Deep drivers of mesoscale circulation in the central Rockall Trough

Abstract

Mesoscale variability in the central Rockall Trough, immediately west of the British Isles, has been investigated using a combination of ship-borne, underwater glider and gridded satellite altimeter measurements. Altimeter observations show that eddies and large-scale circulation cells are ubiquitous phenomena. They have horizontal length scales of order 100 km with vertical scales of over 1000 m and are associated with mean current speeds (over the upper 1000 m) of 15 ± 7 cm s−1. Monthly area averaged surface eddy kinetic energy (EKE) has substantial inter-annual variability, which at times can dominate a mean seasonal signal that varies from a maximum in May (74 cm2 s−2) to a minimum in October (52 cm2 s−2) and has increased gradually since 1992 at about 1.1 cm2 s−2 per year. This increase may be related to the retreat of the sub-polar gyre (SPG). A 5 month glider mission in the trough showed that the cyclonic component of EKE came from cold water features that are located over 1000 m below the surface. The surface currents from altimeters had similar magnitude to the drift currents averaged over 1000 m from the glider in the stratified autumn, but were half the deep water speed during late winter. Although the mesoscale features move in an apparent random manner, they seem to be constrained by submarine topography such as seamounts. Occasionally anti-cyclonic and cyclonic cells combine to cause a coherent westward deflection of the European slope current that warms the Rockall side of the trough. Such deflections contribute to the inter-annual variability in the observed temperature and salinity that are monitored in the upper 800 m of the trough. By combining glider and altimeter measurements it is shown that altimeter measurements fail to observe a 15 cm s−1 northward flowing slope current on the eastern side as well as a small persistent southward current on the western side. There is much to be gained from the synergy between satellite altimetry and in situ glider observations.