Current and temperature variability on the continental slope

Abstract

Observations of temperature and currents have been made in the lower part of the water column on the western slope of the Porcupine Bank southwest of Ireland where the water depth is about 3500 m. Moored and lowered instruments were used to study the processes that may lead to mixing and diapycnal transfer. A poleward Eulerian mean flow of about 2.5 cm s$^{-1}$ is found in the bottom 250 m, with current direction rotating anticlockwise as the bottom is approached. The shear between 30 and 90 m off the bottom is, on average, geostrophic and the temperature variation at a point averaged over several tidal cycles is dominated by along-slope advection. There is, however, considerable variability, with strong semidiurnal oscillations in both current and temperature. There is a downward phase propagation in temperature at these frequencies indicating the presence of baroclinic M$_{2}$ waves. It is possible that these are responsible for the apparent along and off-slope fluxes of heat that are observed. Fluctuations with periods of 5-6 days appear to propagate polewards along slope with phase speeds of about 13 cm s$^{-1}$, but have constant phase lines lying roughly north-south at an acute angle to the mean contours of the slope. Profiles of potential temperature obtained by using a lowered conductivity, temperature and depth (CTD) instrument show the presence of frequent inversions in the lower 150 m, sometimes exceeding 30 m in height. A moored array of platinum resistance thermometers sampling at 10 s intervals indicates that the inversions are associated with the semidiurnal variability in temperature, the most frequent inversions occurring when the temperature 10 m off the bottom is greatest and when the temperature is falling at higher levels with a component of the current directed upslope. Indirect estimates of the vertical diffusivity and turbulent dissipation are derived from the observed scales of inversions and the overall stratification. These are subject to considerable uncertainty but suggest that the contribution to turbulence in the boundary layer from the inversions may exceed that supplied by the shear stress on the sea bed. Sources in instability are discussed, and one is examined analytically. An exact steady or time-dependent solution of flow of a stratified fluid parallel to a sloping boundary is derived. Solutions in which the density remains statically stable are possible only when a rather delicate balance exists between flow, stratification and diffusion.