Strong cyclones in the deep ocean beneath the Gulf Stream have been observed during the period June 13, 1988, to August 7, 1990, near 68 ¿W, 37 ¿N in data from the Synoptic Ocean Prediction (SYNOP) Experiment. These cyclones developed in association with the evolution of large amplitude, quasi-stationary meander troughs in the Gulf Stream. It is likely that baroclinic instability is responsible for cyclone spin-up. The dynamical analysis of these cyclones indicates the process is analogous with atmospheric cyclogenesis from the perspectives of divergence and conservation of potential vorticity, but not in terms of the density field evolution. Large positive vertical velocities in the thermocline over developing low pressure centers at 3500 m are consistent with convergence at depth and divergence in the upper ocean, and with stretching of the lower water column and shortening of the upper water column. The stretching of the lower water column accounts for the generation of positive relative vorticity there. However, the evolution of the density field in the oceanic case does not resemble the atmospheric case. In the atmosphere, density field adjustments in the air column above the low pressure center at the Earth's surface are in the correct sense to account for decreasing pressure there. In the ocean, density field adjustments in the water column fail to account for the developing low pressure centers, so sea surface height depressions must be responsible. These depressions must have an approximate magnitude of 0.5 m depth over a 250 km horizontal extent (the cyclone's diameter). ¿ 1999 American Geophysical Union