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Downloaded 10/01/21 10:31 PM UTC 874 JOURNAL of CLIMATE VOLUME 12 MARCH 1999 VAVRUS 873 The Response of the Coupled Arctic Sea Ice±Atmosphere System to Orbital Forcing and Ice Motion at 6 kyr and 115 kyr BP STEPHEN J. VAVRUS Center for Climatic Research, Institute for Environmental Studies, University of WisconsinÐMadison, Madison, Wisconsin (Manuscript received 2 February 1998, in ®nal form 11 May 1998) ABSTRACT A coupled atmosphere±mixed layer ocean GCM (GENESIS2) is forced with altered orbital boundary conditions for paleoclimates warmer than modern (6 kyr BP) and colder than modern (115 kyr BP) in the high-latitude Northern Hemisphere. A pair of experiments is run for each paleoclimate, one with sea-ice dynamics and one without, to determine the climatic effect of ice motion and to estimate the climatic changes at these times. At 6 kyr BP the central Arctic ice pack thins by about 0.5 m and the atmosphere warms by 0.7 K in the experiment with dynamic ice. At 115 kyr BP the central Arctic sea ice in the dynamical version thickens by 2±3 m, accompanied bya2Kcooling. The magnitude of these mean-annual simulated changes is smaller than that implied by paleoenvironmental evidence, suggesting that changes in other earth system components are needed to produce realistic simulations. Contrary to previous simulations without atmospheric feedbacks, the sign of the dynamic sea-ice feedback is complicated and depends on the region, the climatic variable, and the sign of the forcing perturbation. Within the central Arctic, sea-ice motion signi®cantly reduces the amount of ice thickening at 115 kyr BP and thinning at 6 kyr BP, thus serving as a strong negative feedback in both pairs of simulations. Ice motion causes the near- surface air to cool in both sets of experiments, however, thus representing a positive feedback at 115 kyr BP and a negative feedback at 6 kyr BP. The excess cooling with ice motion at 115 kyr BP is caused by the enhanced, advective spreading of the ice pack into the North Atlantic dominating over the warming tendency from the thinner central Arctic sea ice. The reduced atmospheric warming due to ice dynamics at 6 kyr BP is caused by sea-ice ridging, a thickening process that partially counteracts the orbitally induced atmospheric warming per- turbation. 1. Introduction Sea-ice motion is one process that has often been Polar regions have been identi®ed by modeling stud- omitted from climate models, even though it is regarded ies as the areas expected to experience the most dramatic as the most likely candidate to counter the positive feed- future climate changes from increased levels of atmo- backs inherent in the thermodynamic-only sea-ice codes embedded in most atmospheric GCMs (WCRP 1994). spheric CO2. One of the strongest positive feedbacks cited in these analyses is associated with the retreat and The negative feedbacks effected by ice dynamics are thinning of sea ice in high latitudes, particularly the believed to stem from two primary features that are Arctic (IPCC 1990). Unfortunately polar regions are absent in a stationary ice cover: ice advection and local also among the least understood climatically, due to the ice thickness variations, which include leads (Hibler logistical dif®culties they pose for obtaining data and 1984). Ice motion causes regions of convergence and the added complexities inherent in modeling cryo- divergence within the pack, giving rise to regional ice spheric components. This paper will test the robustness thickness variations. In some areas, such as compressive of enhanced polar climate sensitivity simulated in nu- regions along coasts, this deformation results in the spa- merous GCMs by incorporating a frequently neglected tial pattern of sea-ice thickness being dominated by at- process (sea-ice transport) believed to provide negative mospheric wind forcing rather than thermodynamic feedbacks and by performing data-model comparisons forcing. In these regions, one would expect that exter- of past climate changes in high latitudes. nally forced atmospheric thermal perturbations would have a relatively small bearing on the thickness of the ice cover. The second distinguishing aspect of a dynamic ice pack is the ridging caused by small-scale motion Corresponding author address: Dr. Stephen J. Vavrus, Center for that results in a range of ice thicknesses locally and the Climatic Research, Institute for Environmental Studies, University of WisconsinÐMadison, 1225 West Dayton St., Madison, WI 53706- creation of open water. This variable thickness distri- 1695. bution is important because sea ice grows at a rate in- E-mail: [email protected] versely proportional to its thickness; thus, the overall q 1999 American Meteorological Society Unauthenticated | Downloaded 10/01/21 10:31 PM UTC 874 JOURNAL OF CLIMATE VOLUME 12 FIG. 1. Mean-annual sea-ice drift and sea level pressure (mb) [from R. Colony, University of Washington Polar Science Center, in Barry et al. (1993)]. growth rate of the pack can be dominated by that of the