Abstract
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)
Highlights
IntroductionThese experiments have proven invaluable in identifying the main processes that develop the Tropical
After the pioneering work of Eady (1949) on baroclinic instability at mid-latitudes under the quasi-geostrophic approximation, Stone (1966, 1970) used Eady’s model to study the baroclinic instability in the ageostrophic regime
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
Summary
These experiments have proven invaluable in identifying the main processes that develop the Tropical. Instability Waves (TIWs) in the Pacific Ocean at a 20–30 day period and about a 1000 km wavelength (Masina et al., 1999). -layer model, a zonal current profile similar to the equatorial system in the Pacific and in the Atlantic is barotropically unstable to westward waves, with a period of about 1 month and a zonal wavelength of about 1000 km (Philander, 1978). With this mean flow configuration we performed two sets of experiments; the first set of experiments consists of five sensitivity experiments performed in order to test the dependence of the wave on the frontal layer depth and on stratification 3.1), and the second set consists of three experiments that have been integrated in time in order to examine how the speed of the unstable mean state jet affects the phase speed of the wave
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