PHASE-LOCKED AND ASYMMETRIC CORRELATIONS OF THE WINTERTIME ATMOSPHERIC PATTERNS WITH THE ENSO

Teleconnections between sea surface temperature (SST) anomalies over t he Pacific and the dominant patterns of wintertime Northern Hemisphere 500-hPa height are examined by applying statistical techniques such a s rotated principal component analysis and composite analysis. It is s hown that the Pacific/North American (PNA) patterns in December throug h March are correlated most significantly with the ENSO-related SST an omalies in the previous October, while the western Pacific (WP) patter ns in December through February are most closely linked to the ENSO-re lated SST anomalies in the same season. In addition, the PNA response to the ENSO signal during La Nina events is more significant than that during El Nino events, while the WP response is stronger during El Ni no events than during La Nina events. A composite analysis shows that in the El Nino winters the North Pacific centre of the PNA pattern is located about 10 degrees east of its normal position, leading to a les s significant correlation between the ENSO signal and the PNA pattern in these winters. The ENSO-related SST anomalies include a large centr e of action over the tropical Pacific and an oppositely signed anomaly centre over the North Pacific. The North Pacific centre appears to th e west of the dateline in September and October. This ENSO-related see d of SST anomalies slowly moves eastward in the following months, grad ually cutting off its connection with SST anomalies over the tropical Pacific and being coupled with the PNA pattern. It is pointed out that , although the wintertime SST anomaly over the North Pacific may appea r as a mode linearly independent of the ENSO signal in the same season , it is partially related to the ENSO signal in the preceding autumn. Possible dynamical explanations of the above results are discussed. It is suggested that the WP pattern can be linked to the tropical Pacifi c heat source via advection of vorticity by the upper-tropospheric div ergent/convergent flow, and the intensification of vorticity gradients associated with a stronger east Asian jet is likely to be responsible for a more significant WP pattern response to the ENSO signal in the El Nino winters. On the other hand, the ENSO related PNA pattern could be considered a manifestation of the eastward extension (EI Nino) or westward withdrawal (La Nina) of the east Asian jet stream due to the local Hadley cell over the Pacific. In addition, the ENSO-related seed of extratropical SST anomaly over the western Pacific in autumn may a lso play an important role in the development of the PNA pattern in th e following winter.