on record and an expectation of occurrence less than every 2000 years. Associated with this ENSO has been high sea surface temperatures (SSTs) in the tropical Pacific, with the warmest waters relative to the 1950 to 1979 mean on the equator at 180°.
4) Changes in ENSO alter, through the PNA, changes in atmospheric circulation throughout the North Pacific, as well as the South Pacific (Trenberth and Hurrell, 1994; Hurrell and van Loon, 1994). Thus the warming in the tropical Pacific extends along the coast of North America throughout much of Canada and Alaska, while cooling is present in the central North Pacific and extends to over 400m in depth (Deser et al., 1996). In the southeast U.S., a cooling influence from the Pacific is countered by a warming associated with the NAO.
5) Linear regression shows that 47% of the variance of the December to March Northern Hemisphere temperatures can be accounted for by the combination of the NAO, PNA and ENSO. The latter are highly correlated and the NAO and ENSO together account for 44% of the variance (Hurrell 1996). When the influence of these natural modes is removed from the record (Figs. 3 and 4), very little warming remains.
This does not mean that there is no global warming. It does suggest that it has been amplified by the way the northern planetary waves have set up relative to the land. It also raises questions about how these modes may be influenced by anthropogenic climate change.
In the Northern Hemisphere it is clear that there is a confounding of global warming by the changes in modes that dominate the natural variability (also Palmer 1993, Wallace et al. 1996). It is important to begin more in-depth analysis of model simulations, especially those dealing with increased greenhouse gases, in terms of the natural modes of variability such as ENSO, the PNA and the NAO, and determine how these may be altered by anthropogenic climate change.
References
Deser, C., M. A. Alexander and M. S. Timlin, 1996: Upper ocean thermal variations in the North Pacific during 1970-1991. J. Climate, (accepted).
Hansen, J., R. Ruedy, M. Sato and R. Reynolds, 1996: Global surface air temperature in 1995: Return to pre-Pinatubo Level. Geophys. Res. Lett., in press.
Hurrell, J. W., 1995: Decadal trends in the North Atlantic oscillation regional temperatures and precipitation.
Science, 269, 676-679.
Hurrell, J. W., 1996: Influence of variations in extratropical wintertime teleconnections on Northern Hemisphere temperature. Geophys. Res. Lett., 23, 665-668.
Hurrell, J. W., and H. van Loon, 1994: A modulation of the atmospheric annual cycle in the Southern Hemisphere. Tellus, 46A, 325-338.
IPCC, 1995: Climate Change 1995: The Science of Climate Change. Eds. J. T. Houghton, F. G. Meira Filho, B. A. Callander, K. Maskell, Cambridge Univ. Press, Cambridge, U.K.
Palmer, T. N., 1993: A nonlinear dynamical perspective on climate change. Weather 48, 314-326.
Trenherth, K. E., and T. J. Hoar, 1996: The 1990~1995 El Nino-Southern Oscillation event: Longest on record. Geophys. Res. Lett., 23, 57-60.
Trenberth, K. E., and J. W. Hurrell, 1994: Decadal atmosphere-ocean variations in the Pacific. Glim. Dyn. 9, 303-319.
Wallace, J. M., Y. Zhang, and J. A. Renwick, 1995: Dynamic contribution to hemispheric mean temperature trends. Science, 270, 780-783.
