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Regional-Scale Climate Variability on Decadal to Multi-Century Timescales

Raymond S. Bradley1
Henry Diaz2

1University of Massachusetts, Amherst
2CDC/OAR, NOAA, Boulder

Project duration: 5/1/04 - 4/30/07

This collaborative project addresses key issues in understanding the Earth’s climate system, as highlighted by the U.S. Climate Change Research Program. In particular, we will focus on improving knowledge of decadal- to century-scale natural and human-induced climate patterns, and the linkages between large-scale and regional climate variability. We address one of the main goals of this program: to provide data that will enable an objective assessment of the potential for and consequences of human-induced climate change at regional scales… and on decadal and longer time scales through the continued development and analysis of long-term, observational climate data sets that can be used to test the ability of GCM-based climate models to realistically simulate and predict climate system behavior on [multiple] time and space scales. We address the following issues related to specific DOE Climate Change Research objectives, with a particular focus on studies of natural (solar, volcanoes) versus anthropogenic forcing of global and regional climate change:

  1. Analysis of multiple GCM climate simulations for different time periods, for diagnosis and assessment of regional impacts. We consider the following: Are the spatial signatures of past regional climatic variations, evaluated over different temporal scales (last 100 years, last several centuries, last millennium) consistent with the estimated nature of the climate forcings thought to be responsible for them? The work will involve analyzing a suite of GCM simulations under changing background climate conditions in different regions of the world. We will use signal-to-noise statistical methodology to identify the most robust regional signals of change in relation to large-scale climate forcings. In particular, comparison of very long (500–1000 years) control and externally forced climate simulations will elucidate characteristic patterns of response to both internal and external forcings in surface climate. A specific focus will be on solar and volcanic forcing effects using a regional climate model for East Asia, and verification using historical documentary sources from the region.
  2. Evaluation of key regions where the effect global warming may be amplified. Certain regions of the world have been identified as “critical systems at risk” from future climate change (NRC, 1999). We will address the following: What is the relationship between climatic changes over the past ~100 years and changes in selected ecosystems [e.g., tropical cloud forest, semiarid desert, sky islands (vertically stratified mountain ecosystems)]? Has climate changed more rapidly at higher elevations? We will build on our previous studies of the magnitude and rates of change of temporal trends in climate at multiple timescales ranging from century to decadal scales, for high-risk regions such as UNESCO biosphere reserves, national parks and areas where strong ecotonal boundaries exist (mountainous regions, arid/semi-arid boundary zones, steppe/tundra transitions, etc).
  3. Analysis of changes in the frequency of occurrence and persistence of extreme events. The potential for an increase in the frequency of high-intensity or extreme climatic events under future global warming conditions is a high priority in the US Climate Change Science Plan. We consider the following: What are the changes in the observed frequency distributions of surface temperature and precipitation in the last ~40 years in different regions of the Americas? How do they compare with modeled distributions for this interval and with observations in the earlier part of the record? We will follow the recommendations of the WMO CCL/CLIVAR Joint Working Group on Climate Change Detection who have prioritized a set of parameters that are important in assessing changes in climatic extremes. We will focus on regions in the Americas with unique ecosystems, such as the cloud forests of Central America and the Sonoran Desert of northern Mexico and the Southwest United States, that may be at relatively higher risk from the effects of rapid or amplified climatic change.
  4. Evaluation of persistent climate anomalies. The record of severe and sustained climatic anomalies in the past, defined on varying timescales (from days to decades), demonstrates that hydroclimatic extremes have often had the greatest effect on human society, and these may persist long after the end of a particular climate event. We address the following: What are the key features of persistent (decadal and longer-term) climate changes recorded in different parts of the world during the pre-instrumental period? Are these related to key circulation modes and SST anomalies? We consider the spatial and temporal characteristics of regional climate change in the second half of the 20th century in comparison with the first half of the century using observations and GCM simulations in order to assess possible anthropogenic forcing in the most recent period. We will also examine the paleoclimatic record of the last ~1500 years to understand hydroclimatic anomalies that appear to have far exceeded recent experience.

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