Thermohaline transitions

We here discuss the physical mechanism of transition from one stationary thermohaline structure to another. Such transitions may occur in both oceans and lakes, and may be either temporal, as in the case of climate variability, or spatial, as in the case of frontal regions separating thermohaline regimes. The example presented here uses data obtained during the Canada/U.S. 1994 Arctic Ocean Section to show that a major warming and ventilation of the mid-depth layers of the Arctic Ocean (200-1000 m) occured due to influx of anomalously warm waters from the Atlantic. This transition appears to have begun in the early 1990s; now, anomalously warm waters are found in the Nansen, Amundsen, and Makarov basins, with the largest temperature difference, as much as 1 degrees C, in the core of the Atlantic layer. In the Makarov Basin the core of the Atlantic layer is now shallower than before (200 versus 400 m), causing the displacement of cold, nutrient-rich Pacific waters, presumably into the North Atlantic and the convective gyres of the Greenland and Labrador Sea. The transition is occuring via the formation of multiple intrusions, 40-60 m thick, extending downward through the Atlantic and upper deep waters; these features appear to be laterally coherent across the Arctic Ocean (> 2000 km) and to persist over time-scales of several years. Once formed, the layers can Support both diffusive and salt finger convection. Potential temperature (theta) versus salinity (S) correlation curves for each successive diffusive and salt finger regime are parallel, with higher values of the stability frequency (N) in the diffusive regime than in the salt finger regime.