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Pierpaolo Falco, Annalisa Griffa, Pierre-Marie Poulain, and Enrico Zambianchi

Abstract

The surface transport properties in the Adriatic Sea, a semienclosed subbasin of the Mediterranean Sea, have been studied using a drifter dataset in the period December 1994–March 1996. Three main points have been addressed. First, the exchange between southern and northern regions and between deep and coastal areas have been studied, focusing on the role of topography. A significant cross-topography or cross-shelf exchange has been found, probably due to the direct wind forcing and to the influence of stratification that isolates the surface flow from bottom effects, especially in the open sea. Second, a Lagrangian transport model with parameters derived from the data has been implemented. Simulated particles have been compared with drifter data with positive results. The model is found to be able to reproduce reality with good approximation, except for a specific advective event during the late summer season. Finally, the residence timescale T, that is, the average time spent by a surface particle in the basin, has been estimated. Direct estimates from the data suggest T ≈ 70–90 days, but these values are biased due to the finite lifetime of the drifters. Model results have been used to estimate the bias, and they suggest a “true” value of T ≈ 200 days.

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Stefano Pierini, Pierpaolo Falco, Giovanni Zambardino, Thomas A. McClimans, and Ingrid Ellingsen

Abstract

Various dynamical aspects of nonlinear western boundary currents (WBCs) have been investigated experimentally through physical modeling in a 5-m-diameter rotating basin. The motion of a piston with a velocity up that can be as low as up = 0.5 mm s−1 induces a horizontally unsheared current of homogeneous water that, flowing over a topographic beta slope, experiences westward intensification. First, the character of WBCs for various degrees of nonlinearity is investigated. By varying up, flows ranging from the highly nonlinear inertial Charney regime down to a weakly nonlinear regime can be simulated. In the first case, the dependence of zonal length scales on up is found to be in agreement with Charney’s theory; for weaker flows, a markedly different functional dependence emerges describing the initial transition toward the linear, viscous case. This provides an unprecedented coverage of nonlinear WBC dependence on an amplitude parameter in terms of experimental data. WBC separation from a wedge-shaped continent past a cape (simulating Cape Hatteras) due to inertial overshooting is then analyzed. By increasing current speed, a critical behavior is identified according to which a very small change of up marks the transition from a WBC that follows the coast past the cape to a WBC (nearly dynamically similar to a full-scale Gulf Stream) that separates from the cape without any substantial deflection, as with the Gulf Stream Extension. The important effect of the deflection angle of the continent is analyzed as well. Finally, the qualitative effect of a sloping sidewall along a straight coast is considered: the deflection of the flow away from the western wall due to the tendency to preserve potential vorticity clearly emerges.

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