A DYNAMIC INTERPRETATION OF THE GLOBAL RESPONSE TO EQUATORIAL PACIFIC SST ANOMALIES

The Northern Hemisphere winter 1988/89 was characterized by large persistent anomalies in both the tropics and the extratropics. A strong cold anomaly in the sea surface temperature (SST) was present in the eastern equatorial Pacific; as a response to this, the Walker circulation was very intense over the Pacific. In the northern extratropics, positive geopotential anomalies over western Europe and the eastern Pacific Ocean persisted through January and February; a major amplification of the Pacific ridge occurred at the beginning of February, with the onset of a Pacific block that caused a severe cold spell over the western coast of North America. The role of the SST anomaly in the maintenance of the seasonal anomaly over the northern extratropics has been investigated at ECMWF by comparing results of 90-day integrations with observed and with climatological SST. These results show that the extratropical response to the ''La Nina'' SST pattern accounts for a large proportion of the January-February anomaly, although none of the experiments was able to reproduce the Pacific block. The question of whether midlatitude influences on the tropical circulation played a significant role in the maintenance of the observed tropical anomaly is addressed by a 90-day experiment in which SSTs are set to their climatological values, but the extratropical flow is forced to be close to the observed one by ''relaxing'' wind and temperature fields toward the verifying analysis. The changes in the tropical circulation induced by the extratropical relaxation are clearly positively correlated with those induced by the SST anomaly. A second ''relaxation'' experiment shows that these changes are indeed able to reinforce the extratropical response, suggesting the existence of a positive feedback. In a nonlinear framework, this feedback can be seen as the manifestation of global-scale regimes that exist independently of SST anomalies, but whose frequency of occurrence and stability properties can be significantly altered by a strong, persistent boundary forcing. This hypothesis is supported by the study of a simple five-dimensional dynamical system, which results from the coupling of a three-variable chaotic model with a two-variable linear oscillatory system (representing the qualitative nature of the midlatitude and tropical large-scale circulation, respectively). The regimes of the system are determined by its chaotic component and are only marginally affected by the coupling as far as their position in phase space is concerned; however, the frequency of the regimes can be significantly altered by a forcing applied to the oscillatory component. It is shown that this model can explain a number of qualitative aspects of tropical-midlatitude interactions simulated by the GCM integrations herein.