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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Distribution of Reynolds stress carried by mesoscale variability in the Antarctic Circumpolar Current

Satellite altimeter data collected by the Geosat Exact Repeat Mission were used to investigate turbulent stress resulting from the variability of surface geostrophic currents in the Antarctic Circumpolar Current. The altimeter measured sea level along the subsatellite track. The variability of the along-track slope of sea level is directly proportional to the variability of surface geostrophic currents in the cross-track direction. Because the grid of crossover points is dense at high latitudes, the satellite data could be used for mapping the temporal and spatial variability of the current. Two and a half years of data were used to compute the statistical structure of the variability. The statistics included the probability distribution functions for each component of the current, the time-lagged autocorrelation functions of the variability, and the Reynolds stress produced by the variability. The results demonstrate that stress is correlated with bathymetry. In some areas the distribution of negative stress indicate that eddies contribute to an acceleration of the mean flow, strengthening the hypothesis that baroclinic instability makes important contributions to strong oceanic currents.

Johnson, Thomas J.↗
NASA NTRS1985

Observing large-scale temporal variability of ocean currents by satellite altimetry - With application to the Antarctic circumpolar current

A new method is developed for studying large-scale temporal variability of ocean currents from satellite altimetric sea level measurements at intersections (crossovers) of ascending and descending orbit ground tracks. Using this method, sea level time series can be constructed from crossover sea level differences in small sample areas where altimetric crossovers are clustered. The method is applied to Seasat altimeter data to study the temporal evolution of the Antarctic Circumpolar Current (ACC) over the 3-month Seasat mission (July-October 1978). The results reveal a generally eastward acceleration of the ACC around the Southern Ocean with meridional disturbances which appear to be associated with bottom topographic features. This is the first direct observational evidence for large-scale coherence in the temporal variability of the ACC. It demonstrates the great potential of satellite altimetry for synoptic observation of temporal variability of the world ocean circulation.

Fu, L.-L.↗
NASA NTRS1994

Mean sea surface height of the Antarctic Circumpolar Current from Geosat data: Method and application

The mean sea surface height across the Antarctic Circumpolar Current (ACC) has been reconstructed from height variability measured by the Geosat altimeter without assuming prior knowledge of the geoid. For this study, an automated technique has been developed to estimate mean sea surface height for each satellite ground track using a meandering Gaussian jet model, and errors have been estimated using Monte Carlo simulation. The results are objectively mapped to produce a picture of the mean Subantarctic and Polar Fronts, which together comprise the major components of the Antarctic Circumpolar Current. The meandering jet model explains between 40% and 70% of the height variance along the jet axes. The results show that the fronts are substantially steered by topography and that the jets have an average Gaussian width of about 44 km in the meridional direction and meander about 75 km to either side of their mean locations. The average height difference across the Subantarctic Front (SAF) is 0.7 m and across the Polar Front (PF) 0.6 m. The mean widths of the fronts are correlated with the size of the baroclinic Rossby radius.

Gille, Sarah T.↗
NASA NTRS2007

Antarctic Circumpolar Current Transport Variability during 2003-05 from GRACE

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 cm H2O (1 cm H2O approx. 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 cm H2O 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 equiv 10(exp 6)cu m/s) of barotropic transport variability per cm H2O of BP change is estimated. Wind stress data time series from the Quick Scatterometer (QuikSCAT), averaged monthly, zonally, and over the latitude band 40(deg)- 65(deg)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.

Gravity Recovery and Climate Experiment (GRACE)↗
NASA NTRS1984

Temporal variability of the Antarctic Circumpolar Current observed from satellite altimetry

Sea level measurements by the Seasat altimeter were used to study the temporal variability of the Antarctic Circumpolar Current between July and October 1978. Large-scale zonal coherence in the cross-stream sea level difference was observed, indicating a general increase in the surface geostrophic velocity of the current around the Southern Ocean. The result demonstrates the power of satellite altimetry to monitor the variability of large-scale ocean currents.

