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Is Climate Variability over the North Pacific a Linear Response to ENSO?

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  • 1 Department of Atmospheric Science, University of Washington, Seattle, Washington
  • | 2 Tokyo University of mercantile Marine, Koto-ku, Tokyo, Japan
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Abstract

Indices of the dominant spatial patterns of wintertime Northern Hemisphere 500-mb height and North Pacific sea surface temperature are strongly correlated with one another on interannual and interdecadal timescales, and both am correlated with indices of the El Niñ-Southern Oscillation. One possible interpretation of these relationships is that the tropical SST anomalies associated with ENSO force the atmospheric circulation over the North Pacific, and these atmospheric anomalies, in turn, give rise to the observed SST anomalies over the extratropical North Pacific.

In this study, linear relationships between ENSO and extratropical variables are examined in two different ways. First, the component of the observed extratropical variability that is linearly dependent upon ENSO is removed. The dominant spatial patterns in the residual variability of 500-mb height and SST anomalies over the North Pacific are shown to be similar to their counterparts in the total fields and remain strongly coupled on both interannual and interdecadal timescales. Second, the 44 winters used in the analysis are divided into strong ENSO and weak ENSO groups in accordance with the absolute magnitude of ENSO SST anomalies during that winter. Consistent with the analysis of the residual fields, the dominant patterns in extratropical 500-mb height and SST over the North Pacific are strongly coupled, even during winters in which tropical Pacific SST anomalies are weak. An alternative analysis, in which a 15-year record of MSU tropical precipitation data is used as a basis for defining the ENSO signal, yields similar results. The linear relation between SST in the western tropical Pacific and extratropical circulation anomalies is also examined and found to be weak.

Abstract

Indices of the dominant spatial patterns of wintertime Northern Hemisphere 500-mb height and North Pacific sea surface temperature are strongly correlated with one another on interannual and interdecadal timescales, and both am correlated with indices of the El Niñ-Southern Oscillation. One possible interpretation of these relationships is that the tropical SST anomalies associated with ENSO force the atmospheric circulation over the North Pacific, and these atmospheric anomalies, in turn, give rise to the observed SST anomalies over the extratropical North Pacific.

In this study, linear relationships between ENSO and extratropical variables are examined in two different ways. First, the component of the observed extratropical variability that is linearly dependent upon ENSO is removed. The dominant spatial patterns in the residual variability of 500-mb height and SST anomalies over the North Pacific are shown to be similar to their counterparts in the total fields and remain strongly coupled on both interannual and interdecadal timescales. Second, the 44 winters used in the analysis are divided into strong ENSO and weak ENSO groups in accordance with the absolute magnitude of ENSO SST anomalies during that winter. Consistent with the analysis of the residual fields, the dominant patterns in extratropical 500-mb height and SST over the North Pacific are strongly coupled, even during winters in which tropical Pacific SST anomalies are weak. An alternative analysis, in which a 15-year record of MSU tropical precipitation data is used as a basis for defining the ENSO signal, yields similar results. The linear relation between SST in the western tropical Pacific and extratropical circulation anomalies is also examined and found to be weak.

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