INTRODUCTION

The major modes of atmospheric circulation that influence seasonal climate over the Americas exhibit dramatic variability on interannual to decadal timescales. The resulting regional climate variability has enormous socioeconomic impacts as vividly demonstrated by the disastrous flooding in eastern Argentina and the extended drought and massive wildfires in Mexico during the 1997-1998 El Nino event. At decadal scales, the prolonged shift in sea surface temperature patterns over the north and south Pacific Ocean after 1976 (Graham 1995) has resulted in ocean and atmospheric changes that have caused costly changes in commercial fish populations in the eastern north Pacific and a greatly reduced carrying capacity for commercially important Patagonian grasslands. These coherent inter-hemispheric changes in annual and decadal climate patterns appear to have been driven by fundamental changes in the hydrologic cycle of the tropical Pacific Ocean.

Hemispheric-scale networks of instrumental and proxy climate data are needed to document and help understand these changes in the ocean-atmosphere system and their impact on the Americas. Substantial recent effort has been devoted to the development of ocean-atmospheric monitoring arrays in the tropical Pacific (e.g., TOGA/TAO, TOPEX/POSEIDON). The cost of these arrays has already been justified by the economic benefits provided by the long-lead climate forecasting associated with the recent El Niño-Southern Oscillation (ENSO) warm event. However, there is clear instrumental and paleoclimatic evidence that, for example, the frequency of warm and cold ENSO events has been subject to substantial changes over the past several centuries. The available instrumental meteorological records are simply too short to clearly define the important temporal and spatial modes inherent in the low-frequency dynamics of ENSO and other major circulation systems. As these decade-scale changes in atmospheric circulation have strong impacts on regional climates and society, understanding these phenomena will improve the skill of long-range climate forecasting.

 

OBJECTIVES

To address these large scale issues we propose a Collaborative Research Network that will develop a global scale transect of proxy climate records from the Western Cordillera of the Americas and document climate variability at a wide range of timescales for the past several hundred years. Specifically we identify four major objectives.

1. To develop a network of exactly-dated annual tree-ring chronologies from climatically sensitive treeline sites that extend from the arctic to sub-antarctic in the western American cordillera (Fig 1). These chronologies would be used to reconstruct and compare regional climate variability along the transect, and to define the natural interannual-decadal scale modes of variability present in the major circulation systems that affect this region, including the Pacific/North America Pattern (PNA), the Pacific Decadal Oscillation (PDO), the Mexican Monsoon (MM), the El Nino/Southern Oscillation (ENSO), and the Trans-Polar Index (TPI).

 

2. To establish an international collaborative network of scientists who will execute this project and foster training and the development of dendrochronology and paleoclimatic research in the Americas. This will be achieved by the establishment of collaborative research, scientific exchange and training programs among the member institutions. We will strengthen existing facilities by the establishment of dedicated research positions at four Latin American centres including two new laboratories to be developed in Bolivia and Mexico where there is high potential for dendroclimatic research (Fig. 1). This will enhance the scientific expertise in dendrochronology and climatic analysis in Latin America for both students and established scientists. A series of Annual Science Meetings will be developed to promote collaboration and training within the CRN plus the exchange of information with other IAI groups.

Collaborating Institutions:

 

ARGENTINA: Instituto de Nivología, Glaciología y Ciencias Ambientales (Mendoza);

 

BOLIVIA: Universidad Mayor de San Andrés (La Paz);

 

CHILE: Universidad de Chile (Santiago), Universidad Austral de Chile (Valdivia);

 

CANADA: University of Victoria (Victoria), University of Western Ontario (London);

 

MEXICO: Instituto Nacional de Investigaciones Forestales y Agropecuarias (San Luis Potosi);

 

USA: University of Arizona (Tucson), University of Arkansas (Fayetteville),

Lamont-Doherty Earth Observatory (NY),

NOAA (Boulder,CO),

Ohio State University (Columbus)

Scripps Institute of Oceanography (San Diego)

 

3. To develop applied, climate related time-series from these tree-ring data that will be of particular value in planning, the management of resources and impact assessments. Such socially relevant products would include for example, the long-term reconstruction of streamflow, precipitation and drought in agriculturally important regions; documentation of changes in the frequency of natural hazards (floods, drought, fire, etc); investigation of the interaction between natural climate variability and human impacts on watersheds; and the development of volume production chronologies from commercially important forest regions. We would actively engage the impacts community in the development and use of these materials through the Annual Science Meetings and individual collaborative efforts.

4. To produce a database of tree-ring chronologies and environmental reconstructions that will be freely and permanently accessible to the scientific community for climatic analyses and human impact assessments. This will include a central website for our proposed project, and contributions to the International Tree-Ring Databank (ITRDB) and the paleoclimatic database maintained by the U.S. National Geophysical Data Center in Boulder, Colorado (USA).

