As part of the WOCE Atlantic Climate Change Experiment (ACCE) we and our colleagues at WHOI deployed over 100 isopycnal floats on the 27.5 kgm-3 surface, some west of the mid-Atantic ridge (we) and others east of the ridge (WHOI). The objective was to map out the circulation across the subpolar North Atlantic in this mid-pycnocline surface. Interestingly, this surface also separates the north- and south-flowing AMOC, i.e., it is the maximum in the AMOC stream function in density coordinates. This means that somewhere somehow water must pass from the upper to the lower layer. That is exactly what some of our floats revealed, how do we know that?
This density surface outcrops in the Irminger Sea. As warm salty North Atlantic water weaves its way from the Iceland Basin in east around the Reykjanes Ridge it gradually increases in density due to the continual loss of heat to the atmosphere. By the time the water reaches the Irminger Sea most water less dense than 27.5 kgm-3 has disappeared which means that water on this surface will now be exposed to the atmosphere. In float terms this means they must surface and that they do.
Throughout their missions the floats measured pressure, temperature, and dissolved oxygen. In an earlier blog (Dec. 19, 2023) I wrote about how oxygen varies as the isopycnal the float drifts on outcrops in winter, it’s quite dramatic. Equally dramatic were the vertical movements a float experienced while in the mixed layer. While P, T, and O2 were measured only at the end of each listening period the recorded pressures indicated substantial vertical movement in the mixed layer. This figure shows the history of one such float (#567).
It was deployed north of the subpolar front where the 27.5 kgm-3 isopycnal lies at about 150 m depth. After ~130 days drifting north in the Irminger Current it surfaces and continues north throughout the winter until early May when it begins to subduct below the surface at 65°N, but not due to warming as you might expect but the admixture of fresher water from the nearby Greenland shelf. The float now takes off (point A) and races at barely 50 m depth past the southern tip of Greenland to point B in 50 days (days 350 to 400). At end of November 1999 (day 490) the float surfaces again on the western side of the Labrador Sea and starts bobbing in the mixed layer, now clearly due to intense cooling. The pressure record shows the float bobbing to nearly 50 m depths. Almost certainly they are being dragged down by plumes or blobs of water cooled at the surface (Kearns and Rossby, 1993).
This is another example of the enormous descriptive power of the Lagrangian technique, it tells you how stuff moves about in the ocean and in this case what happens when an isopycnal breaches the surface. We found it instructive to sit down and discuss the life cycle of floats - where they went and why. They may surface, they may follow bathymetry, they may get trapped in eddies for extended periods of time, they many describe planetary wave motion; these are the stories floats can tell! Ensembles of floats may tell us allowable pathways for fluid motion, how water gets from one place to another, or not. Or about domains of occupation as discussed in the previous blog.
Kearns, E. and T. Rossby. A Simple Method for Measuring Deep Convection. J. Atmospheric and Oceanic Technology, 10, 609-617, 1993.