Fu, L.-L.↗
NASA NTRS2015

An Assessment of Antarctic Circumpolar Current and Southern Ocean Meridional Overturning Circulation During 1958-2007 in a Suite of Interannual CORE-2 Simulations

In the framework of the second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II), we present an analysis of the representation of the Antarctic Circumpolar Current (ACC) and Southern Ocean Meridional Overturning Circulation (MOC) in a suite of seventeen global ocean-sea ice models. We focus on the mean, variability and trends of both the ACC and MOC over the 19582007 period, and discuss their relationship with the surface forcing. We aim to quantify the degree of eddy saturation and eddy compensation in the models participating in CORE-II, and compare our results with available observations, previous fine-resolution numerical studies and theoretical constraints. Most models show weak ACC transport sensitivity to changes in forcing during the past five decades, and they can be considered to be in an eddy saturated regime. Larger contrasts arise when considering MOC trends, with a majority of models exhibiting significant strengthening of the MOC during the late 20th and early 21st century. Only a few models show a relatively small sensitivity to forcing changes, responding with an intensified eddy-induced circulation that provides some degree of eddy compensation, while still showing considerable decadal trends. Both ACC and MOC interannual variability are largely controlled by the Southern Annular Mode (SAM). Based on these results, models are clustered into two groups. Models with constant or two-dimensional (horizontal) specification of the eddy-induced advection coefficient show larger ocean interior decadal trends, larger ACC transport decadal trends and no eddy compensation in the MOC. Eddy-permitting models or models with a three-dimensional time varying show smaller changes in isopycnal slopes and associated ACC trends, and partial eddy compensation. As previously argued, a constant in time or space is responsible for a poor representation of mesoscale eddy effects and cannot properly simulate the sensitivity of the ACC and MOC to changing surface forcing. Evidence is given for a larger sensitivity of the MOC as compared to the ACC transport, even when approaching eddy saturation. Future process studies designed for disentangling the role of momentum and buoyancy forcing in driving the ACC and MOC are proposed.

Southern Ocean dynamics↗
NASA NTRS2006

Antarctic Circumpolar Current Transport Variability during 2003-05 from GRACE

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 cm(sub H2O) (1 cm(sub H2O) similar to 1 hPa), slightly lower than the 1.9 cm(sub H2O) 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 cm(sub H2O) (1.1 and 0.6 cm(sub H2O) 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 equivalent to 10(exp 6) m(exp 3) s(exp -1)) of barotropic transport variability per cm(sub H2O) of BP change is estimated. Wind stress data time series from the Quick Scatterometer (QuikSCAT), averaged monthly, zonally, and over the latitude band 40 de - 65 deg 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.

oceanographic regions↗
NASA NTRS1995

Dynamics of the Antarctic Circumpolar Current. Evidence for Topographic Effects from Altimeter Data and Numerical Model Output

Geosat altimeter data and numerical model output are used to examine the circulation and dynamics of the Antarctic Circumpolar Current (ACC). The mean sea surface height across the ACC has been reconstructed from height variability measured by the altimeter, without assuming prior knowledge of the geoid. The results indicate locations for the Subantarctic and Polar Fronts which are consistent with in situ observations and indicate that the fronts are substantially steered by bathymetry. Detailed examination of spatial and temporal variability indicates a spatial decorrelation scale of 85 km and a temporal e-folding scale of 34 days. Empirical Orthogonal Function analysis suggests that the scales of motion are relatively short, occuring on 1000 km length-scales rather than basin or global scales. The momentum balance of the ACC has been investigated using output from the high resolution primitive equation model in combination with altimeter data. In the Semtner-Chervin quarter-degree general circulation model topographic form stress is the dominant process balancing the surface wind forcing. In stream coordinates, the dominant effect transporting momentum across the ACC is bibarmonic friction. Potential vorticity is considered on Montgomery streamlines in the model output and along surface streamlines in model and altimeter data. (AN)

Gille, Sarah T.↗
NASA NTRS1983

Interaction of the antarctic circumpolar current with bottom topography - An investigation using satellite altimetry

Theoretical studies on the interaction of a zonal current with a zonal ridge, an isolated bump, and a meridional ridge compare favorably with hydrographic observations within the eastward flowing Antarctic Circumpolar Current where it flows over similar topographic features. However, the existing hydrographic data are insufficient for examining the temporal stability and kinematic behavior of the resulting mesoscale structures. In this study, some of these transient features have been compared with patterns in sea surface variability, derived from collinear satellite altimetric data. When these features occurred near the crossing point of two satellite ground traces, it was possible to characterize their length scales, dynamic height relief, and translational and surface geostrophic velocities.