Past, present and future climate variability are central issues within Global Change and have enormous socioeconomic and ecological implications that policy-makers must address. The scope and scale of this project clearly address the IAI Science Agenda and the focus on education, training and infrastructure development also supports the basic mission of IAI to enhance scientific capacity in Latin America.

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FIG 2. LEFT. Surface temperature differences ([1977-1982 mean]-[1971-1975 mean]; after Graham, 1995, Fig 4a) showing changes induced by the 1976 shift in the PDO over the Pacific Ocean (Mantua et al., 1997). This suggests that the changes in average temperature observed during the 1970's along the extratropical Americas were driven almost entirely by changes in tropical sea surface temperatures. Is this common forcing in both hemispheres a unique event or has this teleconnection pattern been recurrent in the past? RIGHT Comparison of annual (April to March) temperature fluctuations in Alaska (a) and Northern Patagonia (d) with 7 coastal Alaskan tree-ring chronologies (b) and 3 northern Patagonian tree-ring chronologies (e) plus the winter glacier mass balance from Peyto Glacier, Alberta (c). All records clearly show the effects of the 1976 transition from cold to warm conditions over the central Pacific. This indicates that tree-ring records from high latitude sites in both hemispheres have a comparable sensitivity to sea surface temperature changes in the Pacific (Villalba et al., 1988).

 

PROBLEM AND NEED

Development of skilful long-range climate forecasting in certain key areas of the world comes at a time when society has become increasingly vulnerable to climatic variability. It is socially, economically and scientifically vital that the nature of climate variability be defined with the greatest possible precision, particularly for those regions where the potential predictability of climate is high. ENSO has demonstrated value for climate forecasting in portions of North and South America, and other major circulation systems in the Americas are linked in part to ENSO variability and may contain other predictable components. Persistent changes in the PDO, for example, appear to have been associated with a major redistribution of surface water mass in both the north and south Pacific after 1976 (Figure 2; Graham 1995). These ocean-air anomalies have had huge environmental and social consequences, including a coherent decade-scale response of climatically-sensitive tree-ring chronologies in Alaska and Patagonia (Figures 2-4). Point correlation analyses comparing gridded sea-surface temperature (SST) fields over the entire Pacific basin suggest that these persistent large-scale SST changes and the associated coherent climate/tree growth responses at the temperate high latitudes of North and South America may have their origin in the equatorial Pacific (Figures 3-4).

The proposed inter-hemispheric network of tree-ring chronologies represents the only currently feasible prospect for documenting and analysing large-scale climate anomalies across the Americas at annual and seasonal resolutions for the past several hundred years. Contributions from other important proxy climate records (tropical and temperate ice cores, annually banded corals, varved marine and lacustrine sediments) will also be essential to define paleo-circulation systems and past environmental changes, but presently they are not sufficiently abundant, widespread, or precisely dated to substitute for the proposed latitudinal tree-ring array.

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3. Spatial correlation patterns (1857-1988) between annual SST anomalies over the Pacific Ocean and annual SST anomalies on (a) the Alaska coast (57.5° N, 142.5° W), (b) the Chilean coast (42.5° S, 72.5° W), and tree-ring variations from (c) coastal Alaska and (d) northern Patagonia. Grid cell locations indicated by white dots (a and b): tree-ring sites shown by white triangles (c and d). The similar spatial correlation patterns between Pacific SSTs and both Alaskan and South American tree-ring records indicate that Pacific SSTs are a common forcing of climate variations in the extra-tropics. These strong correlation patterns also suggest that suitable networks of tree-ring chronologies could be used to reconstruct SST variations for selected areas in the Pacific prior to the period of instrumental records.

FIG 4. Upper diagram. Reconstruction of SOI based on tree-rings from Texas, S.W. USA, Mexico and Java (Stahle et al., in press). Lower diagram. The most significant decadal-scale oscillatory modes (centred at 12.8 years) extracted by singular spectrum analysis from the first principal component of temperature-sensitive tree-ring chronologies from northern Patagonia (blue) and coastal Alaska (red) (Villalba et al., 1998). If decadal-scale climatic variations forced by the tropical Pacific had affected past temperature changes along the extra-tropical coasts of North and South America, the tree-ring records from Alaska and Northern Patagonia should present similar oscillatory modes. Although significantly correlated over most of the record (r=0.60), the relationships change over time (r = 0.73 1592-1849; r =0.02 1850-1988) possibly due to a major reorganisation in the Tropical Pacific Ocean and its extratropical teleconnections. These two completely independent proxy climate records show major changes in both decadal and interannual climate variability around 1850. These observations have important implications in relation to our knowledge of ENSO and ENSO-related climate variability.

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