EVOLUTION OF THE TORTUGAS GYRE AND ITS INFLUENCE ON RECRUITMENT IN THE FLORIDA-KEYS

Moored current measurements, satellite-tracked drifters, shipboard hyd rography and a time sequence of satellite derived surface thermal imag es are used to show the formation and evolution of cold, cyclonic gyre s coupled to large offshore meanders of the Florida Current in the sou thern Straits of Florida (SSF). Gyre formation is dependent upon the o rientation of the Loop Current as it enters the SSF. With a well devel oped Loop Current the flow overshoots the entry to the SSF causing the formation of a cold recirculation off the Dry Tortugas, approximately 200 km in size, that persists over time scales of about 100 days. The demise of the gyre occurs as it moves to the east at about 5 km.d-1, reducing to half its original size off Big Pine and Marathon Keys and unobservable off the northern keys. Previously observed local gyres be tween Key West and Islamorada, FL (Lee et al., 1992) are identified as a latter stage in the downstream evolution of the gyres formed off th e Tortugas. When the Loop Current is not developed, flow from the Yuca tan Channel turns anticyclonically into the SSF, causing strong eastwa rd flow over the slope off the Dry Tortugas and lower Florida Keys, an d gyre formation does not occur. Gyre formation provides enhanced food supply, retention and shoreward transports for successful recruitment in the western and lower Florida Keys of locally spawned snapper and grouper larvae. A previously unknown potential retention area for larv al invertebrates and fish on the southwest Florida shelf has been iden tified from a satellite tracked drifter trajectory, and a local lobste r recruitment pathway combining advective influences from the Tortugas gyre, Loop Current and shelf circulation are proposed.