At high northern latitudes, there is a strong symbiotic relationship between the very non-zonal Atlantic storm track and the ocean beneath, close to key regions of deep water formation. Taken together with the jet-stream/polar front and stratospheric polar vortex, these form a remarkably ‘tall' fluid system. Here the ocean circulation contributes substantially to the column heat balance of the atmosphere, and to the coupled air/sea mode of meridional fresh-water transport, which is narrowly channeled. Either through an accelerating hydrologic cycle, increasing outflows from Arctic to Atlantic, or decline of the ocean circulation there have occurred 4 decades of decreasing salinity throughout the subpolar ocean.
The complex passages, sills, and ridges of the subpolar zone are being instrumented to explore Arctic-Atlantic transports of volume, heat and fresh-water. Seaglider autonomous undersea vehicles now are collecting hydrographic and bio-optical sections of the subpolar Atlantic which may, in combination with satellite altimetry and complementary deep ocean observations, finally show us the three-dimensional structure of the meridional overturning circulation and its variability. Indications of declining ocean circulation have been seen in altimetric/hydrographic study of the cyclonic subpolar Atlantic gyre and in the easternmost deep overflows. At the same time, projection of the circulation onto the potential temperature/salinity plane articulates the description, bringing out key water masses, mixing, and regions of sinking and diapyncal transport. These are richer than simple estimates of bulk volume transport. This teasing apart of the circulation onto the č/S phase plane, together with dynamical studies of key elements of subpolar geophysical fluid dynamics, challenges climate models to reveal and upgrade their high-latitude physics.
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