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Using an Ocean Model to Examine ENSO Dynamics

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  • 1 Department of Oceanography, The Florida State University, Tallahassee, Florida
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Abstract

An equatorial ocean model with the Kelvin and first six Rossby waves and n vertical modes is developed to simulate the equatorial sea surface temperature anomalies (SSTAs) and, more importantly, zonal movement of the western equatorial Pacific warm pool associated with an ENSO episode. When forced by observed wind stress, the model reproduces well the SSTAs in both the central and eastern equatorial Pacific. The two gravest baroclinic modes are needed to model the equatorial thermocline depth, zonal current, and SSTAs. The SSTA near the equatorial South American coast is mostly due to upwelling and that in the eastern Pacific from about 150°–100°W is mainly due to both zonal advection and upwelling. The SSTA in the equatorial Pacific near the edge of the warm pool (160°E–160°W) mostly results from the advection of mean SST by the anomalous zonal current and Newtonian damping.

Model calculations show that the growing ENSO disturbance, which propagates eastward from the western equatorial Pacific during an El Niño year, is largely halted by zonal currents advecting the western equatorial Pacific warm pool and the associated atmospheric convection back to their mean position. These zonal currents are mostly associated with equatorial Kelvin and Rossby ocean waves reflected from the eastern and western Pacific boundaries. The model solutions suggest that eastern and western boundary reflections contribute comparably to these negative feedback zonal currents at the crucial region near the edge of the warm pool. The solutions also show that the equatorial central Pacific zonal current due to the wind forcing in the far western Pacific (west of 160°E) only plays a minor role in the negative feedback during El Niño and is negligible during La Niña.

Current affiliation: Department of Oceanography, Naval Postgraduate School, Monterey, California

Corresponding author address: Dr. Allan J. Clarke, Dept. of Oceanography 4320, The Florida State University, Oceanography/Statistics Bldg., West Call St., Tallahassee, FL 32306-4320. Email: clarke@ocean.fsu.edu

Abstract

An equatorial ocean model with the Kelvin and first six Rossby waves and n vertical modes is developed to simulate the equatorial sea surface temperature anomalies (SSTAs) and, more importantly, zonal movement of the western equatorial Pacific warm pool associated with an ENSO episode. When forced by observed wind stress, the model reproduces well the SSTAs in both the central and eastern equatorial Pacific. The two gravest baroclinic modes are needed to model the equatorial thermocline depth, zonal current, and SSTAs. The SSTA near the equatorial South American coast is mostly due to upwelling and that in the eastern Pacific from about 150°–100°W is mainly due to both zonal advection and upwelling. The SSTA in the equatorial Pacific near the edge of the warm pool (160°E–160°W) mostly results from the advection of mean SST by the anomalous zonal current and Newtonian damping.

Model calculations show that the growing ENSO disturbance, which propagates eastward from the western equatorial Pacific during an El Niño year, is largely halted by zonal currents advecting the western equatorial Pacific warm pool and the associated atmospheric convection back to their mean position. These zonal currents are mostly associated with equatorial Kelvin and Rossby ocean waves reflected from the eastern and western Pacific boundaries. The model solutions suggest that eastern and western boundary reflections contribute comparably to these negative feedback zonal currents at the crucial region near the edge of the warm pool. The solutions also show that the equatorial central Pacific zonal current due to the wind forcing in the far western Pacific (west of 160°E) only plays a minor role in the negative feedback during El Niño and is negligible during La Niña.

Current affiliation: Department of Oceanography, Naval Postgraduate School, Monterey, California

Corresponding author address: Dr. Allan J. Clarke, Dept. of Oceanography 4320, The Florida State University, Oceanography/Statistics Bldg., West Call St., Tallahassee, FL 32306-4320. Email: clarke@ocean.fsu.edu

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