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Nathan J. Mantua and David S. Battisti

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.

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Nathan J. Mantua and David S. Battisti

Abstract

Observed surface winds from 1961 through September 1992 are used to force a reduced gravity shallow water ocean model. Results from the hindcast of ocean variability are found to be consistent with the results Li and Clarke presented in a previous study of sea level and zonal wind variability in the tropical Pacific. In this note the apparent discrepancies are reconciled between “delayed oscillator” theory and calculated lag correlations between the observationally based records of western boundary Kelvin mode amplitude (ηW) and zonal wind forcing. Evidence for sensitivity in these “delayed oscillator” lag correlations is presented from a variety of sources, including: the hindcast data, output from the standard version of the Zebiak and Cane coupled ocean-atmosphere model, and through three cases with idealized time series of ENSO variability. The authors demonstrate that the low lag correlations for ηW leading the zonal wind forcing by 1 to 1½ years is not inconsistent with the hypothesized role of western boundary reflections as the ultimate termination mechanism for ENSO anomalies. The lack of regularity in the system studied guarantees a degraded correlation for ηW leading the zonal wind by 1 to 1½ years. This lack of regularity is not contained in, nor explained by, delayed oscillator theory.

A robust feature in all of the records examined in this work is the existence of upwelling Kelvin signals in the far western equatorial Pacific Ocean due to the developing warm (ENSO) events. The amplitude of the observed Kelvin signals in the western Pacific is sufficient to terminate the developing ENSO events via the delayed oscillator mechanism.

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Nathan J. Mantua, Steven R. Hare, Yuan Zhang, John M. Wallace, and Robert C. Francis

Evidence gleaned from the instrumental record of climate data identifies a robust, recurring pattern of ocean–atmosphere climate variability centered over the midlatitude North Pacific basin. Over the past century, the amplitude of this climate pattern has varied irregularly at interannual-to-interdecadal timescales. There is evidence of reversals in the prevailing polarity of the oscillation occurring around 1925, 1947, and 1977; the last two reversals correspond to dramatic shifts in salmon production regimes in the North Pacific Ocean. This climate pattern also affects coastal sea and continental surface air temperatures, as well as streamflow in major west coast river systems, from Alaska to California.

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Dillon J. Amaya, Michael A. Alexander, Antonietta Capotondi, Clara Deser, Kristopher B. Karnauskas, Arthur J. Miller, and Nathan J. Mantua
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Michael G. Jacox, Michael A. Alexander, Nathan J. Mantua, James D. Scott, Gaelle Hervieux, Robert S. Webb, and Francisco E. Werner
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Matthew Newman, Michael A. Alexander, Toby R. Ault, Kim M. Cobb, Clara Deser, Emanuele Di Lorenzo, Nathan J. Mantua, Arthur J. Miller, Shoshiro Minobe, Hisashi Nakamura, Niklas Schneider, Daniel J. Vimont, Adam S. Phillips, James D. Scott, and Catherine A. Smith

Abstract

The Pacific decadal oscillation (PDO), the dominant year-round pattern of monthly North Pacific sea surface temperature (SST) variability, is an important target of ongoing research within the meteorological and climate dynamics communities and is central to the work of many geologists, ecologists, natural resource managers, and social scientists. Research over the last 15 years has led to an emerging consensus: the PDO is not a single phenomenon, but is instead the result of a combination of different physical processes, including both remote tropical forcing and local North Pacific atmosphere–ocean interactions, which operate on different time scales to drive similar PDO-like SST anomaly patterns. How these processes combine to generate the observed PDO evolution, including apparent regime shifts, is shown using simple autoregressive models of increasing spatial complexity. Simulations of recent climate in coupled GCMs are able to capture many aspects of the PDO, but do so based on a balance of processes often more independent of the tropics than is observed. Finally, it is suggested that the assessment of PDO-related regional climate impacts, reconstruction of PDO-related variability into the past with proxy records, and diagnosis of Pacific variability within coupled GCMs should all account for the effects of these different processes, which only partly represent the direct forcing of the atmosphere by North Pacific Ocean SSTs.

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