for all climate variables is too short to permit the detection of these phenomena. Prior to the 1950s, the only maritime instrumental records useful for these studies are those for SST and air temperature. Although SST data are available along the major global shipping routes from about 1900 (see, Pan and Oort, 1990), spatial and temporal coverage for these variables is adequate only across the North Atlantic (Figure 2). For the continental areas, potentially useful data are available for more of the climate variables as far back as the mid-1800s. These data, however, are uneven in their spatial distribution. Prior to World War II there are essentially no instrument-based data for the state of the atmosphere above the surface. For the ocean, subsurface data are limited to infrequent and isolated transects through the ocean, usually across the North Atlantic. Thus, prior to the 1950s the instrumental data records exist for only a few key variables in isolated regions, and provide only a blurred glimpse at climate variability. The data are insufficient for deducing the attending atmospheric and oceanic circulations and heat transport, and the energy exchange between the media.

The post-World War II instrumental data base for the

FIGURE 2

The climatological data base for sea-surface temperature. Shown in (a) is the number of months in which there exists at least one observation in a 2° latitude by a 2° longitude area during the decade 1920-29. A dot indicates 1-12 months (out of 120) with data, a slash 13-60 months, and a plus sign 61-120 months. Panel (b): as in (a), but for 1950-59. (From Pan & Oort. 1990; reprinted with permission of Springer-Verlag.)