Near-Surface Shear Flow in the Tropical Pacific Cold Tongue Front
M. F. Cronin and W. S. Kessler
Journal of Physical Oceanography, 39, 1200-1215.
Abstract
Near-surface shear in the Pacific cold tongue front at 2°N, 140°W was measured using a set of five moored
current meters between 5 and 25m for nine months during 2004Ð05. Mean near-surface currents were
strongly westward and only weakly northward (3cm/s). Mean near-surface shear was primarily westward
and, thus, oriented to the left of the southeasterly trades. When the southwestward geostrophic shear was
subtracted from the observed shear, the residual ageostrophic currents relative to 25m were northward and
had an Ekman-like spiral, in qualitative agreement with an Ekman model modified for regions with a
vertically uniform front. According to this "frontal Ekman" model, the ageostrophic Ekman spiral is forced
by the portion of the wind stress that is not balanced by the surface geostrophic shear. Analysis of a composite
tropical instability wave (TIW) confirms that ageostrophic shear is minimized when winds blow along the
front, and strengthens when winds blow oblique to the front. Furthermore, the magnitude of the near-surface
shear, both in the TIW and diurnal composites, was sensitive to near-surface stratification and mixing. A
diurnal jet was observed that was on average 12cm/s stronger at 5m than at 25m, even though daytime
stratification was weak. The resulting Richardson number indicates that turbulent viscosity is larger at night
than daytime and decreases with depth. A "generalized Ekman" model is also developed that assumes that
viscosity becomes zero below a defined frictional layer. The generalized model reproduces many of the
features of the observed mean shear and is valid both in frontal regions and at the equator.
Download the paper as it appeared in JPO (May 2009):
(1.3Mb, 16-page pdf file)
Dr. William S. Kessler
NOAA / PMEL / OCRD
7600 Sand Point Way NE
Seattle WA 98115 USA
