Observations of long Rossby waves in the northern tropical
Pacific
Kessler, W.S.
Journal of Geophysical Research, 95(C4), 5183-5217
(1990)
Long baroclinic Rossby waves are potentially important in the
adjustment of the tropical Pacific pycnocline to both annual and
interannual wind stress curl fluctuations. Evidence for such waves
is found in variations of the depth of the 20°C isotherm in
the northern tropical Pacific during 1970-1987. A total of 199,067
bathythermograph profiles have been compiled from the archives of
several countries; the data coverage is dense enough that westward
propagating events may be observed with a minimum of zonal
interpolation. After extensive quality control, 20°C depths
were gridded with a resolution of 2° latitude, 5°
longitude, and bimonths; statistical parameters of the data were
estimated. A simple model of low-frequency quasi-geostrophic
pycnocline variability allows the physical processes of Ekman
pumping, the radiation of long (nondispersive) Rossby waves due to
such pumping in midbasin, and the radiation of long Rossby waves
from the observed eastern boundary pycnoline depth fluctuations.
Although the wind stress curl has very little zonal variability at
the annual period in the northern tropical Pacific, an annual
fluctuation of 20°C depth propagates westward as a long Rossby
wave near 5°N and 14°-18°N in agreement with the
model hindcast. Near the thermocline ridge at 10°N, however,
the annual cycle across the basin is dominated by local Ekman
pumping. The wave-dominated variability at 5°N weakens the
annual cycle of geostrophic transport of the North Equatorial
Countercurrent in the western Pacific. El Niño events are
associated with westerly wind anomalies concentrated in the central
equatorial Pacific; upwelling wind stress curl is generated in the
extraequatorial tropics by these westerlies. Long upwelling Rossby
waves forced by this curl pattern were observed to raise the
western Pacific thermocline well outside the equatorial waveguide
in the later stages of El Niños, consistent with the simple
long-wave model. The observations suggest that although simple
reflection of the long Rossby waves from the western boundary is
not the major process affecting subsequent development on the
equator, it is likely that the extraequatorial waves play some role
in setting the timescale of ENSO events.
Dr. William S. Kessler
NOAA / PMEL / OCRD
7600 Sand Point Way NE
Seattle WA 98115 USA
