Coupled climate models exhibit considerable sensitivity to changes in the parameterization of the stratus clouds and show change in basin-scale model winds and SST when parameterizations are altered . However, there have been few measurements of tropical and Peruvian stratus decks. The weak observational foundation is currently limiting our ability to better understand and model this region. Thus, an observational focus on eastern Pacific stratus has been recommended for the EPIC (Eastern Pacific Investigation of Climate) program and plans for two phases of investigation have been outlined. Phase 1 will provide an assessment of the present understanding of the stratus decks, including select observations to improve understanding of the air-sea coupling and investigations of the performance of existing models in replicating these observations. Phase 2 would be an intensive observational phase to follow later, pending the results of Phase 1.

PROJECT GOALS:
Our work is a central element of Phase 1 and revolves around the deployment of a well-instrumented air-sea interaction surface mooring under the stratus clouds off northern Chile. The immediate goals of the deployment are to obtain time series of:

• accurate air-sea fluxes and surface meteorology
• the temporal evolution of the vertical structure of the upper ocean temperature, salinity, and horizontal velocity fields

The goals of the analyses of this data will be to:

• examine the temporal evolution of the upper ocean heat content,
• quantify the roles, at time scales from minutes to seasonal, of atmospheric forcing, and local, 1-D process in that evolution,
• investigate how the atmosphere drives the ocean under the stratus deck,
• examine the relative importance of shortwave, longwave, and latent heat flux variability related to the cloud cover, and
• explore the possible feedback mechanisms that would link the evolution of the atmospheric and oceanic boundary layers.

In addition, after calibration, the data will be made available and used in ground-truthing remote sensing, as benchmark time series for atmospheric, oceanic, and coupled models, and to develop improved air-sea flux fields in that region.

METHODOLOGY:
A well-instrumented air-sea interaction buoy will be placed under the persistent stratus cloud deck found west of Peru and Chile for three years. The site, 85°W, 18 to 20°S, is chosen because it is representative of the stratus region and was occupied by a NOAA NDBC buoy from 1985-1995, which will provide some historical context. The mooring will provide, with good vertical and temporal resolution, upper ocean velocity, temperature, and salinity profiles. It will also collect a complete set of surface meteorological time series, allowing the air-sea fluxes of momentum, heat, and freshwater.

The site will be occupied for 3 years by consecutive 12-month deployments. The surface buoy will have 2 IMET systems measuring air and sea surface temperatures, humidity, barometric pressure, wind speed and direction, incoming shortwave and longwave radiation, and precipitation. The ocean measurements will include a floating SST sensor (5 cm depth), 16 temperature and 10 conductivity/temperature loggers in the upper 200 m, 2 Vector Measuring Current Meters, and 1 300 kHz ADCP to measure velocity in the upper 150 m. Subsurface acoustic rain gauges on the mooring will be deployed by Jeff Nyusten (UW APL). The surface meteorological data will be telemetered and provided to modeling and data assimilation efforts as well as to the other investigations working in the stratus region.

RESULTS AND ACCOMPLISHMENTS:
The mooring will be deployed for the first time in October 2000. After that time, this report will be periodically updated.

FUTURE WORK:
The field work and analysis activities for the next three years have been described above. The intent is to use this data to gauge our level of understanding of how the stratus clouds modulate the surface fluxes and air-sea coupling. It is anticipated that we will find that atmospheric, oceanic, and coupled models do not presently perform well in this region and fail to accurately represent the vertical structure and temporal evolution of the upper ocean and lower atmosphere in this location. This information will be used to motivate a more comprehensive field program to better understand the coupled system in the stratus cloud regime during Phase 2, starting in about 2003.

PUBLICATIONS RESULTING FROM THIS RESEARCH:
To be updated as the project proceeds.

Principal Investigators:
Robert A. Weller
rweller@whoi.edu
Phone: (508) 289-2508
Fax: (508) 457-2163

Steven P. Anderson
sanderson@whoi.edu
Phone: (508) 289-2876
Fax: (508) 457-2163

Institution:
Woods Hole Oceanographic Institution
Woods Hole, Massachusetts 02543

LINKS:

http://www.whoi.edu

http://uop.whoi.edu

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