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Sarah T. Gille

Abstract

Four years of ocean vector wind data are used to evaluate statistics of wind stress over the ocean. Raw swath wind stresses derived from the Quick Scatterometer (QuikSCAT) are compared with five different global gridded wind products, including products based on scatterometer observations, meteorological analysis winds from the European Centre for Medium-Range Weather Forecasts, and reanalysis winds from the National Centers for Environmental Prediction. Buoy winds from a limited number of sites in the Pacific Ocean are also considered.

Probability density functions (PDFs) computed for latitudinal bands show that mean wind stresses for the six global products are largely in agreement, while variances differ substantially, by a factor of 2 or more, with swath wind stresses indicating highest variances for meridional winds and for zonal winds outside the Tropics. Higher moments of the PDFs also differ. Kurtoses are large for all wind products, implying that PDFs are not Gaussian. None of the available gridded products fully captures the range of extreme wind events seen in the raw swath data.

Frequency spectra for the five gridded products agree with frequency spectra from swath data at low frequencies, but spectral slopes differ at higher frequencies, particularly for frequencies greater than 100 cycles per year (cpy), which are poorly resolved by a single scatterometer. In the frequency range between 10 and 90 cpy that is resolved by the scatterometer, spectra derived from swath data are flatter than spectra from gridded products and are judged to be flatter than ω −2/3 at all latitudes.

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Sarah T. Gille

Abstract

Autonomous Lagrangian Circulation Explorer (ALACE) floats are used to examine eddy fluxes in the Southern Ocean. Eddy fluxes are calculated from differences between ALACE float data and mean fields derived from hydrographic atlas data or objectively mapped float observations. Heat fluxes indicate an average poleward eddy heat transport across the Antarctic Circumpolar Current (ACC) of about 3–7 kW m−2 at 900-m depth. Because analysis of current meter data suggests that ALACE's 9–25-day averaging underestimates the total heat flux, the initial ALACE estimates are rescaled to account for this undersampling. This results in a total corrected heat flux of 5–10 kW m−2 at 900 m, depending on the mean field used for the calculations. If the cross-ACC heat flux is assumed to vary exponentially through the water column with an e-folding depth of 1000 m, then the implied net poleward heat flux across the ACC is between 0.3 ± 0.1 and 0.6 ± 0.3 (×1015 W). These estimates are in agreement with previous Southern Ocean eddy flux estimates, which have suggested a cross-ACC heat fluxes ranging between 0.05 and 0.9 (×1015 W). Cross-stream fluxes vary geographically, with the largest fluxes occuring in the Indian Ocean sector, near the Agulhas Retroflection. Statistically significant poleward fluxes also occur along the core of the ACC. Along-stream fluxes are comparable in size to cross-stream fluxes. Momentum fluxes observed by ALACE are isotropic and do not indicate statistically significant eddy–mean flow interactions.

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Sarah T. Gille

Abstract

Potential vorticity (PV) is used as an indicator of the forcing processes and dissipation at work in the Southern Ocean. Output from the Semtner–Chervin model run with quarter-degree resolution is considered on isopycnal surfaces along Montgomery streamfunctions. Numerical results are compared with hydrographic measurements. Although simple hypotheses might suggest that subsurface PV should be unaffected by wind forcing and constant along streamlines, these results indicate that even at about 1000-m depth, PV varies along mean streamlines in both the numerical model output and in the in situ observations. The changes in PV are largely represented by stratification changes rather than shifts in the Coriolis parameter or in relative vorticity. In the numerical model output, a combination of mechanisms is responsible for these changes in PV, including transient tracer fluxes, transient momentum fluxes, diffusive processes, and long-term tracer drift.

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Sarah T. Gille

Abstract

Long-term trends in the heat content of the Southern Hemisphere ocean are evaluated by comparing temperature profiles collected during the 1990s with profiles collected starting in the 1930s. Data are drawn both from ship-based hydrographic surveys and from autonomous floats. Results show that the upper 1000 m of the Southern Hemisphere ocean has warmed substantially during this time period at all depths. Warming is concentrated within the Antarctic Circumpolar Current (ACC). On a global scale, this warming trend implies that the ocean has gained heat from the atmosphere over the last 50 to 70 years. Although the data do not preclude the possibility that the Southern Ocean has warmed as a result of increased heat fluxes, either into the ocean or within the ocean, in general the strong trend in the Southern Ocean appears regionally consistent with a poleward migration of the ACC, possibly driven by long-term poleward shifts in the winds of the region, as represented by the southern annular mode.

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Sarah T. Gille

Abstract

Autonomous Lagrangian Circulation Explorer (ALACE) floats are used to examine mean flow and eddy fluxes at 900-m depth in the Southern Ocean. Mean temperature and dynamic topography from float data are consistent with earlier estimates from hydrographic surveys, although floats imply warmer temperatures and narrower frontal structures than do atlas data. Differences between hydrographic and ALACE dynamic topography suggest the presence of eastward bottom velocities of about 2 cm s−1 below the eastward-flowing jets of the Antarctic Circumpolar Current. Flow is steered by bathymetry and can be represented as an equivalent barotropic system with an e-folding depth of about 700 m.

