Motivated by the substantial sensitivity of eddies in two-layer quasi-geostrophic (QG) turbulence models to the strength of bottom drag, this study explores the sensitivity of eddies in more realistic ocean general circulation model (OGCM) simulations to bottom drag strength. The OGCM results are interpreted using previous results from horizontally homogeneous, two-layer, flat-bottom, f-plane, doubly periodic QG turbulence simulations and new results from two-layer -plane QG turbulence simulations run in a basin geometry with both flat and rough bottoms. Baroclinicity in all of the simulations varies greatly with drag strength, with weak drag corresponding to more barotropic flow and strong drag corresponding to more baroclinic flow. The sensitivity of the baroclinicity in the QG basin simulations to bottom drag is considerably reduced, however, when rough topography is used in lieu of a flat bottom. Rough topography reduces the sensitivity of the eddy kinetic energy amplitude and horizontal length scales in the QG basin simulations to bottom drag to an even greater degree. The OGCM simulation behavior is qualitatively similar to that in the QG rough bottom basin simulations in that baroclinicity is more sensitive to bottom drag strength than are eddy amplitudes or horizontal length scales. Rough topography therefore appears to mediate the sensitivity of eddies in models to the strength of bottom drag. The sensitivity of eddies to parameterized topographic internal lee wave drag, which has recently been introduced into some OGCMs, is also briefly discussed. Wave drag acts like a strong bottom drag in that it increases the baroclinicity of the flow, without strongly affecting eddy horizontal length scales.

Application of dynamical systems tools has recently revealed in surface ocean currents produced by a Hybrid-Coordinate Ocean Model (HYCOM) simulation the presence of a persistent large-scale Lagrangian coherent structure (LCS) on the southern portion of the west Florida shelf (WFS). Consistent with satellite-tracked drifter trajectories, this LCS constitutes a cross-shelf barrier for the lateral transport of passive tracers. Because of the constraints that the above LCS, as well as smaller-scale LCSs lying shoreside, can impose on pollutant dispersal and its potentially very important biological consequences, a study was carried out on the nature of the surface ocean Lagrangian motion on the WFS. The analysis is based on the same simulated surface ocean velocity field that has been able to sustain the aforementioned persistent cross-shelf transport barrier. Examination of several diagnostics suggests that chaotic stirring dominates over turbulent mixing on time scales of up to two months or so. More specifically, it is found on those time scales that tracer evolution at a given length scale is governed to a nonnegligible extent by coarser-scale velocity field features, fluid particle dispersion is spatially inhomogeneous, and the Lagrangian evolution is more irregular than the driving Eulerian flow.

The temporal response of the length of a partially-mixed estuary to changes in freshwater discharge, , and tidal amplitude, , is studied using a 108 day time series collected along the length of the Hudson River estuary in the spring and summer of 2004 and a long-term (13.4 year) record of , , and near-surface salinity. When was moderately high, the tidally-averaged length of the estuary, , here defined as the distance from the mouth to the up-estuary location where the vertically-averaged salinity is five psu, fluctuated by more than 47 km over the spring-neap cycle, ranging from 28 km to >75 km. During low flow periods, varied very little over the spring-neap cycle and approached a steady length. The response is quantified and compared to predictions of a linearized model derived from the global estuarine salt balance. The model is forced by fluctuations in and relative to average discharge, , and tidal amplitude, , and predicts the linear response time scale, τ, and the steady-state length, , for average forcing. Two vertical mixing schemes are considered, in which a) mixing is proportional to and b) dependence of mixing on stratification is also parameterized. Based on least-squares fits between and estuary length predicted by the model, estimated τ varied by an order of magnitude from a period of high average discharge ( = 750 ms, τ = 4.2 days) to a period of low discharge ( = 170 ms, τ = 40.4 days). Over the range of observed discharge, ∝ , consistent with the theoretical scaling for an estuary whose landward salt flux is driven by vertical estuarine exchange circulation. Estimated τ was proportional to the discharge advection time scale (/, where is the cross-sectional area of the estuary). However, τ was three to four times larger than the theoretical prediction. The model with stratification dependent mixing predicted variations in with higher skill than the model with mixing proportional to . This model provides insight into the time dependent response of a partially-stratified estuary to changes in forcing and explains the strong dependence of the amplitude of the spring-neap response on freshwater discharge. However, the utility of the linear model is limited because it assumes a uniform channel and because the underlying dynamics are nonlinear and the forcing, and , can undergo large amplitude variations. River discharge, in particular, can vary by over an order of magnitude over timescales comparable to or shorter than the response timescale of the estuary.