Colton, M. T.↗
NASA NTRS1991

Mesoscale and large-scale variability of the Antarctic circumpolar current

This investigation of the physical oceanography of the Southern Ocean will carry out two parallel efforts during the years preceding the launch of TOPEX/POSEIDON. First, the Geosat data will be used to develop a preliminary descriptive picture of the mesoscale and large-scale, low-frequency surface circulation of the Southern Ocean. Some of this analysis of Geosat data has already begun. For example, as a measure of the geographical distribution of mesoscale variability, a color-coded map of the standard deviation of sea level from two years of Geosat data is shown. Efforts are presently under way to investigate the seasonal and year-to-year variability of this mesoscale energy. The data are also being used to generate low-pass filtered fields of sea level from which the temporal evolution of large-scale variability in the Southern Ocean may be investigated. The second parallel effort is the development and test of modeling and data assimilation techniques that will later be applied to TOPEX/POSEIDON data during the postlaunch phase. One objective of the modeling and data assimilation is to investigate the relation between mesoscale sea level variations and eddy flux in the Southern Ocean. Uncertainties in present estimates of the various components of meridional oceanic heat transport are large. The evidence presented indicates that the very energetic mesoscale variability in the ACC apparently accounts for much of the estimated 0.45x10(exp 15) watts of poleward heat transport across the ACC required to balance the heat budget. Eddy variability is strongly coherent vertically in the ACC, at least in the vicinity of Drake Passage where nearly all of the historical in situ data have been collected.

Chelton, Dudley B.↗
NASA NTRS1976

Characteristics of Antarctic sea ice as determined by satellite-borne microwave imagers

The electrically scanning microwave radiometers on Nimbus 5 (wavelengths of 1.55 cm) and Nimbus 6 (wavelengths of 0.81 cm) produce images with a resolution of about 30 km. Weekly maps of sea ice coverage are generated, showing the percent of open water in each 2.5 x 2.5 degree cell. More detailed studies of the dynamics and thermodynamics of the sea ice and its interaction with the ocean and the atmosphere are carried out, using 3-day average pseudo-color images and maps of ice boundaries. Features of the Antarctic ice discerned from microwave images are discussed, and the important features of Antarctic currents are illustrated.

Zwally, H. J.↗
NASA NTRS1991

Analysis of altimeter data jointly with seafloor electric data (vertically integrated velocity) and VCTD-yoyo data (detailed profiles of VCTD)

We propose simultaneous analyses of the TOPEX/POSEIDON altimetry data, in situ data--mainly permanent seafloor electric recordings--and velocity, conductivity, temperature, density (VCTD)-yoyo data at several stations in areas of scientific interest. We are planning experiments in various areas of low and high energy levels. Several complementary and redundant methods will be used to characterize the ocean circulation and its short- and long-term variability. We shall emphasize long-term measurement using permanent stations. Our major initial objectives with the TOPEX/POSEIDON mission are the Confluence area in the Argentine Basin and the Circumpolar Antarctic Current. An early experiment was carried out in the Confluence zone in 1988 and 1990 (Confluence Principal Investigators, 1990) to prepare for an intensive phase later one. This intensive phase will include new types of instrumentation. Preliminary experiments will be carried out in the Mediterranean Sea (in 1991) and in the North Atlantic Ocean (in 1992, north of the Canary Islands) to test the new instrumentation.

Tarits, Pascal D.↗
NASA NTRS1991

Ocean transport and variability studies of the South Pacific, Southern, and Indian Oceans

