Over its ten-year lifetime, the Tropical Ocean-Global Atmosphere (TOGA) program (1985-1995) made major strides toward understanding the El Niño Southern Oscillation (ENSO) phenomenon, which impacts surface air temperature and rainfall over many regions of the globe. In particular, TOGA demonstrated the feasibility of operational seasonal-to-interannual climate prediction of equatorial Pacific sea surface temperature anomalies based on numerical models that simulate, in a rudimentary manner, the physics of the coupled tropical ocean-atmosphere system, and it clarified the nature of the remote, planetary-scale atmospheric response to these anomalies. The U.S. Global Ocean-Atmosphere-Land System (GOALS) program is predicated on the belief that the skill of operational climate prediction can be further increased by continued research on ENSO and by efforts to understand other elements of the climate system that contribute to the observed seasonal-to-interannual variability. The Pan American Climate Studies (PACS) program is a component of the U.S. GOALS program in the 1995-2005 time frame, and PACS provides a phenomenological context for some of the GOALS research.
The overall goal of PACS is to extend the scope and improve the skill of operational seasonal-to-interannual climate prediction over the Americas. Particular emphasis is placed on warm season rainfall, which is not yet predictable. In the context of PACS, climate prediction is concerned not only with seasonal mean rainfall and temperature, but also with the frequency of occurrence of significant weather events such as hurricanes or floods over the course of a season or seasons.
The scientific objectives of PACS are to promote a better understanding and more realistic simulation of (1) the boundary forcing of seasonal-to-interannual climate variations over the Americas, (2) the evolution of tropical sea surface temperature anomalies, (3) the seasonally varying mean climate over the Americas and adjacent ocean regions, (4) the time-dependent structure of the Intertropical Convergence Zone (ITCZ)/cold tongue complex, and (5) the relevant land surface processes.
To state these objectives more explicitly:
Boundary forcing: The atmospheric circulation, in isolation, is not predictable beyond a week or two. Hence, prospects for improved climate prediction on the seasonal-to-interannual time scale hinge on the ability to exploit the relationships between the planetary-scale atmospheric circulation and the slowly evolving and potentially predictable boundary forcing; i.e., the fields of sea surface temperature, vegetation, and soil moisture, in which the season-to-season "memory" of the coupled ocean-atmosphere-land system resides. Improved understanding of boundary-forced atmospheric anomalies is of central importance, not only to PACS, but to the entire GOALS program.
Evolution of tropical sea surface temperature anomalies: For climate prediction of a season or more in advance, it is necessary to take into account the evolution of the boundary forcing of the atmosphere. In keeping with the general strategy of GOALS, PACS will seek to advance the state-of-the-art of the prediction of sea surface temperature anomalies in the tropical Atlantic and Pacific, both of which are known to influence climate variability over parts of the Americas.
Seasonally varying mean climate: Regional rainfall anomalies over the Americas are largely a reflection of the intensification or weakening, or subtle displacements in positions of the climatological-mean features that organize the rainfall, i.e., the monsoons, the oceanic ITCZs, and the tropical and extratropical cyclone tracks. An understanding of these robust climatological-mean features and their seasonal evolution is a prerequisite for the interpretation and prediction of the anomalies.
Structure of the ITCZ/cold tongue complex: A major stumbling block in the validation of the models used to simulate tropical atmosphere-ocean interaction in ENSO is the lack of observational data for defining the structure of the ocean mixed layer and the overlying atmosphere in the ITCZ/cold tongue complex. PACS will address this deficiency through its field projects and the related modeling studies that will make use of these data.
Land surface processes: The distribution of rainfall over the Americas is shaped, not only by sea surface temperature patterns but also by land surface processes, particularly during the warm season, when vegetation and soil moisture are highly influential. Orography and coastal geometry mediate these effects and leave a distinctive mesoscale imprint upon the rainfall patterns. These issues will be addressed in collaboration with the Global Energy and Water Experiment (GEWEX) and its regional programs, with PACS supplying the atmospheric modeling expertise and GEWEX the hydrological expertise. Much of this research will require the use of mesoscale models, applied in a climatological setting.
PACS scientific objectives (1) and (2) directly address the scientific objectives of the GOALS program: (1) relates to atmospheric prediction and (2) to prediction of tropical sea surface temperature; in this sense they may be viewed as primary. Objectives (3)-(5) play a supportive role by advancing understanding of the mechanisms that give rise to and limit the predictability of the coupled global ocean-atmosphere-land system: (3) and (4) relate to the prediction of ENSO and other phenomena that give rise to tropical sea surface temperature anomalies, and (5) relates to the prediction of continental rainfall.
PACS encompasses a broad range of activities, including empirical studies, data set development, modeling, climate monitoring, and more intensive, limited-term field experiments. In order to avoid being spread too thin, the field studies will focus on different regions of the Pan-American climate system in sequence. During the first five years they will focus on atmosphere-ocean interaction in the tropical eastern Pacific, in association with the ENSO cycle and the climatological-mean annual march. During the second half of PACS the emphasis will shift to the tropical Atlantic Ocean where the sea surface temperature anomalies are more subtle and more diverse in terms of horizontal structure than in the Pacific, but no less important in terms of their influence upon precipitation in the adjacent continental regions.
This scientific prospectus/implementation plan provides the motivation and scientific basis for PACS and describes the research that will be carried out in pursuit of PACS scientific objectives. Section 1 gives a scientific description of phenomena in the ocean-atmosphere system that are likely to be of importance for understanding and predicting seasonal-to-interannual climate variations over the Americas. The climate and weather variations over the Americas that provide the practical and scientific motivation for PACS are discussed in Section 2, while Sections 3-5 describe the empirical studies, data set development, and modeling that will be conducted under its auspices. Section 6 describes field studies envisioned for PACS, starting with projects already funded and going on to describe activities likely to be proposed for the 1997-2000 time frame. Section 7 discusses linkages with GEWEX and its regional programs and NASA's Tropical Rainfall Measurement Mission (TRMM). Section 7 also describes the anticipated linkages with emerging national and international programs, including the Scripps-Lamont Consortium for the Ocean's Role in Climate (CORC), the Pilot Research Moored Array in the Tropical Atlantic (PIRATA), the anticipated World Climate Research Programme (WCRP) Climate Variability and Predictability (CLIVAR) international programs on the Variability of the American Monsoon Systems (VAMOS), and Pacific and Atlantic basinwide extended climate studies (BECS). Program management is discussed in Section 8. Within the U.S., interagency support is being sought for PACS, with coordination by the GOALS Project Office. The Inter-American Institute for Global Change Research serves as a vehicle for coordinating international cooperation.