We have spent much of today in fog, edging to the northwest, making vertical profiles every 15 miles as we go. The focus for the time being is on ocean circulation. We will gradually be crossing the Subpolar Front, a major boundary between warm southern waters and cool northern waters. Along this boundary flows the North Atlantic Current, a continuation of the Gulf Stream and the supplier of the warm water that keeps the climate of Western Europe relatively mild. The Subpolar Front crosses the Mid-Atlantic Ridge close to the twin east-west gashes of the Charlie-Gibbs Fracture Zone. The valleys also provides a deep connection between the eastern and western basins of the North Atlantic via which deep water of Arctic origin seeps to the west. So the topography of this area makes it a crossroads of the North Atlantic circulation.
Our present route follows the track of an orbiting satellite, TOPEX/POSEIDON, which carries an altimeter able to accurately measure the elevation of the ocean surface. At first glance, it appears that the ocean’s surface is flat – at least, if you look beyond the effect of waves and tides. Of course the ocean is curved because the planet is curved, but this can still be regarded as flat if the surface is not tilted when compared with the direction of gravity. In fact the ocean is not flat and the shape of its topography tells us about ocean currents in just the same way that a pressure chart of the atmosphere tells us about winds and weather systems. When the ocean surface is elevated, the weight of this extra water creates high pressure about which currents at the surface flow in a clockwise direction (in the Northern Hemisphere). As our ship track takes us across the North Atlantic Current, the ocean surface will drop by about half a meter. It’s not much! We can neither see nor measure this directly, but by making our shipboard measurements of currents and profiles of density we can infer the slope of the surface and compare this with the satellite measurements. We will also see the deeper structure that is invisible to the satellite. This comparison is the research focus for Jane Read of NOC, Southampton who is leading this aspect of our work
So for now it’s one profile after another as we slowly piece together a transect revealing the structure of the currents and differing water types of this region. There’s not much to be seen as we push forward into the fog.