Vertical motion in the Gulf Stream and its relation to meanders

Vertical motion at 700 m in the Gulf Stream main thermocline is computed using the adiabatic heat equation and a combination of data from inverted echo sounders and moored current meter measurements of T and (u, v) at 400, 700, 1000, and 3500 m. Daily maps of all data on a regular grid were created using optimal estimation; from these maps the tendency and advection terms in the heat equation were calculated. The mapped vertical motions w agree well with w observed contemporaneously along isopycnal RAFOS float tracks. Strong events occur frequently within the Gulf Stream: diagnosed vertical motions greater than 2 mm s(-1) that persist for days are common, occurring roughly every week within the 250-km mapped region. Upwelling and downwelling regions have horizontal scales of about 100 km both along and across stream. Case studies and animation elucidate the relationship between meander properties and the and amplitude of the w fields: typically, the largest ur is positioned near the steepest thermocline slope, downwelling/upwelling occurs entering/exiting troughs, strongest vertical motions occur in slowly propagating features, and vertical motions are usually weaker for rapidly propagating features. This observed dependence upon phase speed is opposite to predictions of recent kinematic and conceptual models, The main contribution to observed w (and the factor missing in those models) is the existence of strong abyssal flow, serving as a barotropic reference for the entire water column, with a component which may flow orthogonal to the baroclinic front along sloping isopycnals. The strongest vertical and cross-frontal components are usually associated with slowly moving, or stalled, steep meanders, associated with which abyssal cyclones or anticyclones have developed. The vertical motion which is forced in these cases can significantly alter the vertical vorticity and/or frontal strength through tilting of the strong vertical shear and of the strong vertical stratification. Vertical and cross-frontal transports of 5 Sv occur in steep meanders.