ENSO Simulated by Intermediate Coupled Models and Evaluated with Observations over 1970–98. Part II: Role of the Off-Equatorial Ocean and Meridional Winds

C. Cassou Ocean Science Element, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California

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C. Perigaud Ocean Science Element, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California

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Abstract

Introducing new parameterizations of subsurface temperature and atmospheric convection in the Cane and Zebiak model allows the reproducing of oscillations with a period close to 4 yr; equatorial wind anomalies that are located close to the date line; and realistic amplitudes of SST, wind, and thermocline anomalies in the equatorial Pacific both in warm and cold phases. When the weight given to the atmospheric convection term is increased, the simulated wind maximum along the equator is displaced farther west, and the amplitude and duration of the warm events decrease. Compared to the simulations with the standard parameterization, the ENSO-like oscillations are not lost when the friction is increased with a decay time ranging from 30 to 12 months. Off-equatorial baroclinic ocean and wind anomalies are much less strong, but still necessary for the system to oscillate. Unrealistic easterlies are still present in the eastern Pacific.

Replacing the atmospheric model by a statistical relationship between SST and wind stress anomalies allows further reducing of these deficiencies. The simulated ENSO-like events are in good agreement with observed oceanic and atmospheric fields in terms of amplitude and spatial patterns within 15° of the equator. Sensitivity tests to the statistics prescribed in the atmospheric component show that the model simulates warm events with duration and amplitude that increase with the eastern penetration of the westerlies in the central Pacific, whereas cold events do not last longer and easterlies remain located at the date line whatever their strength. After warm events, the model simulates a gain of heat content on average in the north, and similar features are found in the 1983 and 1998 El Niño events, whereas the Cane and Zebiak model simulates a loss. The gain is due to northerlies along the ITCZ that are associated with an anticyclonic curl and a “recharge” of the oceanic heat content, whereas the loss is explained by the unrealistic easterlies in the eastern Pacific and by the weakness of the meridional winds.

Corresponding author address: Dr. Claire Perigaud, Jet Propulsion Laboratory, MS 300/323, 4800 Oak Grove Drive, Pasadena, CA 91109-8099.

Email: cp@pacific.jpl.nasa.gov

Abstract

Introducing new parameterizations of subsurface temperature and atmospheric convection in the Cane and Zebiak model allows the reproducing of oscillations with a period close to 4 yr; equatorial wind anomalies that are located close to the date line; and realistic amplitudes of SST, wind, and thermocline anomalies in the equatorial Pacific both in warm and cold phases. When the weight given to the atmospheric convection term is increased, the simulated wind maximum along the equator is displaced farther west, and the amplitude and duration of the warm events decrease. Compared to the simulations with the standard parameterization, the ENSO-like oscillations are not lost when the friction is increased with a decay time ranging from 30 to 12 months. Off-equatorial baroclinic ocean and wind anomalies are much less strong, but still necessary for the system to oscillate. Unrealistic easterlies are still present in the eastern Pacific.

Replacing the atmospheric model by a statistical relationship between SST and wind stress anomalies allows further reducing of these deficiencies. The simulated ENSO-like events are in good agreement with observed oceanic and atmospheric fields in terms of amplitude and spatial patterns within 15° of the equator. Sensitivity tests to the statistics prescribed in the atmospheric component show that the model simulates warm events with duration and amplitude that increase with the eastern penetration of the westerlies in the central Pacific, whereas cold events do not last longer and easterlies remain located at the date line whatever their strength. After warm events, the model simulates a gain of heat content on average in the north, and similar features are found in the 1983 and 1998 El Niño events, whereas the Cane and Zebiak model simulates a loss. The gain is due to northerlies along the ITCZ that are associated with an anticyclonic curl and a “recharge” of the oceanic heat content, whereas the loss is explained by the unrealistic easterlies in the eastern Pacific and by the weakness of the meridional winds.

Corresponding author address: Dr. Claire Perigaud, Jet Propulsion Laboratory, MS 300/323, 4800 Oak Grove Drive, Pasadena, CA 91109-8099.

Email: cp@pacific.jpl.nasa.gov

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