number of warm and cold events in the tropical Pacific is unprecedented.
Whether the unusual 1977-to-1988 imbalance can be ascribed in part to some identifiable contributor, or merely reflects natural variability, is a very difficult question to answer. The major change that occurred in March-to-April 1988, a transition from El Niño to a very strong La Niña (Figure 4), apparently ended the climate regime, although the underlying ocean currents and heat storage must be still perturbed and the pattern could reemerge. Indeed, the 19911992 ENSO event was noted for its exceptionally warm water along the west coast of both North and South America in early 1992.
We wish to especially thank Dennis Shea for preparing some of the figures. This research is partially sponsored by the Tropical Oceans/Global Atmosphere Project Office under grant NA86AANRG0100.
Lamont-Doherty Earth Observatory
Dr. Trenberth has demonstrated in this study how complex the middle-latitude system is. We have to deal with seasonality (there is a nice diagram in his paper that actually emphasizes the seasonality in this very-low-frequency phenomenon that he discussed). We also have to deal with the fact that middle latitudes filter the signal that comes out of the tropics, if it does indeed come out of the tropics as suggested here. Actually, the Pacific may be a more complex environment in which to study mid-latitude interactions than the Atlantic, where the effect of ENSO or of the tropics seems to be weaker.
One of the biggest issues in mid-latitude interaction is whether the ocean and the atmosphere are really coupled. It has been suggested that the ocean is being forced by the atmosphere and no feedback is involved. Local interactions like mixing and heat exchange, or maybe some non-local interactions due to the currents and transports, could be responsible for the fact that the pattern is long-lived. But why is it seen also in summer when the forcing from the atmosphere disappears, as the paper emphasizes?
If there is feedback in mid-latitude interactions, can we learn about it from GCMs? As it turns out, we are dealing with a very confusing set of results presented in several papers. They are confusing not only because of the use of super-anomalies versus regular anomalies, but also because different kinds of models and different kinds of methodologies have been used to run the models. We have seen perpetual experiments run, and we have seen experiments of a more transient nature where the seasonal cycle varies and where the SSTs vary continuously.
The coupling between the mid-latitude ocean and the atmosphere is one of the unresolved challenges today. It remains to be seen whether it is really a two-way interaction, and whether feedback in the middle latitudes is involved.
BRYAN: This all reminds me of the Hoskins and Karoly ideas of about 12 years ago about the connection between the tropics and mid-latitudes.
TRENBERTH: Well, that may be one of the ways in which the whole system is tied together, but the statistical results indicate that it's probably more than just Southern Oscillation. The transients in the middle latitudes seem to be adding a considerable chaotic component to any teleconnections, and the forcing itself is different for every ENSO.
DICKSON: I worry a bit that your picture doesn't take into account the overlap of the North Atlantic and the Southern Oscillation signals that Rogers described in his 1984 paper. The biggest anomaly gradient of the lot sits on the boundary between them on the U.S. eastern seaboard, where there's such a strong land/ sea temperature contrast.
TRENBERTH: We can't treat those signals as linear and independent; I think the statistics indicate that there is a highly variable and sometimes strong connection between them. And I think there's a lot of feedback involved in the East Coast baroclinicity.
RASMUSSON: Yes, the correlation of all the oscillations in that area is fascinating. But the mean patterns in the North Pacific and