The objectives of this study are to analyze ocean dynamics in the western South Pacific and the adjacent Southern Ocean and the eastern Indian Ocean. Specifically, our objectives for these three regions are, for the South Pacific Ocean: (1) To estimate the volume transport of the east Australian Current (EAC) along the Australian coast and in the Tasman Front, and to estimate the time variability (on seasonal and interannual time scales) of this transport. (2) To contribute to estimating the meridional heat and freshwater fluxes (and their variability) at about 30 deg S. Good estimates of the transport in the western boundary current are essential for accurate estimates of these fluxes. (3) To determine how the EAC transport (and its extension, the Tasman Front and the East Auckland Current) closes the subtropical gyre of the South Pacific and to better determine the structure at the confluence of this current and the Antarctic Circumpolar Current. (4) To examine the structure and time variability of the circulation in the western South Pacific and the adjacent Southern Ocean, particularly at the Tasman Front. For the Indian Ocean: (5) To study the seasonal interannual variations in the strength of the Leeuwin Current. (6) To monitor the Pacific-Indian Ocean throughflow and the South Equatorial and the South Java Currents between northwest Australia and Indonesia. (7) To study the processes that form the water of the permanent oceanic thermocline and, in particular, the way in which new thermocline water enters the permanent thermocline in late winter and early spring as the mixed layer restratifies. For the Southern Ocean: (8) To study the mesoscale and meridional structure of the Southern Ocean between 150 deg E and 170 deg E; in particular, to describe the Antarctic frontal system south of Tasmania and determine its interannual variability; to estimate the exchanges of heat, salt, and other properties between the Indian and Pacific Oceans; and to investigate the annual ventilation of the Antarctic Intermediate Water and Subantarctic Mode Water Masses.

Church, John A.↗
NASA NTRS1990

Variations of global mesoscale eddy energy observed from Geosat

Data on oceanic-current variability were extracted from Geosat altimeter observations for 44 17-day repeat cycles, using the Sandwell and Zhang (1989) technique to process the altimeter data and to produce a sea-surface-slope profile having an estimated accuracy of 0.2 microrad. These were used to generate a series of global eddy kinetic energy maps, each averaged over 3 months, together with their mean. It was found that the maximum mean eddy kinetic energy per unit mass exceeds 2000 sq cm/sq sec for most of the western boundary currents; for the Antarctic Circumpolar Current, however, this value reaches only 500 sq cm/sq sec.

Shum, C. K.↗
NASA NTRS1999

Determining the Ocean's Role on the Variable Gravity Field on Earth Rotation

A number of ocean models of different complexity have been used to study changes in the oceanic mass field and angular momentum and their relation to the variable Earth rotation and gravity field. Time scales examined range from seasonal to a few days. Results point to the importance of oceanic signals in driving polar motion, in particular the Chandler and annual wobbles. Results also show that oceanic signals have a measurable impact on length-of-day variations. Various circulation features and associated mass signals, including the North Pacific subtropical gyre, the equatorial currents, and the Antarctic Circumpolar Current play a significant role in oceanic angular momentum variability.

Ponte, Rui M.↗
NASA NTRS2000

Determining the Ocean's Role on the Variable Gravity Field and Earth Rotation

A number of ocean models of different complexity have been used to study changes in the oceanic angular momentum (OAM) and mass fields and their relation to the variable Earth rotation and gravity field. Time scales examined range from seasonal to a few days. Results point to the importance of oceanic signals in driving polar motion, in particular the Chandler and annual wobbles. Results also show that oceanic signals have a measurable impact on length-of-day variations. Various circulation features and associated mass signals, including the North Pacific subtropical gyre, the equatorial currents, and the Antarctic Circumpolar Current play a significant role in oceanic angular momentum variability. The impact on OAM values of an optimization procedure that uses available data to constrain ocean model results was also tested for the first time. The optimization procedure yielded substantial changes, in OAM, related to adjustments in both motion and mass fields,as well as in the wind stress torques acting on the ocean. Constrained OAM values were found to yield noticeable improvements in the agreement with the observed Earth rotation parameters, particularly at the seasonal timescale.

Ponte, Rui M.↗
NASA NTRS1989

Estimation of the two-dimensional ocean current shear field with a synthetic aperture radar

A double inverse method to convert SAR data to estimates of current shear was developed theoretically. Problems of applying the technique to real SAR data were examined. The first step of the inversion process involved elimination of noise in the (x-vector, k-vector) wave spectra deduced from SAR images. The second step, estimation of current shear, required the existence of well defined swell and a favorable single of interaction between swell and ocean current. Application of the method in the Antarctic circumpolar current encountered sea conditions that both satisfied and violated these requirements. Where the wave-current geometry was favorable, realistic surface current velocity fields were obtained, suggesting the presence of a substantial mesoscale eddy. Another attempt to apply the method in the Gulf Stream produced wave spectral estimates that showed no clear peak and hence produced no current estimates.

Barnett, Tim P.↗