Monitoring a shelf edge current using bottom pressures or coastal sea-level data

Abstract

Low-frequency fluctuations in ocean currents can be monitored by their effects on water level and bottom pressure gradients due to geostrophy. A predominantly barotropic current along the Norwegian continental slope is studied to show how routine monitoring of coastal sea-level, air pressure and coastal hydrography can be used to monitor fluctuations of an important source of salt and heat to arctic regions. These data are also needed as forcing boundary conditions for running models of the adjacent shelf seas. The first attempt showed a promising validation to monthly average transports measured 1300 km upstream. The evidence suggests that changes in the slope current are established on the time scale of a few days along the entire continental slope, apparently forced by sea-level changes in the northeast Atlantic. Correcting for atmospheric pressure and the buoyancy of the coastal water near the tide gauge at Bodø, we obtain five-day running average coastal sea-level fluctuations that correlate to within 15% of observed current and bottom pressure gradient fluctuations 300 km away. The extension of the derived relation for monitoring this flow over large distances, with a time resolution of 5 days is demonstrated. Improved quality of the measurements is needed to reduce the inherent sources of noise. Part of the noise is due to mesoscale variability in the coastal current. This can be reduced by averaging over longer times. The method fails at higher time resolutions due to atmospheric variability, basin modes and other processes that are unaccounted for in the simple geostrophic model.