Observations of Atmospheric Boundary Layer Flow Across the North Side of the Cold Tongue in the Eastern Tropical Pacific

As part of the EPIC field experiment, atmospheric and oceanic observations were made in the eastern tropical Pacific from the R/V New Horizon. The atmospheric measurements included wind speed and direction, temperature, humidity, incoming short and long wave radiation, and turbulent fluxes of heat and momentum. The oceanic observations included temperature and salinity measured from the surface down to a depth of 250 m and horizontal velocity with a ship-mounted ADCP. As part of the experiment, underway observations were made at a speed of 4 m/s along 95W from 10N to 1S and back to 10N. The east-west surface temperature front at the north side of the equatorial cold tongue was located just south (0.35 deg S) of the equator and had a magnitude of 1.7 C over a distance of a 700 meters when we crossed from south to north. Surface salinity changed 0.2 psu across the front which contributed to the density change. The temperature front had a dramatic effect on the structure of the northward atmospheric boundary layer flow across the front. The wind speed at 10-m height south of the front was 4 m/s with a low level of turbulence, consistent with stable or near-neutral stratification. North of the front, wind speed gradually increased over a distance of 40 km to 9 m/s with a high level of turbulence, consistent with unstable stratification. The higher wind speed north of the front suggests that enhanced mixing, associated with unstable stratification, transports northward momentum from aloft toward the surface and that stable stratification inhibits momentum transfer south of the front. Fluxes of sensible and latent heat reached 40 and 170 W/m^2 respectively north of the front but were near zero south of the front. Sonic anemometer measurements of velocity and temperature were analyzed to illustrate changes in spectra across the front.