thin ice portion of the spectrum (Maykut 1982). There- fore, if mechanical forcing produces a region of thin ice, then the relatively rapid thermodynamic growth helps to restore the ice toward its original thickness. Likewise, if thermodynamic forcing causes a thinner ice pack, the relative ease with which thin ice can ridge will favor a mechanically induced thickness restoration (Hibler 1984). The importance of ice motion in shaping the modern Arctic sea-ice cover is apparent from a consideration of FIG. 2. Arctic sea-ice draft (m) during (a) summer and (b) winter its average circulation characteristics, as described by [from Bourke and Garrett (1987), in Wadhams (1994)]. Barry et al. (1993) (Fig. 1). The motion of the interior ice pack strongly resembles the implied surface wind stress, consisting of a clockwise circulation around the Ice motion is necessary to reproduce the observed pileup Beaufort Gyre and a drift of ice away from the Siberian of thick ice along the North American coast and the ice coast. After sea ice crosses the North Pole some of it tongue along Greenland (Hibler and Walsh 1982; Pol- converges along the North American coast; the remain- lard and Thompson 1994). Even small amounts of leads, der exits the basin through the Fram Strait and into the which are created by moving ice, cause substantial East Greenland Current. The presence of these two path- warming at the Arctic Ocean surface (Ledley 1991) and ways is strikingly apparent in ice thickness ®elds (Fig. in the lower troposphere of both polar regions (Vavrus 2), which show a buildup of sea ice north of Greenland 1995; Simmonds and Budd 1991). The inclusion of and the Canadian Archipelago and a tongue of ice ex- leads also improves the simulation of Antarctic sea-ice tending along the east coast of Greenland. extent (Washington et al. 1976), whose seasonal cycle Model simulations of the modern Arctic and Antarctic is dif®cult to reproduce without the inclusion of ice ice packs are also strongly affected by ice dynamics. advection (Hibler and Walsh 1982). Furthermore, ice Unauthenticated | Downloaded 10/01/21 10:31 PM UTC MARCH 1999 VAVRUS 875 transport from the Arctic Ocean prevents spurious en- change in Arctic sea-ice characteristics upon the inclu- hancement of the pycnocline strength (Ranelli and Hib- sion of ice transport in a doubled CO2 experiment, but ler 1991), and ice export from the Arctic and Antarctic the response of Antarctic sea ice was signi®cantly mut- Oceans results in considerable thinning of sea ice in ed. both hemispheres (Ledley 1991). Ice dynamics also ap- As a means of clarifying the role of dynamic sea ice pear to have important consequences for climate vari- in shaping high-latitude climate changes, this paper will ability; Hibler and Zhang (1994) report that the absence assess the effect of ice motion on Arctic climate sen- of Arctic ice advection results in a spurious poleward sitivity and will evaluate the ability of a GCM to sim- displacement of the Marginal Ice Zone in the North ulate two time periods (6 kyr and 115 kyr BP) whose Atlantic. climates were strongly affected by alterations of Earth's The importance of ice motion in affecting the sen- orbital con®gurations. This pair of paleoclimates are sitivity of sea ice to climate perturbations has been high- noteworthy because they represent the response of the lighted in simulations ranging in sophistication from high-latitude Northern Hemisphere to a signi®cant in- stand-alone one- and two-dimensional sea-ice models solation surplus (6 kyr BP) and de®cit (115 kyr BP) to three-dimensional regional and global GCMs. Al- relative to present. The approach used here allows for though the consensus view that has emerged from these a number of improvements over previous studies. First, studies is that ice dynamics have a potent stabilizing this methodology provides an opportunity to test the effect on sea-ice variations, the sensitivity tests have in¯uence of ice motion over a range of external forc- usually focused on warming perturbations; very little ings, from the warm perturbation at 6 kyr BP to the consideration has been given to the in¯uence of mobile cold anomaly at 115 kyr BP. Global snapshots of these ice in cooling scenarios. Early simulations by Hibler paleoclimates have been simulated before but without (1984) of the Weddell Sea found that a dynamic-ther- dynamic sea ice and usually with less realistic surface modynamic sea-ice model showed less sensitivity than boundary conditions (e.g., prescribed SSTs and sea-ice a thermodynamic-only model to atmospheric warming extent). Second, because the model is driven by actual perturbations. Later simulations over the same domain insolation perturbations known to have occurred, this by Lemke (1987) showed that ice dynamics mitigate the approach allows for comparison with paleoenvironmen- response of sea ice to snowfall variations, a conclusion tal evidence and is more realistic than sea-ice sensitivity corroborated for Arctic sea ice by Holland et al.
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