cal time series. The relationships pointed out in the previous paragraph appear to be valid during earlier decades. In particular, note the weak coupling between the tropical and extratropical time series on the interannual time scale.
It has been suggested that the prevalence of the warm phase of the ENSO cycle during the late 1970s and 1980s has contributed to the global warming observed during this period (e.g., see Trenberth, 1990; Trenberth and Hurrell, 1995). Inspection of Figure 8 suggests that some of the warming observed within the tropical belt from the late 1970s onward could be attributed to the absence of a well-defined cold phase of the ENSO cycle between 1976 and 1987. On the other hand, this feature could equally well be interpreted as a gradual upward temperature trend that is independent of the ENSO variability. The latter interpretation is supported by the fact that equatorial Pacific SST (Figure 6) was substantially colder during 1988 than at any time since the early 1950s, yet tropical land-surface temperatures (Figure 8), although cool relative to other periods during the 1980s, were not as cool as those observed on a number of occasions during the 1960s and early 1970s.
The time series of extratropical Northern and Southern
Hemisphere surface air temperature in Figure 8 are not particularly well correlated with one another. The only common element is the upward trend during the past decade. Since the ENSO signal is common to the northern and southern tropics, it contributes to the correlation between the two hemispheres when the spatial average is extended all the way to the equator, as in many published time series (e.g., Spencer and Christy, 1992, Figure 12). It is evident from our figure that when the tropical belt is excluded in defining the hemispheric means, the correlation between the hemispheric time series drops dramatically.
Hence, we can conclude that the interannual variations in zonally averaged temperature associated with the ENSO cycle are largely restricted to the tropics, even though the ENSO signal is apparent in global mean temperature.
Low-frequency variability in temperatures aloft over the extratropical Northern Hemisphere exhibits the same type of structural characteristics and seasonality as variability in