Gulf Stream warm core rings: Data analysis and model simulations.

摘要

The large-scale flow patterns of Gulf Stream warm core rings (WCRs), the eddy stability and propagation, and the eddy - shelf/slope interaction are investigated. The Oleander ADCP data are analyzed to construct the flow patterns of WCRs and a numerical model is applied to study the basic processes of WCRs.; With the aid of SSHA, SST and GLOBEC drifter data, the repeat Oleander ADCP transects are mapped to examine the WCR flow structures and their interaction with the Gulf Stream and the shelf and slope seas. The ring center for each ADCP transect is determined from all available data sources (SSHA, SST and ADCP). The spatial flow patterns of WCRs are constructed by co-locating ADCP transects with respect to a common ring center. The re-mapped ADCP transects are fitted to stream function based on multivariate optimal interpolation to produce coherent large-scale flow patterns. Four WCRs are examined. The derived flow patterns show complex flow features involving the WCR, the Gulf Stream, shelf and slope seas, and the cold and warm streamers. The concurrent buoy trajectories also are analyzed, which indicate significant changes in the shape and translational velocity of WCRs propagating along the continental margin.; In previous model studies, a large isolated eddy generally cannot maintain its original structure and strength. In this study, a three-dimensional, primitive equation model is used to examine the eddy stability and propagation in the open ocean. We consider two types of vertical velocity structures: compensated and corotating eddies. Our results show that a compensated eddy rapidly (100 days) evolves into baroclinic instability and breaks apart, and a corotating eddy is more stable but is subject to significant decay in the presence of even a modest bottom slope. Including a large bottom drag coefficient however significantly stabilizes the model eddy. The large bottom dissipation destroys the deep-penetrating unstable cyclones, yet leaves the surface eddy unaffected. A WCR generally propagates in the southwest direction under the planetary beta and nonlinear effects. The eddy trajectory is not affected by modest bottom slope (∼10-3), but can be significantly modified when encountering steep submarine ridge and seamount. The eddy - shelf/slope interaction also is examined. The eddy is initialized far off the shelfbreak, and is allowed to propagate onto the shelf/slope. Our study suggests that the propagation of a compensated eddy along the shelf/slope is determined by the competition of the planetary beta effect, the nonlinear effect and the image effect. The eddy collision with the shelfbreak causes small-scale flow features, including small cyclones/anticyclones, slope jet and strong upwelling and subduction. These flow features significantly affect the shelf and slope circulation, and may play a major role in the water exchange and the biological production in the shelf/slope frontal zone.