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8 MODES OF VARIABILITY SUMMARY Radiatively induced greenhouse warming is not the only effect of the buildup of greenhouse gases. There is a growing body of evidence that suggests that human activities may also be capable of changing the time averaged states of the natural modes of variability of the climate system, most notably the El Nino-Southern Oscillation (ENSO) and the high-latitude Northern and Southern Hemisphere annular modes. An understanding of these modes and how they react to anthropogenic forcing is essential for detection and attribution of global climate change and for interpreting the role of feedbacks. The natural variability of these modes on the year-to-year time scale provides a testbed for model parameterizations of feedbacks. The planetary-scale atmospheric circulation exhibits preferred modes of month-to-month and year-to-year variability that exert a strong influence on regional climate and may be capable of influencing climate sensitivity. The most important of these modes are ENSO, which modulates the mean tropical tropospheric temperature (Angel!, 1988; Newell and Weare, 1976; NRC, 2000a), the mean rainfall and vegetation over the tropical continents, and mean rate of increase of atmospheric carbon dioxide (Keeling and Revelle, 1985, Prentice et al., 2001~; and the Northern and Southern Hemisphere annular modes, which modulate temperature, precipitation and winds, and high-latitude stratospheric ozone concentrations (Hurrell, 1995), and sea-ice concentrations (Rigor et al., 2002~. These planetary-scale modes appear to have exhibited secular trends during the past few decades. Two very strong El Nino events have occurred 107

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108 UNDERSTANDING CLING TE CHANGE FEEDBACKS since 1980 and barometric pressure has tended to be above normal on the western side of the tropical Pacific, indicative of the warm polarity of the ENSO cycle, which favors reduced upwelling in the equatorial Pacific and abnormally dry conditions over its tropical continents. The ENSO cycle has exhibited a bias toward the warm polarity from 1977 onward. The annular modes in both hemispheres have exhibited trends toward the high-index polarity, characterized by below normal sea-level pressure over the polar cap regions, westerly wind anomalies at subarctic latitudes, above normal winter temperatures over most of Eurasia, a thinning of the springtime stratospheric ozone layer, and a thinning and enhanced summer melting of Arctic sea ice (Wallace and Thompson, 2001~. Whether these trends are secular in nature or merely a reflection of decadal-to-century-scale climate variability remains to be seen. In any case they have been large enough over the past few decades to significantly impact the statistics that are commonly used to assess the extent of global climate change. For example, analyses indicate that there have been substantial average increases in precipitation over the tropical oceans since the late 1 970s related to increasing frequency and intensity of El Nino events (Trenberth et al., 2002~. The cooling over the Antarctic continent and the rapid warming over the Antarctic peninsula is largely a consequence of the trend in the Southern Hemisphere annular mode (Thompson and Solomon, 2002~. Much of the wintertime warming over the Eurasian continent, the thinning of the stratospheric ozone layer, and the retreat of Arctic sea ice is a consequence of the trend in the Northern Hemisphere annular mode (Wallace and Thompson, 2001~. An awareness of these modes and an understanding of their behavior is essential for a proper attribution of the observed climatic changes in studies of climate sensitivity. For example, in diagnosing ice-albedo feedbacks it is important to know whether the observed retreat and thinning of sea ice from the 1980s to the l990s (Rothrock et al., 1999) was a direct thermodynamic consequence of global warming, or whether it was due to the enhanced cyclonic circulation around the periphery of the Arctic observed in association with the trend toward the high-index polarity of the Northern Hemisphere annular mode (Rigor et al., 2002~. In a similar manner ENSO-induced changes in the tropics need to be taken into account in diagnosing cloud, water vapor, and static stability feedbacks It has been proposed that the observed trends in ENSO and the annular modes may be anthropogenically induced. The former may be the result of cloud-albedo feedback and enhanced warming in the eastern equatorial Pacific (Timmermann et al., 1999, Meehl and Washington, 1996~. The latter may be the result of either the destruction of stratospheric ozone by

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MODES OF VARIABILITY 109 Chlorofluorocarbons (CFC's) (Volodin and Galin, 1999) or by high-latitude stratospheric cooling induced by the buildup of greenhouse gases (Shindell et al., 1999), or by some combination of the two. If these hypotheses are correct, the trends in these modes should be viewed as an integral part of human-induced climate change. The tropical Pacific SST (e.g., NIN03 index measured through the TOGA/TAO array) and pycnocline depth should continue to be used as a metric to evaluate model performance and to diagnose and monitor decadal and longer-term changes in ENSO statistics, which have the potential to be modulated by global climate change. In the context of this report the month-to-month and year-to-year variations in these natural modes of variability provide an opportunity for verifying model parameterizations of the processes that govern the feedbacks discussed in previous chapters. For example, a faithful simulation of the year-to-year changes in tropical mean temperature, humidity, cloudiness, and rainfall that occur in association with the ENSO cycle requires a realistic treatment of many of the same physical processes that determine the sensitivity of these parameters to global warming. However, unlike forecasts of greenhouse warming, it can be verified on a year-by-year basis. In a similar manner, observed year-to-year changes in stratospheric ozone, sea ice, and snow cover that occur in association with natural fluctuations in the annular modes can be used to diagnose the treatment of processes relevant to ice-albedo feedbacks in climate models. The observational requirements for defining the evolution of the principal modes of variability of the coupled atmosphere-ocean system are defined in the planning documents for the World Climate Research Program (WCRP) on climate variability and predictability (CLIVAR), as described in NRC (2001d). Studies such as the Pacific Basin Extended Climate Study (Davis et al., 2000) could help test hypotheses concerning the nonstationary behavior of natural modes. On the national level and in some cases even on the agency level, program planning for climate and global change and that for diagnosing and predicting natural climate variability on seasonal to decadal time scales has been carried out largely by mutually exclusive communities of scientists with relatively little coordination between them. Clearly, there is an opportunity for synergy between the research program on climate sensitivity and feedbacks outlined in this report and the research on climate prediction described in the CLIVAR planning documents.