GULF-STREAM PATH AND THERMOCLINE STRUCTURE NEAR 74-DEGREES-W AND 68-DEGREES-W

The SYNoptic Ocean Prediction.(SYNOP) experiment had the goal of Provi ding a physical understanding of energetic mesoscale eddy processes in the Gulf Stream. In the SYNOP Inlet Array off Cape Hatteras and in th e Central Array near 68 degrees W moored observations were collected f rom October 1987 through August 1990. The Inlet Array measured the sur face path and bottom currents where the Gulf Stream leaves the contine ntal margin to enter the deep water regime; small amplitude propagatin g and growing meanders characterized the variability there. The Centra l Array measured velocity and temperature (as a proxy for density) at four levels in the water column, as well as the upper and deep level s treamfunctions, all with mesoscale resolution. Near 70 degrees W the p ath envelope exhibited a relative node, confined within a 40-km band 5 5% of the time. Near 68 degrees W the path envelope was over 3 times a s wide, due to several elongated (''steep'') meander troughs and relat ively steep meander crests. The crests typically propagated downstream without much growth. The troughs often stalled near 68 degrees W, ste epened, and persisted for one to several months. Two cases evolved int o ''S-shaped'' paths and subsequently formed rings. Even the time-aver aged fields showed a small trough in the mean path and thermocline str ucture. Whereas meanders df 20- to 60-day periods had similar spectral levels throughout 70 degrees-67 degrees W, meanders with long periods (> 85 day) accounted for the local minimum in variance at 70 degrees W. Bottom pressure and velocity observations revealed repeated periods of intense (swirl speeds > 0.30 m s(-1)) abyssal eddies; the time-ave raged deep currents exhibited a mean cyclone centered 30 km offshore a nd downstream of the upper layer mean trough. The cross-stream slope o f the thermocline steepened linearly with path curvature, consistent w ith gradient wind balance. Structures are illustrated in the mapped fi elds consistent with baroclinic instability wherein troughs steepen an d rings form.