Cyclogenesis in the deep ocean beneath the Gulf Stream - 1. Description

One of the primary scientific results of the Synoptic Ocean Prediction (SYN OP) observational program was the discovery of strong cyclones in the deep ocean beneath the large amplitude Gulf Stream meander troughs that routinel y form at about 68 degrees W. These strong well-organized cyclones extend t o at least 3500 m below the sea surface and are an important component of t he overall dynamical variability of the Gulf Stream and adjacent deep water s. Typically, a small amplitude Gulf Stream meander "stalls" near 68 degree s W and begins to amplify. As the amplitude of troughs in the Gulf Stream j et increases, the currents at 3500 m strengthen and turn, forming a cycloni c circulation pattern. During SYNOP, six well-defined instances of meander trough amplification and deep cyclogenesis occurred. The cyclones were char acterized by strong swirl speeds (up to 0.5 m s(-1)) and were long-lived (t ypically lasting 6-9 weeks) frequent occurrences (present 35% of the time d uring the 25 month deployment period). The structure of the cyclones at 350 0 m is characterized by increasing velocity from the cyclone center out to some radius of maximum velocity and decreasing velocity beyond that radius. This structure was robust over the lifetime of an event and from event to event. Cyclone radius and the radius to maximum velocity were consistently similar or equal to 130 km and similar or equal to 55 km, respectively. Evi dence of cyclones at upper measurement levels and the low vertical shear va lues apparent in the deep water below the thermocline indicate that the cyc lones extended throughout the entire water column; from the benthic boundar y layer, through the thermocline, and to the ocean's surface.