The Norwegian Current carries part of the warm water flowing northward past Ireland into the Norwegian and Greenland (Nordic) Seas, while the subpolar gyre carries the rest westwards towards and past Greenland to the Labrador Basin.
Estimates of flow speeds in the subpolar gyre suggest a transit time of about a decade for a parcel of water that enters the transformation pipeline east of Newfoundland with a temperature of 12 [degrees] to 14 [degrees] C, travels counterclockwise, and emerges from the pipeline in the Labrador Basin at temperatures colder than 4 [degrees] C.
The LSW within the Labrador Basin has a "residence time." The total volume of LSW is only partially replaced each year through the addition of transformed warm water and export of LSW from the Labrador Basin to the rest of the North Atlantic.
These observations suggest that the supply of transforming warm water to our Labrador Basin jacuzzi was running warmer in the 1950s and 1960s compared to the succeeding decades.
Dan Cayan of the Scripps Institution of Oceanography has shown that a high NAO index tends to enhance liberation of heat from ocean to atmosphere over the Labrador Basin while a low NAO index tends to diminish it.
In the last stage of this transformation, deep wintertime convection occurs in the Labrador Basin between North America and Greenland where strong westerly winds cool the surface waters, making them denser than the underlying deep water.
In the North Atlantic, the LSW is very thick, but as the circulation carries it away from the Labrador Basin, mixing with other thinner water masses progressively erodes its thickness.
LSW properties - temperature, salinity, and thickness - have changed significantly through time, and continued measurements in the Labrador Basin since the 1950s enable us to create the time series shown in the figure above right.
The westerlies, which blow cold, dry air from Canada across the Labrador Basin, are a significant factor in determining the depth of Labrador Basin wintertime convection.
Because this surface water required extraordinary cooling to make it denser than the underlying water, it completely inhibited convection in the Labrador Basin. These two events, referred to as the "Great Salinity Anomaly" and the "Lesser Great Salinity Anomaly," eventually moved around the subpolar gyre (see "If Rain Falls" on page 4).
The subpolar LSW, carrying the imprinted climate signals, enters the subtropics along two principal pathways: The deep western boundary current transports LSW from the Labrador Basin to the Caribbean Islands, and the Gulf Stream and North Atlantic Currents carry it from the western boundary out into the interior of the ocean.