Long-Lived Air Temperature Anomalies in the Midlatitudes Forced by the Surface

Abstract

A study of long records of monthly mean air temperature (MMAT) for many stations in the Netherlands indicates that the atmosphere's response to surface boundary forcing is often of a very simple local nature. In the Dutch area, the atmosphere seems to respond to a sea surface temperature (SST) anomaly in the North Sea with an air temperature anomaly of the same sign. Because of the abrupt change in lower boundary forcing near the coastline, very small spatial scales are introduced in air temperature anomalies at long time scales. Over the sea MMAT anomalies have much larger time scales than over the land; a similar increase in time scale can be found in the delay of the climatologically normal temperature with respect to the solar forcing. When extended to the United States, the study showed very similar results; that is, monthly mean surface air temperature (MMAT) anomalies live longest in areas where the air temperature response is slowest to the annual cycle in incoming radiation. Apart from boundary forcing by the oceans, the Gulf of Mexico and the Great Lakes, there is sonic evidence of forcing by snow-cover in the Northeast. Since surface boundary forcing by SST anomalies can be quite persistent, MMAT anomalies are more predictable over the sea and in the coastal zone than in the interior of big land masses. This explains why Madden and Shea (1978) found that the potentially predictable part of the interannual variance in MMAT is largest in predominantly coastal areas, California, in particular. A sizable fraction of the potential predictability in these areas can be effected by such simple tools as linear regression onto antecedent MMAT.