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Wave Driving in the Tropical Lower Stratosphere as Simulated by WACCM. Part II: ENSO-Induced Changes for Northern Winter

Masakazu TaguchiDepartment of Earth Sciences, Aichi University of Education, Kariya, Japan

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Abstract

This study investigates ENSO-induced changes in the tropical lower stratosphere for northern winter as simulated by the Whole Atmosphere Community Climate Model (WACCM). A comparison is made between two 3650-day perpetual January experiments forced with La Niña– and El Niño–like sea surface temperature conditions over the equatorial Pacific. The present analysis includes an extension of the diagnostic framework used for the climatological annual cycle in . A comprehensive description of the ENSO-induced changes, together with their heat and zonal momentum budget diagnoses, demonstrates that the changes are consistently characterized by cooling, locally accelerated Brewer–Dobson circulation (upwelling and poleward flow) and strengthened tropical/subtropical wave driving for the El Niño–like condition. The cooling broadly peaks near the equator with general hemispheric symmetry, and the strengthenings in the poleward flow and wave driving take place in both hemispheres. An important role in the strengthened wave driving is played by changes in tropical/subtropical stationary waves. The changes notably include a dumbbell-shaped height pattern over the Pacific or a modulation of equatorial Rossby waves in response to redistributed convective heating with the ENSO-like perturbation.

Corresponding author address: Masakazu Taguchi, Hirosawa 1, Igaya-cho, Kariya 448-8542, Japan. Email: mtaguchi@auecc.aichi-edu.ac.jp

Abstract

This study investigates ENSO-induced changes in the tropical lower stratosphere for northern winter as simulated by the Whole Atmosphere Community Climate Model (WACCM). A comparison is made between two 3650-day perpetual January experiments forced with La Niña– and El Niño–like sea surface temperature conditions over the equatorial Pacific. The present analysis includes an extension of the diagnostic framework used for the climatological annual cycle in . A comprehensive description of the ENSO-induced changes, together with their heat and zonal momentum budget diagnoses, demonstrates that the changes are consistently characterized by cooling, locally accelerated Brewer–Dobson circulation (upwelling and poleward flow) and strengthened tropical/subtropical wave driving for the El Niño–like condition. The cooling broadly peaks near the equator with general hemispheric symmetry, and the strengthenings in the poleward flow and wave driving take place in both hemispheres. An important role in the strengthened wave driving is played by changes in tropical/subtropical stationary waves. The changes notably include a dumbbell-shaped height pattern over the Pacific or a modulation of equatorial Rossby waves in response to redistributed convective heating with the ENSO-like perturbation.

Corresponding author address: Masakazu Taguchi, Hirosawa 1, Igaya-cho, Kariya 448-8542, Japan. Email: mtaguchi@auecc.aichi-edu.ac.jp

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