Formation of the Cold Tongue and ENSO in the Equatorial Pacific Basin

Ming Cai Department of Meteorology, University of Maryland at College Park, College Park, Maryland

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Abstract

This paper proposes a mechanism that explains how coupled dynamics alone can spontaneously give rise to a realistic west–east asymmetric mean state and an ENSO-like interannual variability without requiring the existence of an external preexisting west–east asymmetry in circulation. The essence of the newly proposed mechanism is that the basinwide ocean–atmosphere coupling acts to reduce the effective restoring force. As a result, the coupled oceanic waves travel more and more slowly within the equatorial ocean basin as the coupling strength increases. When the coupling strength reaches a critical value, the zonally leveled thermocline becomes unstable as a result of the weakening of the effective restoring force, at which the theoretical limit of the traveling timescale would be infinite without nonlinearity. Due to nonlinearity in the coupled system, this primary air–sea interaction instability leads to a west–east asymmetric mean state in which the atmosphere has a prevailing easterly and the ocean basin has a deep-in-west–shallow-in-east thermocline with a warm-west–cold-east sea surface temperature. The direction of the west–east asymmetry in the mean state is dictated by a planetary factor of the earth, namely, that the Coriolis parameter changes sign at the equator. As the coupling strength further increases, the asymmetry in the mean state amplifies and the phase speeds of the coupled equatorial oceanic waves begin to decrease gradually toward an asymptotic limit equal to the full speed in the uncoupled situation.

Using the coupling coefficient that is consistent with the observation, the fully coupled model can produce a realistic mean state in which the basinwide SST (thermocline depth) difference is 4.2°C (116 m) and the westward wind stress at the central Pacific basin is 0.54 dyn cm–2. The self-sustained oscillation has a primary period of 3.7 yr. The SST in the west (east) oscillates between 27.5° and 28.5°C (between 25.2° and 22.5°C).

Corresponding author address: Dr. Ming Cai, Department of Meteorology, University of Maryland at College Park, College Park, MD 20742. Email: cai@atmos.umd.edu

Abstract

This paper proposes a mechanism that explains how coupled dynamics alone can spontaneously give rise to a realistic west–east asymmetric mean state and an ENSO-like interannual variability without requiring the existence of an external preexisting west–east asymmetry in circulation. The essence of the newly proposed mechanism is that the basinwide ocean–atmosphere coupling acts to reduce the effective restoring force. As a result, the coupled oceanic waves travel more and more slowly within the equatorial ocean basin as the coupling strength increases. When the coupling strength reaches a critical value, the zonally leveled thermocline becomes unstable as a result of the weakening of the effective restoring force, at which the theoretical limit of the traveling timescale would be infinite without nonlinearity. Due to nonlinearity in the coupled system, this primary air–sea interaction instability leads to a west–east asymmetric mean state in which the atmosphere has a prevailing easterly and the ocean basin has a deep-in-west–shallow-in-east thermocline with a warm-west–cold-east sea surface temperature. The direction of the west–east asymmetry in the mean state is dictated by a planetary factor of the earth, namely, that the Coriolis parameter changes sign at the equator. As the coupling strength further increases, the asymmetry in the mean state amplifies and the phase speeds of the coupled equatorial oceanic waves begin to decrease gradually toward an asymptotic limit equal to the full speed in the uncoupled situation.

Using the coupling coefficient that is consistent with the observation, the fully coupled model can produce a realistic mean state in which the basinwide SST (thermocline depth) difference is 4.2°C (116 m) and the westward wind stress at the central Pacific basin is 0.54 dyn cm–2. The self-sustained oscillation has a primary period of 3.7 yr. The SST in the west (east) oscillates between 27.5° and 28.5°C (between 25.2° and 22.5°C).

Corresponding author address: Dr. Ming Cai, Department of Meteorology, University of Maryland at College Park, College Park, MD 20742. Email: cai@atmos.umd.edu

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