Various investigators have proposed that disturbances propagating into the
tropical Pacific from the extratropics or the Indian Ocean can trigger or
modify the ENSO cycle (see, e.g., Barnett et al. [1989] and section
3.2.1). More recently, preliminary investigations of the
effect of changing atmospheric CO
concentration on the tropical Pacific
coupled interannual variability have been reported by Meehl et al.
[1993], Graham et al. [1995], and Knutson and Manabe [1995].
However, in these studies of ENSO in an increased CO
environment, there
are serious numerical and methodological issues that arise, and the
relevance of the results from these studies to Nature is not certain.
Finally, the definitive study of Mass and Portman [1989] eliminates volcanic
eruptions as a forcing of ENSO.
It is not clear, however, to what extent the spectrum of interannual variability in the tropical Pacific is stationary: the direct observational data does not exist to adequately define even the multidecadal variability in the tropical Pacific. The low frequency modulation of ENSO could come about from nonlinearity in the coupled atmosphere/ocean/land system, involving either the upper ocean, or changes in the (shallow) thermohaline circulation in the subtropics. Unfortunately, the present intermediate models of the coupled atmosphere/ocean system are inappropriate for such studies, and it is not yet possible to investigate the coupled GCMs for the thousands of model years that are required to examine these issues.
It may be possible to address these issues, however, from the increasing variety and geographical extent of the proxy data from paleoclimate studies. For example, proxy data exists for equatorial upwelling [ Lea et al., 1989] and extrema in wind stress anomalies [ Shen et al., 1992a,b; Cole et al., 1993] from tropical corals. Proxy indicators have been derived for the Southern Oscillation Index from coral [ Cole and Fairbanks, 1990] and tree ring data [ Stahle and Cleaveland, 1993]. Finally, there is a connection between ENSO and high dimethylsulfide (DMS) marine emissions at high southern latitudes [ Legrand and Feniet-Saigne, 1991].