Lateral exchange within Gulf Stream warm-core ring surface layers

Abstract

The radial exchange of near-surface properties in Gulf Stream anticyclonic, warm-core rings is studied with the use of hydrographic and satellite data in coordination with some simple models. Analysis of physical data is restricted to the salinity field because it is the most conservative property available in the near-surface ring. A calculation of the azimuthal averaged salinity as a function of radius from ring center for cruises in April and June 1982 in ring 82B gives lower bounds on the lateral exchange in the ring of 1.07 × 10 7 kg s −1. The exchange rate reaches a maximum just inside the location of maximum surface velocities at a radius of 40 km. Similar rates of exchange can be deduced from earlier data taken in ring 81D. This inward flux of low salinity Slope Water enhances the near-surface stratification. Such a transport of fluid can be more important than surface heating in the determination of the near-surface stability except in very strong winter storms. Horizontal transport of fluid into the ring can be explained by a horizontal diffusivity of ≈ 90 m 2 s −1 if adjective effects are ignored. Adjective terms make the actual strength of diffusion uncertain. Shear dispersion or inward radial advection can reduce the necessary diffusivity while an outward radial flow would demand higher diffusivities. The importance of lateral exchange to biological properties is examined with a simple model where the ring, its surrounding frontal zone, and the far field Slope Water make up three interacting components. If the ring frontal zone is assumed to have higher “production”, either due to increased vertical mixing or through active concentration of organisms tied to convergence, the core of the ring can eventually exceed the far field Slope Water concentrations. Edge enhancement in the ring frontal zone is consistent with a time series of Coastal Zone Colour Scanner (CZCS) chlorophyll images over the April–June 1982 period as well as other observations in ring 82B.