Processes associated with steep meander development in the Gulf Stream near 68 degrees West

From June 1988 until August 1990 an array of 12 Inverted Echo Sounders (IES), 12 additional IES with pressure gauges to measure the abyssal pressure signal, and 12 tall current meter moorings instrumented at 400, 700, 1000, and 3500m, was deployed in the Gulf Stream near 68$\sp\circ$W. The current meter array spanned an area of approximately 260 by 200km with nominal horizontal instrument spacing of 56km in the cross-stream direction and 65km in the downstream direction. The measurements were all 40-hr low-pass filtered to focus on mesoscale variability. During the two year deployment six steep troughs formed in the array with two eventually pinching off as cold core rings. Strong cyclones in the abyssal flow developed concurrently with the steep meander troughs. The development of these features is similar to idealizations of baroclinically growing upper level wave and lower level cyclone systems in the atmosphere. The three dimensional mesoscale ageostrophic flow is diagnosed during two steep meander trough events and one steep crest event. In addition to strong along-stream horizontal ageostrophic flow associated with the curvature of the main jet (cyclostrophic speeds greater than 25$cms\sp{-1}$ are not unusual on trough axes) there is significant horizontal cross-stream ageostrophic flow, which can substantially modify the large (${\cal O}(30cms\sp{-1}))$ barotropic cross-stream flow manifested in the abyssal vortices. The horizontal ageostrophic flow is shown to have large divergent and rotational components. The vertical motion diagnostics show upwelling and downwelling speeds of greater than 3 $mms\sp{-1}.$ With such strong vertical motion it is necessary to include vertical momentum advection in the diagnostics of the horizontal ageostrophic flow. Vorticity equation diagnostics show that the balance is primarily between vorticity advection and vertical stretching for the steep trough events. Other terms are, however, important in the balance at 400 and 700m and act generally to reduce zonal vorticity advection of the vorticity maxima for the steep troughs. The diagnostics point out the importance of the presence of the abyssal cyclones for creating additional vertical stretching, which acts to steepen and slow down developing meanders