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Yuan Zhang, John M. Wallace, and Naoto Iwasaka


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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Youichi Tanimoto, Kimio Hanawa, Yoshiaki Toba, and Naoto Iwasaka


Temporal evolution and spectral structure of sea surface temperature (SST) anomalies in the North Pacific over the last 37 years are investigated on the three characteristic time scales: shorter than 24 months (HF), 24–60 months (ES), and longer than 60 months (DC). The leading empirical-orthogonal function (EOF) for the DC time scale is characterized by a zonally elongated monopole centered at around 40°N, 180°. The leading EOF for the HF time scale is somewhat similar to that for the DC time scale, although there are two centers of action with the same polarity at the mid and western Pacific. The leading EOF for the ES time scale, however, exhibits a different pattern whose center of action at the mid Pacific is located farther southeastward.

In the time evolution of the SST anomalies associated with the leading EOF of the DC time scale, several anomaly periods can be identified that last five years or longer. The transition from a persistent period to another with the opposite polarity is generally very brief, except for the one that lasts throughout the late 1960s.

The EOF analysis was repeated separately on these persistent anomaly periods and the long transition period. The spatial structure of the leading EOF of the SST variability with the ES time scale is found to be sensitive to the polarity of the decadal anomaly. These results are suggestive of the possible influence of the decadal SST variability upon the spatial structure of the variability with shorter time scales.

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Fumiaki Kobashi, Shang-Ping Xie, Naoto Iwasaka, and Takashi T. Sakamoto


The North Pacific subtropical front (STF) is a zone of high sea surface temperature (SST) gradients located around 25°N in the western basin and is most pronounced in spring. The STF’s atmospheric effects are investigated using satellite observations and an atmospheric reanalysis. During April–May along the STF, surface wind stress curl turns weakly cyclonic in the general background of anticyclonic curls. Atmospheric column-integrated water vapor displays a pronounced meridional maximum along this surface trough, suggesting a deep vertical structure. Cyclonic wind curls occur intermittently at intervals of a few days along the STF in subsynoptic low pressure systems accompanying larger, synoptic highs in the main storm track to the north. In the subsynoptic surface lows, convective rain takes place with deep upward motion moistening the entire troposphere. The lows are enhanced by condensational heating, leading to the formation of weak cyclonic wind curls. The lows display vertical structure characteristic of baroclinic instability, suggesting that they are triggered by the passage of synoptic migratory highs and grow on the baroclinicity anchored by the SST front. The cyclonic wind curls appear to be related to a cloud/rainband associated with the so-called pre-baiu/meiyu front in May.

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