During El Ni–o event when positive sea surface temperature (SST) anomalies form in the equatorial eastern Pacific, SST anomalies also tend to develop in the North Pacific. This SST linkage between the tropics and the North Pacific has been emphasized in many studies (Alexander 1990, 1992; Lau and Nath 2001). In general, North Pacific anomaly pattern associated with El Ni–o features cold water in the central Pacific and warm water in the eastern Pacific. To date, however, there is no consensus on the well-defined and detailed anomaly pattern in the mid latitudes during El Ni–o episode.

This study suggests that the North Pacific SST anomaly pattern associated with El Ni–o has experienced a significant change around 1996-1997. The cyclo-stationary empirical orthogonal function (CSEOF) analysis of tropical Pacific SST (30°S-20°N, 100°E-280°E) anomaly reveals a convectional ENSO feature as the first loading vector. On the other hand, the CSEOF first mode of the North Pacific SST anomaly (20°N-70°N, 100°E-280°E) shows positive SST anomalies over the eastern North Pacific and negative SST anomalies over the central North Pacific. This North Pacific pattern has thought to be closely related with ENSO event in previous studies. The correlation coefficient between the first CSEOF PC time series of tropical Pacific and North Pacific SST anomalies, however, reveals that these two components exhibit strong correlation in recent decade (1997-2010, correlation: 0.83) than in the epoch before 1997 (1980-1996, correlation: 0.27).

Regression on the first CSEOF of tropical SST anomaly for these two periods (i.e., 1980-1996 and 1997-2010) shows remarkable different patterns of North Pacific SST anomaly, confirming that the link between the tropical Pacific and the North Pacific has either changed significantly in recent decade or is very volatile.  Strong positive SST anomaly in the eastern North Pacific is extended westward up to 200°E while negative SST anomaly appear over the central to the western North Pacific in 1997-2010. In 1980-1996, on the other hand, positive SST anomaly in the eastern North Pacific appears very weak compared with the 1997-2010 case. Meanwhile, positive SST anomaly over the western North Pacific is a distinguishing feature of the 1980-1996 period. Different extra-tropical SST anomaly patterns associated with ENSO for the two periods may result in distinct atmospheric response through air-sea interaction over the North Pacific. Preliminary results indicate that mid-latitude (40°-60°N) atmospheric responses for the two periods are substantially different.  In this study, details of this difference will be discussed and plausible physical mechanism will be addressed.