Publication Abstract

Recent changes in the North Atlantic

R.R. Dickson, R. Curry and I. Yashayaev

It has long been recognised that the Atlantic Meridional Overturning Circulation (MOC) is potentially sensitive to greenhouse gas- and other climate forcing, and that changes in the MOC have the potential to cause abrupt climate change. However the mechanisms remain poorly understood and our ability to detect these changes remains incomplete. Four main (inter-related) types of ocean change in particular are associated in the literature with greenhouse gas forcing. These are 1) a slowing of MOC overturning rate 2) changes in Northern Seas which might effect a change in Atlantic overturning, including changes in the freshwater flux from the Arctic, and changes in the transport and/or hydrographic character of the northern overflows which ventilate the deep Atlantic, 3) a change in the trans-ocean gradients of steric height (both zonal and meridional) which might accompany a change in the MOC and 4) an intensification of the global water cycle. Though we have as yet no direct measure of the freshwater flux passing from Arctic to the Atlantic via either the Canadian Arctic Archipelago or along the East Greenland Shelf, and no direct measure yet of the Atlantic overturning rate, we examine a wide range of time series from the existing hydrographic record for eviden9 of the other anticipated responses. Large amplitude and sustained changes are found over the past 3 - 4 decades in the southward transport of fresh waters along the Labrador shelf and slope, in the hydrography of the deep dense overflows from Nordic Seas, in the transport of the eastern overflow through Faroe-Bank Channel, and in the global hydrologic cycle. Though the type and scale of changes in ocean salinity are consistent with an amplification of the water cycle, we find no convincing evidence yet of any significant, concerted slowdown in the Atlantic overturning circulation. We describe what ocean observing strategv might be annronriate in the light of these findings.