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Vassilis Zervakis and Murray D. Levine

Abstract

Wind-generated inertial currents can radiate from the mixed layer as horizontally and vertically propagating new-inertial internal gravity waves. To study the timescale of the decay of mixed layer energy and the magnitude of the energy transfer to the ocean below, the authors developed a numerical, linear model on a β plane, using baroclinic modes to describe the velocity field. The model is unforced-wave propagation is initiated by specifying the mixed layer currents that would he generated by a moving atmospheric front. The numerical results are interpreted using concepts of modal interference and modal departure that can be evaluated analytically, thereby permitting predictions Of some features of wave field evolution without the need to run the numerical model. The energy exchange with the pycnocline and deep ocean is explored as a function of the propagation speed and direction of the front, the horizontal extent of the storm, and the background stratification.

The timescale of energy transfer from the mixed layer to the pycocline due to modal interference is greatly affected by the β effect, causing much faster energy transfer for currents generated by southward propagating fronts. The timescale is typically not a strong function of mixed layer depth; however. the magnitude of the energy transfer is. Besides modal interference, vertical energy propagation occurs when low modes leave the area- a possibility for storms of finite horizontal extent. The deep stratification and f also affect the timescale; climatological examples indicate faster wave evolution at low latitudes.

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Murray D. Levine and Vassilis Zervakis

Abstract

Observations of near-inertial oscillations collected during the Ocean Storms Experiment in the northeast Pacific Ocean are compared with results from a linear, numerical model on a β plane, developed by Zervakis and Levine. A slab mixed layer model, forced by the observed wind time series, is used to identify three isolated events of local generation in October, January, and March for detailed analysis. Synoptic storm track maps are used to estimate the initial horizontal wavenumber of the mixed layer currents that are used as initial conditions to the modal. A comparison of the modal with the observed currents reveals some differences and similarities. Overall the January and March events are better represented by the model than the October event. The timescale of the initiation of vertical propagation of energy from the mixed layer occurs almost immediately in October rather than after 8 days in January and March-this difference cannot be explained by the model. The observed vertical and temporal structure indicates that the near-Inertial energy propagated as a “beam” of energy through the pycnocline, especially in October. In the model the wave energy appears to accumulate at the top of the pycnocline. Physical processes that might be responsible for the deficiency of the model are discussed.

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Michael Sparrow, Olaf Boebel, Vassilis Zervakis, Walter Zenk, Alán Cantos-Figuerola, and W. John Gould

Abstract

The Eurofloat experiment was a joint initiative to examine the large-scale spreading of Mediterranean Water (MW) and Labrador Sea Water in the northeast North Atlantic. RAFOS float data from the southern (MW) portion of the Eurofloat experiment have been examined in conjunction with historical float data in order to calculate quasi-Eulerian means in an effort to separate and quantify the constituents of the spreading of the MW tongue east of the Mid-Atlantic Ridge. While recent studies focussed chiefly on the role of meddies in the shaping of the MW tongue, this analysis also examines the tongue's second constituent, that is, the “background” (non-meddy advective and diffusive) flow. The results suggest the existence of two regimes approximately to the north and south of the 36°N parallel (i.e., the latitude of the Gulf of Cadiz), which are distinguished by different types of dominant spreading mechanisms for MW. To the south of the Gulf of Cadiz, the background flow shows an incoherent and weak mean, whereas the mean velocity of the salt enhanced meddies is strong and to the southwest. In contrast, to the north of 36°N the mean velocity of the meddies seems to be less pronounced and the background flow is shown to be a major component in the northwestward spreading of the MW tongue. The two regimes are separated by the Azores Current, which previously has been hypothesized to act as a dynamic barrier to the southward advective spreading of the background regime, which the meddies are able to penetrate because of their high kinetic energy. Overall, the meddies are calculated to contribute to approximately half of the total salinity anomaly flux.

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