David Greenland

Part II of this book deals with the quasi-quintennial timescale that is dominated by the El Nino-Southern Oscillation (ENSO) phenomenon. During the last 50 years, ENSO has operated with a recurrence interval between peak values of 2-7 years. The term quasi-quintennial is chosen to recognize that climatic events other than ENSO-related ones might occur at this timescale. The general significance of the ENSO phenomenon lies in its influence on natural and human ecosystems. It has been estimated that severe El Nino-related flooding and droughts in Africa, Latin America, North America, and Southeast Asia resulted in more than 22,000 lives lost and more than $36 billion in damages during 19971998 (Buizer et al. 2000). The specific significance of ENSO within the context of this book is that it provides fairly well-bounded climatic events for which specific ecological responses may be identified.

In the other chapters in part II, we first look at the U.S. Southwest. The Southwest is home to an urban LTER site, the Central Arizona-Phoenix (CAP) site. Tony Brazel and Andrew Ellis describe the clear ENSO climatic signal at this site and identify surprising responses that cascade into the human/economic system. Ray Smith, Bill Fraser, and Sharon Stammerjohn provide more details of the fascinating ecological responses of the Palmer Antarctic ecosystem to ENSO. World maps of ENSO climatic signals do not usually show the Antarctic, and the LTER program provides some groundbreaking results at this location, with Smith and coworkers (see the Synthesis at the end of this part) providing such maps (figures S.1 and S.2). Kathy Welch and her colleagues present equally new discoveries related to freshwater aquatic ecosystems from the other Antarctic LTER site at the McMurdo Dry Valleys.

This chapter gives a general introduction to ENSO and its climatic effects. How ever, these general patterns may mask the detailed responses that occur at individual locations. This is one reason for presenting the principal results of previous findings related to El Niños and LTER sites and one particular analysis focused on LTER sites. This analysis for the period 1957-1990 investigates the response of monthly mean temperature and monthly total precipitation standardized anomaly values to El Niño and La Niña events as indicated by the Southern Oscillation Index (SOI) (Greenland 1999). The chapter then reviews some of the ENSO-related responses occurring at LTER sites. Some of these responses are treated in more detail in other chapters in this section and this book. The goal here is to provide an introduction to climate variability and ecosystem response at the quasi-quintennial scale. Finally, this chapter addresses some of the framework questions of this book.

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