Evidence of Cross-Frontal Exchange Processes in the Gulf Stream Based on Isopycnal RAFOS Float Data

Abstract

A unique set of Lagrangian observations has recently been collected in the Gulf Stream using the newly developed isopycnal RAFOS float. Between January, 1984 and October, 1985, thirty-seven of these drifters were launched in the main thermocline of the current off Cape Hatteras and lacked acoustically downstream for 30 or 45 days. Temperature and pressure were also recorded along each float trajectory. The isopycnal capability of this drifter allows it to follow fluid parcel pathways quite accurately along the sloping density surfaces of the Gulf Stream. The RAFOS drifters revealed a striking pattern of vertical and cross-stream motion in Gulf Stream meanders. Floats were consistently observed to upwell (downwell) and move onshore (offshore) as they approached anticyclonic (cyclonic) meander crests (troughs). The rms vertical velocity in the center of the stream was observed to be 0.08 cm s⁻¹ on the 12°C surface. No mean vertical motion was detected in the main thermocline of the Gulf Stream between 70° and 55°W. Using a model of the mean cross-stream thermal structure of the current, rms cross-stream velocities were estimated to be 8–10 cm s⁻¹. This meander-induced circulation represents an important mechanism for cross-frontal exchange. When the Gulf Stream is meandering, fluid parcels from the center of the current are brought to the edges and often escape completely. In fact, 60% of the floats launched at Cape Hatteras escaped from the Gulf Stream at least once before reaching 65°W. These losses were not evenly distributed in the vertical; retention was greater in the upper thermocline (11–16°C) than in the lower thermocline (7°–11°C). Comparison of goat trajectories with infrared satellite imagery shows that the meander-induced cross-frontal fluid exchange is enhanced by ring—current interaction and time evolution of the Gulf Stream path.