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  • Author or Editor: Richard D. Rosen x
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Rui M. Ponte
and
Richard D. Rosen

Abstract

The ocean's angular momentum (M) and torques about the Polar axis are analyzed using output from the global, eddy-resolving model of Semtner and Chervin. Seasonal variability in M is dominated by the annual cycle, whose magnitude appears capable of helping explain the residual in the solid earth-atmosphere annual momentum budget. Planetary (M Ω) and relative (Mr ) ocean angular momentum components have comparable seasonal amplitudes. Most of the mean signal in Mr , results from flows in the Antarctic Circumpolar Current region, but flows as far north as approximately 30°S am needed to explain the seasonal cycle. Local1y, the strongest variability in relative angular momentum is found in the Tropics at all depths, a manifestation of the zonal, recirculating character of the tropical circulation. The time rate of change of M is very small compared to the applied wind torque. Calculation of bottom pressure torques using the geostrophic relation reveals a dominant balance between them and the surface wind torques in the model, implying a rapid transfer of angular momentum between the atmosphere and the solid earth through the ocean. The torque balance holds for latitudes totally blocked by continental boundaries as well as for latitudes that are only partially blocked (e.g., Drake Passage), suggesting the same angular momentum transfer mechanism for closed basin and Antarctic Circumpolar Current regions. Implications of the results for future ocean modeling efforts are discussed.

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Rui M. Ponte
,
David A. Salstein
, and
Richard D. Rosen

Abstract

A barotropic shallow-water model is used to study the large-scale sea level response to realistic barometric forcing at periods ranging from 1 day to 1 year. Results are presented from coarse resolution “open” ocean experiments (i.e., no shallow continental shelf regions or marginal seas) with coastal geometries and bottom topography representative of the North Atlantic and Pacific basins. The validity of the inverted barometer (IB) approximation is examined in detail, including nonlocal effects which result from taking into account the constant volume of the ocean. These effects are found to be important at low latitudes, where a considerable part of the sea level variability is related to pressure forcing over higher latitudes.

Root-mean-square deviations from an IB response in the range of 1–3 cm are typical, with most of the variance occurring at high frequencies. Basin-averaged estimates yield IB deviations of only a few percent at time scales longer than 1 week increasing to 5%–20% over the range from 1 week to 2 days, but significantly larger departures from isostatic behavior are possible locally. Over these time scales, deviations in the Pacific are generally larger than in the North Atlantic. For periods shorter than 2 days, the IB approximation is not reliable, with the largest departures from equilibrium occurring at frequencies where basinwide resonances are present. Model results agree, in general, with findings of previous studies and are interpreted using simple dynamical ideas.

Some of the implications of our results are explored. Preliminary calculations suggest that nonisostatic effects in the ocean may significantly contribute to variations in the earth's rotation at short time scales (1 to 2 weeks or shorter). Issues concerning the need to interpret satellite altimeter estimates of surface height in the presence of pressure-driven variability are also briefly discussed.

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