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Victor Zlotnicki

Abstract

Geosat altimetric data for November 1986 to December 1988 are used to estimate sea level differences between the Sargasso Sea and the slope waters across the Gulf Stream region, averaged between 73° and 61°W, and comparable areas across the Kuroshio extension region, averaged between 143° and 156°E. Sea level differences (south minus north) in both regions are higher in the fall. Their projection onto an annual cosine peaks in late September-mid-October, with 9 cm (Gulf Stream region) and 6.9 cm (Kuroshio region) amplitudes, and accounts for about 60% of the variance. Residual altimetric errors due to water vapor, orbit, and sea state bias are shown to be minor or negligible contributors, as is uncertainty in the atmospheric pressure used in the inverse barometer correction and uncertainty in the tidal model removed. Ship drift data averaged in a comparable manner appear to respond to time changes in Ekman transport rather than the geostrophic transport measured by the sea level differences. Dynamic heights above 200 m computed from climatological (1940–75) water temperature and salinity data show that, while dynamic heights in both regions rise by 6 to 7 cm in September due to heating above the seasonal thermocline, the south–north difference in dynamic height has a somewhat different seasonal pattern: in the Gulf Stream region the annual cycle of climatological differential dynamic heights (south minus north) accounts for 86% of the variance, has 3 dyn cm amplitude and peaks in late November; in the Kuroshio region it is negligible.

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Victor Zlotnicki, John Wahr, Ichiro Fukumori, and Yuhe T. Song

Abstract

Gravity Recovery and Climate Experiment (GRACE) gravity data spanning January 2003–November 2005 are used as proxies for ocean bottom pressure (BP) averaged over 1 month, spherical Gaussian caps 500 km in radius, and along paths bracketing the Antarctic Circumpolar Current’s various fronts. The GRACE BP signals are compared with those derived from the Estimating the Circulation and Climate of the Ocean (ECCO) ocean modeling–assimilation system, and to a non-Boussinesq version of the Regional Ocean Model System (ROMS). The discrepancy found between GRACE and the models is 1.7 cmH2O (1 cmH2O ∼ 1 hPa), slightly lower than the 1.9 cmH2O estimated by the authors independently from propagation of GRACE errors. The northern signals are weak and uncorrelated among basins. The southern signals are strong, with a common seasonality. The seasonal cycle GRACE data observed in the Pacific and Indian Ocean sectors of the ACC are consistent, with annual and semiannual amplitudes of 3.6 and 0.6 cmH2O (1.1 and 0.6 cmH2O with ECCO), the average over the full southern path peaks (stronger ACC) in the southern winter, on days of year 197 and 97 for the annual and semiannual components, respectively; the Atlantic Ocean annual peak is 20 days earlier. An approximate conversion factor of 3.1 Sv (Sv ≡ 106 m3 s−1) of barotropic transport variability per cmH2O of BP change is estimated. Wind stress data time series from the Quick Scatterometer (QuikSCAT), averaged monthly, zonally, and over the latitude band 40°–65°S, are also constructed and subsampled at the same months as with the GRACE data. The annual and semiannual harmonics of the wind stress peak on days 198 and 82, respectively. A decreasing trend over the 3 yr is observed in the three data types.

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Nikolai Maximenko, Peter Niiler, Luca Centurioni, Marie-Helene Rio, Oleg Melnichenko, Don Chambers, Victor Zlotnicki, and Boris Galperin

Abstract

Presented here are three mean dynamic topography maps derived with different methodologies. The first method combines sea level observed by the high-accuracy satellite radar altimetry with the geoid model of the Gravity Recovery and Climate Experiment (GRACE), which has recently measured the earth’s gravity with unprecedented spatial resolution and accuracy. The second one synthesizes near-surface velocities from a network of ocean drifters, hydrographic profiles, and ocean winds sorted according to the horizontal scales. In the third method, these global datasets are used in the context of the ocean surface momentum balance. The second and third methods are used to improve accuracy of the dynamic topography on fine space scales poorly resolved in the first method. When they are used to compute a multiyear time-mean global ocean surface circulation on a 0.5° horizontal resolution, both contain very similar, new small-scale midocean current patterns. In particular, extensions of western boundary currents appear narrow and strong despite temporal variability and exhibit persistent meanders and multiple branching. Also, the locations of the velocity concentrations in the Antarctic Circumpolar Current become well defined. Ageostrophic velocities reveal convergent zones in each subtropical basin. These maps present a new context in which to view the continued ocean monitoring with in situ instruments and satellites.

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