temperature must be near freezing. The maximum changes due to the feedback are to be expected in subpolar latitudes in winter and in polar or subpolar latitudes in spring when both snow and sea-ice changes are important.

In H1, the snow-ice albedo feedback mechanism is significant even in winter because the maximum ΔT in that prediction is in subpolar regions between 45° N and 70° N. In M3, on the other hand, the snow-ice albedo feedback seems to be most significant in spring when a maximum in ΔT occurs around 65° N.

In M3 there is another, even stronger, maximum in winter near the north pole. This cannot be interpreted as the result of a snow-ice albedo feedback because there is no solar radiation. It has been suggested that it is a result of a sea-ice thickness feedback: When the sea ice in the model becomes sufficiently thin, the surface air becomes strongly coupled by conduction to the ocean immediately below the sea ice, which must be near freezing. This gives a warming effect and therefore a positive feedback. The warming is further enhanced by the circumstance that the polar ice in this model (for quadrupled CO2) is completely melted so that the polar seas beneath the ice in winter will be warmer. In H1, the sea-ice thickness feedback cannot be clearly seen in winter. Instead, H1 shows a maximum ΔT near the north pole in spring when the sea ice is thin enough and the leads wide enough to permit effective atmospheric communication with the ocean. In the annual average, H1 shows a large ΔT poleward of about 45° N, with a flat maximum of about 7°C near 60° N.



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