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Ann. Geophys., 20, 729-740, 2002
www.ann-geophys.net/20/729/2002/
© European Geosciences Union 2002
Instabilities of continuously stratified zonal equatorial jets in a periodic channel model
S. Masina
Istituto Nazionale di Geofisica e Vulcanologia, I-00143, Roma, Italy
Correspondence to: S. Masina (masina@ingv.it)
Abstract. Several numerical
experiments are performed in a nonlinear, multi-level periodic channel model
centered on the equator with different zonally uniform background flows which
resemble the South Equatorial Current (SEC). Analysis of the simulations
focuses on identifying stability criteria for a continuously stratified fluid
near the equator. A 90 m deep frontal layer is required to destabilize a
zonally uniform, 10° wide, westward surface jet that is symmetric about the
equator and has a maximum velocity of 100 cm/s. In this case, the phase
velocity of the excited unstable waves is very similar to the phase speed of
the Tropical Instability Waves (TIWs) observed in the eastern Pacific Ocean.
The vertical scale of the baroclinic waves corresponds to the frontal layer
depth and their phase speed increases as the vertical shear of the jet is
doubled. When the westward surface parabolic jet is made asymmetric about the
equator, in order to simulate more realistically the structure of the SEC in
the eastern Pacific, two kinds of instability are generated. The oscillations
that grow north of the equator have a baroclinic nature, while those generated
on and very close to the equator have a barotropic nature.
This study shows that the potential for baroclinic
instability in the equatorial region can be as large as at mid-latitudes, if
the tendency of isotherms to have a smaller slope for a given zonal velocity,
when the Coriolis parameter vanishes, is compensated for by the wind effect.
Key words. Oceanography: general
(equatorial oceanography; numerical modeling) – Oceanography: physics (fronts
and jets)
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