This is largely because melting sea ice changes the albedo of high latitude oceans, and to a lesser extent because an inversion prevails at high latitudes, especially in winter, whereas at low latitudes the heating is convectively mixed througout the troposphere. (www-das.uwyo.edu)
Mike; As a glaciologist I focus primarily on higher latitude ocean circulation changes, not the tropics. (realclimate.org)
The changes in the return values of daily minimum temperature (Figure 9.29, bottom) are larger than those of daily maximum temperature over land areas and high latitude oceans where snow and ice retreat. (ipcc.ch)