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Sarah T. Gille

Abstract

The momentum balance of the Antarctic Circumpolar Current is investigated using both output from a high-resolution primitive equation model and sea surface height measurements from the Geosat altimeter. In the Semtner–Chervin general circulation model, run with approximately one-quarter degree resolution and time-varying ECMWF winds, topographic form stress is the dominant process balancing the surface wind forcing. Detailed examination of form stress in the model indicates that it is due to three large topographic obstructions located at Kerguelen Island, Campbell Plateau, and Drake Passage. The difference between wind stress and form stress represents the lateral transfer of momentum into and out of the ACC. It is examined both in zonal coordinates to conform to the model architecture and along mean streamlines in order to reduce the effects of standing eddies. In this particular model, in stream coordinates, biharmonic friction dominates the lateral transfer of momentum. Since biharmonic friction is a parameterization of subgrid-scale transient eddy processes, this indicates that the unresolved transient eddy processes play a critical role in fluxing momentum across the ACC in this model. Although the relative importance of individual terms in the momentum balance does not vary substantially along streamlines, elevated levels of eddy kinetic energy are associated with the three major topographic features. In contrast, altimeter data show elevated energy levels at many more topographic features of intermediate scales, suggesting that smaller topographic effects are better able to communicate with the surface in the real ocean than in the model.

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Sarah T. Gille and Leonel Romero

Abstract

Autonomous Lagrangian Circulation Explorer (ALACE) floats were designed to measure subsurface velocities throughout the global ocean. In order to transmit their data to satellite, they spend 24 h at the ocean surface during each 10–25-day cycle. During this time the floats behave as undrogued drifters. In the Southern Ocean, floats tend to advect downwind and, in accordance with Ekman theory, slightly to the left of the wind during their time at the surface. Mean displacements are likely to carry floats northward and, correspondingly, with each cycle, the Southern Ocean floats will move into warmer water with higher dynamic height. Because of large variability, the northward trend may not be discernible for any single float: in 2 years' worth of 10-day cycles, a typical float will be displaced 100 ± 270 km northward relative to a float that never surfaces. Float surface velocities and wind speed are statistically correlated at the 95% confidence level. Compared with drogued drifters, floats tend to move more rapidly, are advected more strongly downwind, and are more sensitive to changes in wind speed. Regression coefficients estimated from the differences between float and drogued drifter velocities suggest that floats may be used to estimate the mean upper ocean currents in regions where drogued drifter data are not available.

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Wilbert Weijer and Sarah T. Gille

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This study addresses the response of the Southern Ocean to high-frequency wind forcing, focusing on the impact of several barotropic modes on the circumpolar transport. A suite of experiments is performed with an unstratified model of the Southern Ocean, forced with a stochastic wind stress that contains a large range of frequencies with synoptic time scales. The Southern Ocean adjustment displays a different character for frequencies below and above 0.2 cpd. The low-frequency range is dominated by an “almost-free-mode” response in the region where contours of f /H are obstructed by only a few bathymetric features; the truly free mode only plays a minor role. Topographic form stress, rather than friction, is the dominant decay mechanism of the Southern Mode. It leads to a spindown time scale on the order of 3 days. For the high-frequency range, the circumpolar transport is dominated by the resonant excitation of oscillatory modes. The “active” response of the ocean leads to strong changes and even discontinuities in the phase relation between transport and wind stress.

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Philip Sura and Sarah T. Gille

Abstract

Sea surface height anomalies measured by the Ocean Topography Experiment (TOPEX)/Poseidon satellite altimeter indicate high values of skewness and kurtosis. Except in a few regions, including the Gulf Stream, the Kuroshio Extension, and the Agulhas Retroflection, that display bimodal patterns of sea surface height variability, kurtosis is uniformly greater than 1.5 times the squared skewness minus an adjustment constant. This relationship differs substantially from what standard Gaussian or double-exponential noise would produce. However, it can be explained by a simple theory in which the noise is assumed to be multiplicative, meaning that a larger background state implies larger random noise elements. The existence of multiplicative noise can be anticipated from the equations of motion, if ocean dynamics are split into a slowly decorrelating deterministic component and a rapidly decorrelating contribution that is approximated as noise. Such a model raises the possibility of predicting the probabilities of extreme sea surface height anomalies from first physical principles and may provide a useful null hypothesis for non-Gaussian sea surface height variability.

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Wilbert Weijer, Sarah T. Gille, and Frédéric Vivier

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The barotropic intraseasonal variability in the Australia–Antarctic Basin (AAB) is studied in terms of the excitation and decay of topographically trapped barotropic modes. The main objective is to reconcile two widely differing estimates of the decay rate of sea surface height (SSH) anomalies in the AAB that are assumed to be related to barotropic modes. First, an empirical orthogonal function (EOF) analysis is applied to almost 15 years of altimeter data. The analysis suggests that several modes are involved in the variability of the AAB, each related to distinct areas with (almost) closed contours of potential vorticity. Second, the dominant normal modes of the AAB are determined in a barotropic shallow-water (SW) model. These stationary modes are confined by the closed contours of potential vorticity that surround the eastern AAB, and the crest of the Southeast Indian Ridge. For reasonable values of horizontal eddy viscosity and bottom friction, their decay time scale is on the order of several weeks. Third, the SW model is forced with realistic winds and integrated for several years. Projection of the modal velocity patterns onto the output fields shows that the barotropic modes are indeed excited in the model, and that they decay slowly on the frictional O(3 weeks) time scale. However, the SSH anomalies in the modal areas display rapid O(4 days) decay. Additional analysis shows that this rapid decay reflects the adjustment of unbalanced flow components through the emission of Rossby waves. Resonant excitation of the dominant free modes accounts for about 20% of the SSH variability in the forced-model run. Other mechanisms are suggested to explain the region of high SSH variability in the AAB.

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