DECADAL VARIABILITY IN ENSO AND EXTRATROPICAL TELECONNECTIONS
Kevin E. Trenberth
National Center for Atmospheric Research1
P.O. Box 3000, Boulder, CO 80307
Perceptions of climate variations and change
One perception on the climate record is that natural variability is the predominant signal and human-induced effects are small. Another perception is that natural variability is noise that masks the anthropogenic signal. In-depth study of the forcings and the issues involved in anthropogenic climate change, in particular the increases in greenhouse gases and atmospheric aerosols, leads to an appreciation that a discernable influence of humans on climate has been identified (IPCC 1995).
There is only one climate record for the planet Earth. We do not have the luxury of watching an ensemble of Earth's evolve and averaging their climates to reduce noise, as we can do in models. The observed temperature record necessarily includes BOTH the anthropogenic signal and natural variability, and the evidence that is emerging indicates that these are less separable than might be desired by either camp. It is a challenge to the scientific community to keep an open mind and begin to adequately address this issue.
This paper illustrates the problem by showing how natural modes of the climate system in the Northern Hemisphere, in particular the North Atlantic Oscillation (NAO), the Pacific-North American (PNA) teleconnection pattern, and El Nino-Southern Oscillation (ENSO) combine to influence the planetary wave structure over the Northern Hemisphere such that most of temperature changes in recent years have been of warming over North America and Eurasia but cooling over the northern oceans. At the surface, the heat capacity of the underlying surface has a major influence in amplifying changes over land while moderating changes over the ocean, so that global warming is enhanced, while the mid troposphere, as manifested in the satellite-borne microwave sounder unit (MSU) retrievals for channel 2R, show a very different pattern. These aspects are a factor in accounting for why the year 1995 was the warmest (or second warmest) on record. They also account for why there was a sharp reversal for the northern winter of 1995-96.
1 The National Center for Atmospheric Research is sponsored by the National Science Foundation
Temperature changes
1) Hansen et al. (1996) have documented the temperatures for 1995, or more specifically December 1994 to November 1995, and shown that it is the warmest on record using their dataset. Some cooling in December, or use of different datasets, relegate 1995 to second warmest on record. The largest anomalies occurred in the first half of the year and were over the northern continents.
2) Hurrell (1995) has documented the northern winter changes in the NAO and the associated changes in precipitation and temperatures. Values of the NAO index (Fig. 1) are at unprecedented high levels after about 1980 and contribute to warming throughout Eurasia and cooling over the western North Atlantic and Greenland.
3) Trenberth and Hoar (1996) have shown that ENSO has also been very unusual with a prolonged ENSO event from the beginning of 1990 to mid 1995 (Fig. 2) as the longest
