland Islands to South Georgia via the Maurice Ewing Bank are used to estimate the flow of circumpolar water into the Argentine Basin, and to study the interaction of the Antarctic Circumpolar Current with the Falkland Plateau.

The estimated net transport of 129 ± 21 Sv (Sv ≡ 106 m3 s−1) across the section is shared between three major current bands. One is associated with the Subantarctic Front (SAF; 52 ± 6 Sv), and the other two with branches of the Polar Front (PF) over the sill of the Falkland Plateau (44 ± 9 Sv) and in the northwestern Georgia Basin (45 ± 9 Sv). The latter includes a local reinforcement (∼20 Sv) by a deep anticyclonic recirculation around the Maurice Ewing Bank. While the classical hydrographic signature of the PF stands out in this eastbound branch, it is less distinguishable in the northbound branch over the plateau. Other circulation features are a southward entrainment of diluted North Atlantic Deep Water from the Argentine Basin over the eastern part of the Falkland Plateau, and an abyssal anticyclonic flow in the western Georgia Basin, opposite to what was generally assumed.

The different behavior of the SAF and PF at the Falkland Plateau (no structural modification of the former and partitioning of the latter) is attributed to the PF being deeper than the sill depth on the upstream side of the plateau, unlike the SAF. It is suggested that the partitioning takes place at a location where the 2500-m and 3000-m isobaths diverge at the southern edge of the plateau. The western branch of the PF crosses the plateau at a distance of ∼250 km to the east of the SAF. Comparison with a section across the Falkland Current farther downstream shows that its deep part subsequently joins the SAF on the northern side of the plateau where the 2000–3000 m isobaths converge in the steep Falkland Escarpment. The result of this two-stage bathymetric effect is a net transfer of at least 10 Sv from the PF to the SAF at the crossing of the Falkland Plateau.

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Madeleine K. Youngs, Andrew F. Thompson, M. Mar Flexas, and Karen J. Heywood

Abstract

The complex export pathways that connect the surface waters of the Weddell Sea with the Antarctic Circumpolar Current influence water mass modification, nutrient fluxes, and ecosystem dynamics. To study this exchange, 40 surface drifters, equipped with temperature sensors, were released into the northwestern Weddell Sea’s continental shelf and slope frontal system in late January 2012. Comparison of the drifter trajectories with a similar deployment in early February 2007 provides insight into the interannual variability of the surface circulation in this region. Observed differences in the 2007 and 2012 drifter trajectories are related to a variable surface circulation responding to changes in wind stress curl over the Weddell Gyre. Differences between northwestern Weddell Sea properties in 2007 and 2012 include 1) an enhanced cyclonic wind stress forcing over the Weddell Gyre in 2012; 2) an acceleration of the Antarctic Slope Current (ASC) and an offshore shift of the primary drifter export pathway in 2012; and 3) a strengthening of the Coastal Current (CC) over the continental shelf in 2007. The relationship between wind stress forcing and surface circulation is reproduced over a longer time period in virtual drifter deployments advected by a remotely sensed surface velocity product. The mean offshore position and speed of the drifter trajectories are correlated with the wind stress curl over the Weddell Gyre, although with different temporal lags. The drifter observations are consistent with recent modeling studies suggesting that Weddell Sea boundary current variability can significantly impact the rate and source of exported surface waters to the Scotia Sea, a process that determines regional chlorophyll distributions.

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Marina Azaneu, Karen J. Heywood, Bastien Y. Queste, and Andrew F. Thompson

Abstract

The dense water outflow from the Antarctic continental shelf is closely associated with the strength and position of the Antarctic Slope Front. This study explores the short-term and spatial variability of the Antarctic Slope Front system and the mechanisms that regulate cross-slope exchange using highly temporally and spatially resolved measurements from three ocean gliders deployed in 2012. The 22 sections along the eastern Antarctic Peninsula and west of the South Orkney Islands are grouped regionally and composited by isobaths. There is consistency in the front position around the Powell Basin, varying mostly between the 500- and 800-m isobaths. In most of the study area the flow is bottom intensified. The along-slope transport of the Antarctic Slope Current (upper 1000 m) varies between 0.2 and 5.9 Sv (1 Sv ≡ 106 m3 s−1) and does not exhibit a regional pattern. The magnitude of the velocity field shows substantial variability, up to twice its mean value. Higher eddy kinetic energy (0.003 m2 s−2) is observed in sections with dense water, possibly because of baroclinic instabilities in the bottom layer. Distributions of potential vorticity show an increase toward the shelf along isopycnals and also in the dense water layer. Glider sections located west of the South Orkney Islands indicate a northward direction of the flow associated with the Weddell Front, which differs from previous estimates of the mean circulation. This study provides some of the first observational confirmation of the high-frequency variability associated with an active eddy field that has been suggested by recent numerical simulations in this region.

