Decadal changes in the thermohaline circulation of the North Atlantic

A set of repeated full-depth and mostly ocean-wide hydrographic sections is used to describe the temporal and spatial changes of temperature and salinity at the intermediate layers of the North Atlantic over the past 40 years. They cover three distinct periods, namely the late 1950s, the early 1980s and the early 1990s but do not constitute a time series. Most parts of the recently observed changes in the subpolar and subtropical gyres are attributed to the changes of the Labrador Sea Water (LSW) properties. The latter is formed in the Labrador Sea and spreads into the North Atlantic. The spreading time of the LSW from the source region is evaluated. It takes less than two years for the LSW to arrive in the Newfoundland Basin and 18-20 yr to reach the Deep Western Basin interior at 24.5 degrees N. Changing characteristics of the LSW are related to the varying intensity of the Meridional Overturning Circulation (MOC) and the Meridional Heat Transport (MHT). The intensification of the MOC and MHT around 1981 coincides with the rapid decrease of the LSW production. The increase of the LSW production, that began just after 1987 and resulted in a dramatic cooling and densification of this water mass, was accompanied by a reduction of both MOC and MHT. Two scenarios of the MOC interaction with the full-depth circulation of the North Atlantic are suggested. These scenarios indicate that the development of the MOC is governed by the changing transports of the deep waters of Arctic origin. The direct contribution of the LSW to the intensity of the MOC is negligible. The LSW is seen as an important part of the feedback loop in the North Atlantic MOC system. (C) 1999 Elsevier Science Ltd. All rights reserved.