Introduction

EPIC stands for ``East Pacific Investigation of Climate Processes in the Coupled Ocean-Atmosphere System''. The scientific objectives are (Weller et al., 1999):

1.

To observe and understand the ocean-atmosphere processes responsible for the structure and evolution of the large-scale atmospheric heating gradients in the equatorial and northeastern Pacific portions of the cold-tongue/ITCZ complex, including:

(a)

mechanisms governing temperature and salinity field evolution in the oceanic cold tongue and in the region of strong meridional gradient in sea surface temperature from the oceanic cold tongue through the ITCZ;

(b)

atmospheric boundary layer structure and evolution from the equator through the ITCZ, primarily in the southerly monsoonal regime; and

(c)

the processes determining the existence, character, and strength of deep convection in the northeast Pacific ITCZ.

2.

To observe and understand the dynamical, radiative, and microphysical properties of the extensive boundary layer cloud decks in the southeasterly tradewind and cross-equatorial flow regime and their interactions with the ocean below.

Achieving the EPIC objectives is expected to improve the performance of coupled atmosphere-ocean models, which currently have difficulties in simulating the equatorially asymmetric response of the climate system to the seasonal march of insolation (Mechoso et al., 1995). The precise reasons for the present difficulties in the simulations are model-dependent and a subject of intensive modeling research. There is a consensus, however, that key atmospheric and oceanic processes, as well as on the way in which those processes interact, are incompletely understood. Given the importance of the east Pacific to climate phenomena such as El Niño and to precipitation in the Americas, understanding and solving these problems merits high priority.

The full 5 year EPIC program is described in the EPIC science and implementation plan (Weller et al., 1999). This program includes intensive observations of the eastern Pacific over short periods as well as less intense monitoring over longer periods. Investigators associated with CLIVAR's international program on Variability of American Monsoon Systems (VAMOS) will participate in EPIC along the entire west coast of Latin America, including the Peruvian and Chilean stratus regimes, with additional focus on coastal ocean-atmosphere-land interactions. VAMOS EPIC (VEPIC) will thus facilitate the collaboration of U. S. EPIC 2001 investigators with on-going and/or planned international efforts in similar areas of interest. EPIC2001 is an intensive process study proposed under the auspices of EPIC for August and September of 2001. The cross-equatorial SST difference is strongest during this period, and stratocumulus coverage along the coasts of North and South America is largest. To keep budgets manageable, CLIVAR decided to first study the east Pacific warm pool and ITCZ along a single longitude rather than launching immediately into a full three-dimensional study of the entire region. EPIC2001 was generated to 1) make atmospheric observations along $95^{\circ} \mbox{ W}$, which is the longitude of the easternmost TAO moorings, and 2) study small to mesoscale oceanic and atmospheric processes at one or two sites. Owing to the current lack of information about the ocean and atmosphere in the east Pacific, this is virtually guaranteed to increase our understanding of processes in the region.

Atmospheric and oceanic observations along $95^{\circ} \mbox{ W}$ will address

1.

the processes that determine the nature of deep convection in and near the ITCZ, including its variability, strength, and location;

2.

the evolution of the vertical structure of the atmospheric boundary layer as the surface winds flow northward over the cold tongue and strong SST gradient of the equatorial front;

3.

how air-sea coupling affects ocean mixed layer dynamics and SST in the east Pacific warm pool; and

4.

the processes in the upper ocean that affect the structure and evolution of the shallow thermocline in this region.

In addition, exploratory observations in the southeastern tropical Pacific during this period will focus on obtaining the dynamical, radiative and microphysical data needed to evaluate models and parameterizations of boundary layer stratiform cloud formation and evolution and to determine feedbacks between the ocean and the stratus clouds.