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Pierre Cauchy, Karen J. Heywood, Nathan D. Merchant, Bastien Y. Queste, and Pierre Testor

Abstract

Wind speed measurements are needed to understand ocean–atmosphere coupling processes and their effects on climate. Satellite observations provide sufficient spatial and temporal coverage but are lacking adequate calibration, while ship- and mooring-based observations are spatially limited and have technical shortcomings. However, wind-generated underwater noise can be used to measure wind speed, a method known as Weather Observations Through Ambient Noise (WOTAN). Here, we adapt the WOTAN technique for application to ocean gliders, enabling calibrated wind speed measurements to be combined with contemporaneous oceanographic profiles over extended spatial and temporal scales. We demonstrate the methodology in three glider surveys in the Mediterranean Sea during winter 2012/13. Wind speeds ranged from 2 to 21.5 m s−1, and the relationship to underwater ambient noise measured from the glider was quantified. A two-regime linear model is proposed, which validates a previous linear model for light winds (below 12 m s−1) and identifies a regime change in the noise generation mechanism at higher wind speeds. This proposed model improves on previous work by extending the validated model range to strong winds of up to 21.5 m s−1. The acquisition, data processing, and calibration steps are described. Future applications for glider-based wind speed observations and the development of a global wind speed estimation model are discussed.

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Andrew F. Thompson, Karen J. Heywood, Sally E. Thorpe, Angelika H. H. Renner, and Armando Trasviña

Abstract

An array of 40 surface drifters, drogued at 15-m depth, was deployed in February 2007 to the east of the tip of the Antarctic Peninsula as part of the Antarctic Drifter Experiment: Links to Isobaths and Ecosystems (ADELIE) project. Data obtained from these drifters and from a select number of local historical drifters provide the most detailed observations to date of the surface circulation in the northwestern Weddell Sea. The Antarctic Slope Front (ASF), characterized by a ∼20 cm s−1 current following the 1000-m isobath, is the dominant feature east of the peninsula. The slope front bifurcates when it encounters the South Scotia Ridge with the drifters following one of three paths. Drifters (i) are carried westward into Bransfield Strait; (ii) follow the 1000-m isobath to the east along the southern edge of the South Scotia Ridge; or (iii) become entrained in a large-standing eddy over the South Scotia Ridge. Drifters are strongly steered by contours of f /h (Coriolis frequency/depth) as shown by calculations of the first two moments of displacement in both geographic coordinates and coordinates locally aligned with contours of f /h. An eddy-mean decomposition of the drifter velocities indicates that shear in the mean flow makes the dominant contribution to dispersion in the along-f /h direction, but eddy processes are more important in dispersing particles across contours of f /h. The results of the ADELIE study suggest that the circulation near the tip of the Antarctic Peninsula may influence ecosystem dynamics in the Southern Ocean through Antarctic krill transport and the export of nutrients.

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Michael P. Meredith, John M. Vassie, Robert Spencer, and Karen J. Heywood

Abstract

Bottom pressure recorders (BPRs) have been deployed at Drake Passage (DP) to monitor changes in the volume transport of the Antarctic Circumpolar Current (ACC) through the passage. The use of inverted echo sounders (IESs) in assisting the interpretation of the BPR data is presented. The initial data processing of the IES data is outlined, and the accuracy of the data described. The most significant limitation on the application of the data is the presence of a sea-state bias with a root-mean-square value of around 0.4 ms. IES data are shown to perform well at determining whether individual changes in bottom pressure are due to changes in cross-passage-averaged barotropic transport or due to the effect of meanders, eddies, and/or lateral shifts of ACC fronts.

The conversion of acoustic travel time to more useful oceanographic parameters (dynamic height, baroclinic pressure, inverse-barometer-corrected sea level) is described. A method for improving the performance of very deep bottom pressure in monitoring ACC barotropic transport changes is described for the case where the BPR–IES instrumentation is deployed near an ACC front. Reasons for the inability of this method to improve the ability of shallower pressure records in monitoring the ACC are discussed, and suggestions for future refinements are outlined.

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