Aperiodic Variability in the Zebiak-Cane Coupled Ocean-Atmosphere Model: Air-Sea Interactions in the Western Equatorial Pacific

Abstract

The behavior of two coupled ocean-atmosphere models of intermediate complexity, the Zebiak-Cane (ZC hereafter) and Battisti version (B88 hereafter) of the ZC coupled ocean-atmosphere model, are reviewed and compared to the observed climate record from the tropical Pacific region. A major difference between each system lies in the modes of variability that each supports. In the observations, variability at timescales shorter than that associated with El Niño and the Southern Oscillation (ENSO) is ubiquitous and difficult to characterize in terms of clearly coupled ocean-atmosphere interactions. The B88 model supports only a single unstable mode: the ENSO mode. In the ZC model two distinct modes are present: the interannual ENSO mode, and the mobile mode: a near-annual, westward propagating instability. It is demonstrated that differences in the basic-state climatology and thermodynamic parameters are keys to understanding the differences between the ZC and B88 coupled model behaviors. It is found that interactions between the ENSO and the mobile mode is the cause for irregular variability in standard ZC model simulations. The mobile mode instability is inherently dependent on shallow-water equatorial wave dynamics: anomalies at the air-sea interface are found to phase lock with the gravest symmetric oceanic Rossby mode. The westward propagating instability generates large disturbances in the dynamic ocean fields even though the surface anomalies are relatively weak. The timescale of the recurrence of the mobile instability is that of the free-equatorial-wave basin mode, which is about 9 mouths. Interactions between the ENSO and mobile modes play an important role in the behavior of the standard ZC model. The phase (cold versus warm) of the model ENSO cycle determines the strength of the mobile mode instability, while ocean disturbances generated by the mobile mode interfere with the model ENSO regularity. It is demonstrated that the coexistence of these distinct, unstable, coupled ocean-atmosphere instabilities is the key element in producing aperiodic behavior in the standard ZC coupled model. The cause for aperiodic variability in the perpetual month simulations is linked to air-sea interactions in the far western Pacific. By explicitly suppressing the air-sea interactions in the far western Pacific, the ZC model ENSO cycle becomes much more periodic than that in the standard model. There is no convincing evidence for robust coupled ocean-atmosphere interactions in the observed western Pacific region. However, uncoupled atmospheric variability associated with intraseasonal oscillations and the south Asian monsoon are very energetic features in this region. The results imply that wind stress variability in the western equatorial Pacific may act as a stochastic forcing that interrupts what might otherwise be a pure ENSO cycle.