A North-Pacific short-wave train during the extreme ENSO phases: A possible link between the subtropical western Pacific and North America

The Pacific-North America (PNA) teleconnection emanating from the SST anomaly center in the equatorial Central Pacific has constituted the major theme of the climate variability research in the past two decades. Note that interannually the tropical Pacific SSTs exhibit an east-west seesaw between the eastern and western tropical Pacific in concert with the El Niño –Southern Oscillation (ENSO) cycle. Furthermore, some studies already suggested that precipitation and the exceptionally cold weather of some winters over North America are related to the convective activity of the western Pacific. Therefore, one may question whether the warm/cold SST anomalies in the western tropical Pacific are able to induce a teleconnection wave pattern to link the climate systems of both sides of the North Pacific.

It is shown by the linear stationary wave theory that only ultralong waves are capable of penetrating to high latitudes. Based on this wave theory, we introduce a Fourier scale separation to divide the ENSO anomalous circulation into two wave regimes: long-wave (wave number 1-3) and short-wave (wave number 4-15). The conventional teleconnection wave train from the equatorial central Pacific is basically formed by the long-wave regime. In contrast, the short-wave regime exhibits a well-organized wave train propagating from the western subtropical Pacific along the North-Pacific rim into North America. In order to understand the formation of this short-wave train, we perform a diagnostic analysis and numerical simulations. It is indicated by the comparison between the anomalous divergent circulation of the short-wave train and rainfall anomalies that the anomalous rainfall/convection over the Philippine Sea induced by cold surge vortices of the East-Asian winter monsoon forms the forcing of this wave train. With an idealized forcing (with a radius of 103 km) centered at (10oN, 135oE), the North-Pacific short-wave train was properly simulated by Version 3 of the NCAR Community Climate Model (CCM3).

As shown by the linear wave theory, short weaves are trapped in the tropics. Nevertheless, the short-wave train stimulated by the western tropical Pacific forcing propagates along the maximum westerlies across the North Pacific in order to reach North America. Therefore, this newly identified short-wave train enables us not only to better understand the formation of the ENSO anomalous circulation pattern across North America, but also to establish a possible link between the climate systems of the western Pacific and North America.