North American Warm-west/Cold-East air temperature pattern and its linkage to different El Nino types

 

In recent years, the surface air temperature (SAT) anomalies in winter over North America show a “warm-West/cool-East” (WWCE) dipole pattern. The underlying mechanism of the North American WWCE dipole pattern has been an important research topic. This study examines the physical cause of the WWCE dipole generation.

It is found that the positive phase (PNA⁺) of the Pacific North American (PNA) pattern can lead to the generation of the WWCE SAT dipole. However, the impact of the PNA⁺ on the WWCE SAT dipole over North America depends on the type of the El Nino SST anomaly. When an Eastern-Pacific (EP) type El Nino occurs, the anticyclonic anomaly center of the PNA⁺ over the North American continent is displaced eastward near 100°W due to intensified midlatitude westerly winds over North Pacific so that its anticyclonic anomaly dominates the whole North America. In this case, the cyclonic anomaly of the PNA⁺ almost disappears over the North America. Thus, the WWCE SAT dipole over the North America is weakened. In contrast, when a central-Pacific (CP) type El Nino appears, the anticyclonic anomaly center of the associated PNA⁺ is located over the North America west coast due to reduced midlatitude westerly winds over North Pacific. As a result, the cyclonic anomaly of the PNA⁺ can appear over the east United States to result in an intensified WWCE SAT dipole over the North America