Finestructure and microstructure in the North Atlantic Current
Additional Document Info
The relationship between intrusive finestructure and optical microstructure was studied by simultaneous CTD Tow-yos and deployments of the shadowgraph profiler SCIMP. Strong thermohaline intrusions, 5 to 50 m thick, were tracked laterally for 5 to 10 km in the front associated with the North Atlantic Current. Some of the intrusions showed significant cross-isopycnal slopes, consistent with the hypothesis that double-diffusion is causing the intrusions to cross density surfaces. The SCIMP shadowgraphs displayed clear images of salt fingers at the lower boundaries of warm salty intrusions, confirming that double-diffusion is an active mixing process in frontal interleaving zones. In addition, significant correlations of the SCIMP optical microstructure with temperature, salinity and velocity finestructure were found. Mixing activity was more likely to occur when the density ratio (R rho = alpha T(x)/beta S(x)) was near one or when the Richardson number was low. These correlations show that double-diffusion was more common than shear instability in the frontal interleaving zone. In contrast, a SCIMP dive in mid-gyre showed much less activity, no interleaving, and few sites with R rho near one. Thus shear instability may account for most of the weak mixing in those mid-gyre regions where the mean T-S profiles do not favor strong double-diffusion activity. The scales of the frontal intrusions are found to be consistent with the double-diffusive generation model of Toole and Georgi (1981). Application of the Joyce (1977) lateral mixing model suggests that intrusions are a significant cross-frontal mixing mechanism.