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Can Reflected Extra-equatorial Rossby Waves Drive ENSO?

William S. KesslerWoods Hole Oceanographic Institution, Woods Hole, Massachusetts

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Abstract

The possibility that the evolution of the ENSO phenomenon is determined by the reflection of extra-equatorial Rossby waves from the western boundary into the equatorial waveguide has been a subject of recent debate. Observations and some wind-driven models suggest an apparent continuity of off-equatorial signals and subsequent waveguide anomalies. On the other hand, coupled model results show that ENSO-like behavior can be simulated with no involvement of the extra-equatorial regions. Linear equatorial wave theory shows that significant reflection can only occur within about 8° of the equator, with a sharp fall-off in the reflectivity poleward of this latitude. Although the amplitude of the thermocline anomalies associated with observed ENSO-forced extra-equatorial Rossby waves can be large, it is the net zonal transport of these waves that is crucial to the reflectivity, and this net transport decreases rapidly as Rossby waves occur farther from the equator. The zonal geostrophic flows associated with observed extra-equatorial Rossby waves in the northern tropical Pacific do not provide a net transport that could make a significant contribution to equatorial Kelvin waves. If the extra-equatorial signals do exert an influence on the equatorial waveguide, it must be through a mechanism other than simple boundary reflection.

Abstract

The possibility that the evolution of the ENSO phenomenon is determined by the reflection of extra-equatorial Rossby waves from the western boundary into the equatorial waveguide has been a subject of recent debate. Observations and some wind-driven models suggest an apparent continuity of off-equatorial signals and subsequent waveguide anomalies. On the other hand, coupled model results show that ENSO-like behavior can be simulated with no involvement of the extra-equatorial regions. Linear equatorial wave theory shows that significant reflection can only occur within about 8° of the equator, with a sharp fall-off in the reflectivity poleward of this latitude. Although the amplitude of the thermocline anomalies associated with observed ENSO-forced extra-equatorial Rossby waves can be large, it is the net zonal transport of these waves that is crucial to the reflectivity, and this net transport decreases rapidly as Rossby waves occur farther from the equator. The zonal geostrophic flows associated with observed extra-equatorial Rossby waves in the northern tropical Pacific do not provide a net transport that could make a significant contribution to equatorial Kelvin waves. If the extra-equatorial signals do exert an influence on the equatorial waveguide, it must be through a mechanism other than simple boundary reflection.

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