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Melinda M. Hall and Nick P. Fofonoff

Abstract

Two CTD sections across the Gulf Stream at 68° and 55°W were acquired in late March of 1988 within 11 days of one another as part of an effort to look at downstream changes in the current. Using complementary current meter measurements, sections of total barotropic and geostrophic baroclinic velocity are constructed and used to calculate transport in potential density classes. Potential vorticity sections are presented for both locations, including the effects of planetary, stretching, and relative vorticity. The data are also used to examine the core properties of recently formed 18°Water at the two sections. It is found that: 1) water parcels in the exposed surface layers experience downstream density and potential vorticity changes consistent with surface forcing; 2) thermocline Gulf Stream transport is conserved downstream and below the exposed layers is conserved within individual density classes; 3) subthermocline Gulf Stream transport increases modestly at levels above the sill depth of the New England Seamounts but quadruples at levels below that; 4) the calculated potential vorticity structure is consistent with the transport distribution and historical observations and displays several distinct layers; and 5) transport and potential vorticity distributions together suggest that five active layers and steep bottom topography are required to fully describe downstream evolution of the Gulf Stream as an open-ocean eastward jet.

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Harry L. Bryden and Nick P. Fofonoff

Abstract

Estimates of horizontal derivatives of velocity made by differencing velocity measurements are used to show that the observed velocity field due to low-frequency mesoscale motions during the preliminary Mid-Ocean Dynamics Experiment (MODE-0) field program is horizontally nondivergent within estimated errors. The errors in horizontal derivatives of 0.15×10−6 s−1 art are too large for direct estimates of horizontal divergence to be made accurately. The vorticity, however, can be estimated from these horizontal derivatives with an error small compared with its magnitude. Over the measurement period of 50 days, the advection of planetary vorticity balances only one-half of the local change of vorticity so these observations cannot be explained in terms of barotropic Rossby waves alone. There are indications that vortex stretching, estimated from a linear heat balance, may balance the remaining local change of vorticity as expected for baroclinic Rossby waves. Based on other measurements in this regions, however, it is likely that the horizontal advection of relative vorticity is also important in the vorticity balance. A positive, but not significantly different from zero, correlation between estimates of relative vorticity and advection of planetary vorticity suggests that the enstrophy of the observed velocity field is decreasing with time. In conjunction with a similar result for the perturbation potential energy obtained in this region, this result supports the view that the MODE region is a region of decay, rather than growth, of the low-frequency mesoscale motions.

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