Observations of Atmospheric Boundary Layer
Flow Across the North Side of the Cold Tongue in the
Eastern Equatorial Pacific
C. A. Paulson, H.
Wijesekera and W. S. Pegau,
D. Rudnick,
Scripps Institution of Oceanography
R. Weller, Woods Hole Oceanographic Institution
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 increase in wind speed north of the front
suggests that enhanced mixing associated with unstable stratification,
transports northward momentum from aloft toward the surface.