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

One of the primary scientific results of the Synoptic Ocean Prediction (SYNOP) observational program was the discovery of strong cyclones in the deep ocean beneath the large amplitude Gulf Stream meander troughs that routinely form at about 68°W. These strong well‐organized cyclones extend to at least 3500 m below the sea surface and are an important component of the overall dynamical variability of the Gulf Stream and adjacent deep waters. Typically, a small amplitude Gulf Stream meander “stalls” near 68°W and begins to amplify. As the amplitude of troughs in the Gulf Stream jet increases, the currents at 3500 m strengthen and turn, forming a cyclonic circulation pattern. During SYNOP, six well‐defined instances of meander trough amplification and deep cyclogenesis occurred. The cyclones were characterized by strong swirl speeds (up to 0.5 m s −1 ) and were long‐lived (typically lasting 6–9 weeks) frequent occurrences (present 35% of the time during the 25 month deployment period). The structure of the cyclones at 3500 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 ≃130 km and ≃55 km, respectively. Evidence of cyclones at upper measurement levels and the low vertical shear values apparent in the deep water below the thermocline indicate that the cyclones extended throughout the entire water column; from the benthic boundary layer, through the thermocline, and to the ocean's surface.