1. Centre for Environment, Fisheries, and Aquaculture Science, Lowestoft NR33 OHT, UK
2. Bedford Institute of Oceanography, Dartmouth, Nova Scotia B2Y 4A2, Canada
3. Institut fur Meereskunde, 22529 Hamburg, Germany
4. Marine Laboratory, PO Box 101, Aberdeen AB11 9DB, UK

Correspondence to: Bob Dickson¹ Correspondence and requests for materials should be addressed to R.R.D. (e-mail: Email: r.r.dickson@cefas.co.uk).

Abstract

The overflow and descent of cold, dense water from the sills of the Denmark Strait and the Faroe–Shetland channel into the North Atlantic Ocean is the principal means of ventilating the deep oceans, and is therefore a key element of the global thermohaline circulation. Most computer simulations of the ocean system in a climate with increasing atmospheric greenhouse-gas concentrations predict a weakening thermohaline circulation in the North Atlantic as the subpolar seas become fresher and warmer^{1, 2, 3}, and it is assumed that this signal will be transferred to the deep ocean by the two overflows. From observations it has not been possible to detect whether the ocean's overturning circulation is changing, but recent evidence suggests that the transport over the sills may be slackening⁴. Here we show, through the analysis of long hydrographic records, that the system of overflow and entrainment that ventilates the deep Atlantic has steadily changed over the past four decades. We find that these changes have already led to sustained and widespread freshening of the deep ocean.