Evidence has been presented and discussed to show a cooling trend over the Northern Hemisphere since around 1940, amounting to over 0.5°C, due primarily to cooling at mid- and high latitudes. Some regions of the middle latitudes have actually warmed while others, such as the central and eastern United States, have experienced sharp cooling. A representative station for this latter region is Lafayette, Ind., which has recorded a drop of 2.2°C in its mean annual temperature from 1940 through 1978. The cooling trend for the Northern Hemisphere has been associated with an increase of both the latitudinal gradient of temperature and the lapse rate, as predicted by climate models with decreased solar input and feedback mechanisms.
Observations and interpretation of sunspot activity have been used to infer a direct thermal response of terrestrial temperature to solar variability on the time scale of the Gleissberg cycle (~90 years, an amplitude of the 11-year cycles). Measurements at the Greenwich Observatory and the Kitt Peak National Observatory, as well as other supportive information and arguments, are presented to hypothesize a physical link between the sunspot activity and the solar parameter. On the time scale of the Gleissberg cycle when the mean annual sunspot number exceeds 50 it is proposed that global cooling may be initiated due to the decreased insolation. This is also supported by umbral-to-penumbral ratios computed and interpreted by Hoyt (1979a).
Observations of sensible heat flux by stationary planetary waves and transient eddies, as well as general circulation modeling results of these processes, have also been examined from the viewpoint of the hypothesis of cooling due to reduced insolation. The westerlies appear to have shifted southward and to have strengthened during the cooling period, which allows for arguments of a preferred wave number for stationary waves due to mountain interaction. This type of interaction may give rise to preferred regions of heat flux as seen observationally, e.g., the warming in the far west regions of the United States and the sharp cooling in central and eastern regions. Cyclone frequencies have also been observed to shift southward, with up to 25% reduction in January and July cyclone frequency during the cooling trend in the western border of the North America continent and in the Gulf of Alaska. This region corresponds to the location of the large amplitude ridge in the planetary wave that has been observed, especially during the winter season when the westerlies are stronger.
