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.
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.
Further suggestions that D / O events in Greenland are generated by shifts in the North Atlantic ocean circulation seem highly implausible, given the weak contribution of
the high latitude ocean to the meridional flux of heat.
(Note vast areas of
the high latitude oceans were covered by ice, during the coldest period and could hence no longer absorb carbon dioxide.)