Ocean–atmosphere System of the Tropical East Pacific

SEPTEMBER 2004 AMERICAN METEOROLOGICAL SOCIETY | T hough much progress has been made in the development of coupled ocean–atmosphere models, many discrepancies remain between observations and model results. The tropical east Pacific (i.e., east of about 140°W) is a region in which the performance of coupled models has been problematic (Mechoso et al. 1995), with particular difficulty in the simulation of the seasonal cycle of sea surface temperatures (SSTs) and the associated atmospheric circulations. In this paper and its companion (Bretherton et al. 2004), we describe the objectives and preliminary results of the East Pacific Investigation of Climate Processes in the Coupled Ocean–Atmosphere System 2001 (EPIC2001), a process study designed to document the characteristics of physical processes important to climate models in tropical east Pacific. In the development of the EPIC2001 field program, the biggest uncertainties in modeling the east Pacific climate system were thought to be due to inadequate representation of certain key physical processes in models, particularly deep cumulus convection, ocean mixing, and stratus region energy balances. The primary purpose of EPIC2001 is to better understand these processes, with the goal of improving climate model performance. In order to provide perspective on these issues, we first describe some of the peculiarities of the regional climate and results of attempts to model it. Figure 1 shows the central characteristic of the seasonal cycle in the eastern tropical Pacific. In spite of the seasonal passage of the sun north and south across EPIC2001 AND THE COUPLED OCEAN–ATMOSPHERE SYSTEM OF THE TROPICAL EAST PACIFIC