Publication Abstract

All change in the Arctic

R.R. Dickson

Since the late-1960s, when convection there was at its weakest and shallowest, we have witnessed are markably sustained re-intensification and deepening of convective activity in the Labrador Sea. From 1966 to 1992, the overall freshening of Labrador Sea Water (LS W) has been equivalent to mixing-in an extra 6 metres of fresh water at the sea surface; its cooling has been equivalent to a continuous loss over the 26 years of 8 Wm^-2; and by 1993-5, the depth of convective overturn had so exceeded its normal limits (2000 m or so) that for the first time in our experience, it began to excavate the top of the colder but saltier sublayer of North Atlantic Deep Water (NAD W) at depths >2300 m. It is as a result of this change that the density of the thick LSW layer underwent an abrupt increase between 1990 and 1994/5, and it is this anomalously cold, fresh, and dense new vintage of LS W that Sy et al. use, together with its chlorofluorocarbon content, to provide modern estimates of trans-ocean spreading rates at LS W depths. As we might almost have predicted by now, the results were quite unexpected, with spreading rates three or four times faster than previous estimates!

Seen in this light, the coolings, freshenings, deepenings and spreadings of Labrador Sea Water no longer seem parochial. Far from being a small deal in the NW Atlantic, these changes may well be playing the major role in the largest deal of all, protracting, repeating or in some way structuring the short term behaviour of the NAO into the long, slow shifts of global change. As the Scientific Steering Group of the Climate Variability (CLIVAR) Program meets in Toronto and Washington to draw up its Initial Implementation Plan (March and April 1997), the oceanographic component will certainly include our best ideas as to how to encompass and quantify change in the mode waters of the North Atlantic. But if our recent experiences are anything to go by, they had better build in some redundancy!

Reference:

R.R. Dickson, 1999. All change in the Arctic. Nature, 386: 649-650