Mesoscale Eddies and Submesoscale, Coherent Vortices: Their Existence Near and Interactions with the Gulf Stream

Abstract

An array of tall current meter moorings, bottom-mounted inverted echo sounders and pressure gauges was deployed for one year in an area about 150-500 km northeast of Cape Hatteras, North Carolina. The Gulf Stream jet was observed by the upper current meters in the array, and the deep western boundary current was observed by several of the deep instruments. Numerous mesoscale eddies and submesoscale, coherent vortices (SCVs) were observed progressing through the region. Two types of mesoscale eddies were seen. Three cold-core Gulf Stream rings were observed, two of which interacted with the Stream, ultimately coalescing with the jet within the array area. Associated with the first ring-Stream interaction was a lateral shift of the position of the Gulf Stream's axis. This shift repositioned the jet about 100 km seaward, and this new path lasted for several months. The second ring-Stream interaction was followed by another shift in the Stream's path. The bottom pressure signal observed during each ring passage through the area gave clear evidence of a vertical “tilt” to each ring's low pressure center. The second type of mesoscale eddy observed was an intriguing, subsurface, anticyclonically swirling warm eddy which moved through the array area along the seaward side of the Gulf Stream in the Sargasso Sea. This 160 km diameter eddy had maximum observed swirl velocities over 50 cm/sec and a positive temperature anomaly of about 4° C., both at about 900 m below the sea sufrace. No clear surface temperature expression was observed by satellite, and the in situ velocity observations suggest that there was little surface velocity, implying that this energetic feature was an interior eddy. Its movement was complex, with a northeastward progression through the array area followed by a seaward turn and another transit through the array area towards the southwest. Progression speed in either direction was about 12-15 km/day. The origin of this eddy and its level of interaction with the Gulf Stream are not clear at this point in the analysis. The SCVs were seen mostly below and beside the Gulf Stream jet, with both cyclonically and anticyclonically swirling SCVs observed. That none were observed in the Gulf Stream jet implies that either SCV transit speeds there were too fast to allow their detection, or that SCVs become sheared apart by the Stream and lose their identity. Of the nineteen SCVs identified, anticyclones outnumbered cyclones almost two to one. The anticyclones typically had larger diameters and higher swirl velocities than the cyclones.