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Evolution of Long Planetary Wave Packets in a Continuously Stratified Ocean

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  • 1 Department of Marine Science, University of South Florida, St. Petersburg, Florida
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Abstract

Recent TOPEX/Poseidon observations show an enhanced (weakened) westward propagation of long planetary Rossby waves at extratropics (Tropics) and they usually propagate faster in the western basin than in the eastern basin in all major oceans. The evolution of a long planetary wave packet in a continuously stratified ocean in response to the various forcing functions is analytically investigated using the wave packet theory. For a wave packet with a large vertical scale, the stratification variation and the vertical shear of the mean zonal current act in concert, causing the wave packet to propagate directly against the mean zonal current—called the counter-Doppler-shift (CDS) effect. It is found that the speed ratio between the zonal baroclinic zonal current and the classic theory increases with the latitude, the eastward zonal current, and the local vertical scale.

The vertical scale of a wave packet plays a critical role in the propagation, the structure, and spatial-scale development of a wave packet. It is found that the β and stratification effects increase (decrease) the vertical spatial scale of a vertically westward (eastward) tilted wave packet. For a wave packet with a large (small) vertical scale, the vertical spatial scale increases (decreases) when the wave packet is tilted westward in an eastward zonal current. The structural change could effectively separate the extratropic oceanic responses into two kinds of systems with two different vertical scales and strengthen the CDS effect, enhancing speeds in western ocean basins. Several analytical solutions for the wave packet are also obtained.

The author concludes that the evolution of a wave packet with a large vertical scale in a zonal current may account for all major features of the sea surface height anomalies observed in the TOPEX/Poseidon data. The possible forcing functions are the atmospheric wind forcing at the sea surface and the ocean topographic forcing on the seafloor, but not the surface cooling or heating.

Corresponding author address: Dr. Huijun Yang, Department of Marine Science, University of South Florida, 140 Seventh Avenue South, St. Petersburg, FL 33701-5016.

Email: yang@marine.usf.edu

Abstract

Recent TOPEX/Poseidon observations show an enhanced (weakened) westward propagation of long planetary Rossby waves at extratropics (Tropics) and they usually propagate faster in the western basin than in the eastern basin in all major oceans. The evolution of a long planetary wave packet in a continuously stratified ocean in response to the various forcing functions is analytically investigated using the wave packet theory. For a wave packet with a large vertical scale, the stratification variation and the vertical shear of the mean zonal current act in concert, causing the wave packet to propagate directly against the mean zonal current—called the counter-Doppler-shift (CDS) effect. It is found that the speed ratio between the zonal baroclinic zonal current and the classic theory increases with the latitude, the eastward zonal current, and the local vertical scale.

The vertical scale of a wave packet plays a critical role in the propagation, the structure, and spatial-scale development of a wave packet. It is found that the β and stratification effects increase (decrease) the vertical spatial scale of a vertically westward (eastward) tilted wave packet. For a wave packet with a large (small) vertical scale, the vertical spatial scale increases (decreases) when the wave packet is tilted westward in an eastward zonal current. The structural change could effectively separate the extratropic oceanic responses into two kinds of systems with two different vertical scales and strengthen the CDS effect, enhancing speeds in western ocean basins. Several analytical solutions for the wave packet are also obtained.

The author concludes that the evolution of a wave packet with a large vertical scale in a zonal current may account for all major features of the sea surface height anomalies observed in the TOPEX/Poseidon data. The possible forcing functions are the atmospheric wind forcing at the sea surface and the ocean topographic forcing on the seafloor, but not the surface cooling or heating.

Corresponding author address: Dr. Huijun Yang, Department of Marine Science, University of South Florida, 140 Seventh Avenue South, St. Petersburg, FL 33701-5016.

Email: yang@marine.usf.edu

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