The forced and intrinsic low-frequency modes in the North Pacific

Conditional maximum covariance analysis is applied to investigate the coherent patterns between the tropical and North Pacific SST and the North Pacific 500-hPa geopotential height anomalies. Two leading modes are identified. One is an intrinsic midlatitude mode, the North Pacific (NP) mode, for which SST anomalies are mainly confined to the extratropical North Pacific. The other is a tropical ocean-atmosphere coupled mode, the ENSO mode, in which an ENSO-like SST pattern dominates the Tropics but extratropical SST anomalies are relatively weak. The NP and ENSO modes exhibit distinct spatial and temporal characteristics. For the NP mode, atmospheric variation leads to changes in SST, while for the ENSO mode the opposite is true. The NP mode displays a persistence barrier during August-September whereas the ENSO mode has a March-April persistence barrier. The upper-tropospheric jet stream associated with the NP and ENSO mode intensifies, respectively, over the central North Pacific and the subtropical northeastern Pacific; consequently, the transient activities maximize in their corresponding jet exit regions. The expansion coefficients of the 500-hPa geopotential height associated with the two modes appear to be significantly correlated. However, by reducing the high-frequency part (e.g., shorter than the interannual time scale) in expansion coefficients, the correlation becomes insignificant, indicating that the significant correlation results from high-frequency signals that are unrelated to the corresponding SST variation. The results presented here suggest that the intrinsic coupled mode in the midlatitude North Pacific may be distinguished from the forced mode by remote ENSO, especially on the interannual time scale.