The Connection Between Trends of Mean Temperature and Circulation at the Surface: Part I. Winter

We have demonstrated that regional temperature trends at the surface in winter are connected with circulation changes on the scale of long waves, and that within a given period the trends change sign both with longitude and with latitude. Since the biggest zonally averaged temperature trends north of about 50°N in our samples exceed the biggest zonally averaged trends over the rest of the Northern Hemisphere by a factor of seven to eight, and since the sign of the zonally averaged trends is not uniform, the sign of the average trend over the subpolar and polar regions in winter becomes decisive for the sign of the average temperature trend of the hemisphere. An important difference between a period when the average temperature of the Northern Hemisphere increased (1900–1941) and one when it decreased (1942–1972), was in the amount of sensible heat transported poleward by the, mean eddies north of the latitude of maximum transport (based on maps of sea level pressure). While the higher latitudes warmed, the poleward transport north of about 55°N (and thus the convergence of heat over the polar cap) was larger than during the period of cooling. This difference was associated with a stronger meridional circulation around the Icelandic low and on the east side of the Siberian high during the warming than during the cooling. The trend of the zonally averaged poleward transport by the mean eddies at sea level, which was positive during the warming and negative during the cooling, amounted at each latitude to a very small fraction of the quantity transported across that latitude.