North Atlantic response to the above-normal export of sea ice from the Arctic

PDF: http://data.dhkim.info/monograph/JGR/2001JC001166.pdf

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. C7, 3224, doi:10.1029/2001JC001166, 2003

North Atlantic response to the above-normal export of sea ice from the Arctic

Oleg A. Saenko

School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada

Edward C. Wiebe

School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada

Andrew J. Weaver

School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada

Abstract

The response of the thermohaline circulation (THC), as well as the freshwater and heat budgets of the northern North Atlantic, to above-normal sea ice export from the Arctic is examined using a global model. The model is not constrained by either open boundary conditions or prescribed atmospheric air temperature and humidity. Two sets of experiments are presented: the transient and the persistent above-normal ice export. In the transient case, ice export is increased by a factor of 2 for 1–5 years. Our century-long simulations do not support the notion that the simulated climate may switch to a new quasi-equilibrium under such perturbations. Rather, in response to the transient positive ice export anomalies the overturning circulation first slows down but then almost completely recovers 15–20 years after the perturbation is removed. However, the budgets of freshwater and heat continue to evolve for up to 40 years in this case. When the simulated North Atlantic freshening reaches a magnitude comparable to that during the Great Salinity Anomaly (GSA), the strength of overturning and heat transport from subtropical to subpolar North Atlantic reduce by no more then 5%. We show that in order to generate a previously reported decrease of overturning and heat transport of as much as 20% the doubled ice export must be sustained for at least 5 years. This would result in a North Atlantic freshening more than 3 times larger than that estimated for the GSA event. In the case of a persistent above-normal export of sea ice from the Arctic the THC also does not collapse, at least within the range of the ice export increase (1.5–3 times) used here. Rather, after about 15–20 years the overturning begins to recover. The stability of the overturning to the persistent above-normal export of sea ice from the Arctic appears to be due to two processes operating on a decadal timescale of 15–20 years. First, the internal (to the coupled system) redistribution of freshwater between the Arctic and North Atlantic, associated with the enhanced export of sea ice, makes the North Atlantic fresher and Arctic Ocean saltier. This, if persistent, decreases the amount of freshwater leaving the Arctic toward the North Atlantic in a liquid form. Second, because the overturning circulation does not collapse, the freshwater anomaly propagates downward within the region of deep water formation, removing the excess of buoyancy from the surface ocean. Also, the use of an active atmospheric component is important for stabilizing the overturning circulation.

Received 9 October 2001; accepted 2 July 2002; published 11 July 2003.

Index Terms: 4215 Oceanography: General: Climate and interannual variability (3309); 4540 Oceanography: Physical: Ice mechanics and air/sea/ice exchange processes; 4207 Oceanography: General: Arctic and Antarctic oceanography; 4255 Oceanography: General: Numerical modeling.