Impacts of the different types of El Niño and PDO on the winter sub-seasonal North American zonal temperature dipole via the variability of positive PNA events

Abstract

In recent years, the winter surface air temperature (SAT) anomaly over North American midlatitudes shows a “warm west/cold east” (WWCE) dipole pattern with warm (cold) extremes occurring in the west (east) of North America. In this paper, the Pacific sea surface temperature (SST) condition linked to the WWCE SAT dipole is investigated. It is found that the winter WWCE dipole can be crudely considered as being a result of the winter mean of sub-seasonal WWCE events. While the sub-seasonal WWCE dipole is related to the positive Pacific North American pattern (PNA⁺), the impact of the PNA⁺ on the WWCE dipole depends on the different type of El Niño SST pattern and the phase of Pacific decadal Oscillation (PDO). For a Central-Pacific (CP) type El Niño, the positive (negative) height anomaly center of PNA⁺ is located in the western (eastern) North America to cause a strong WWCE dipole, though the positive PDO favors the WWCE dipole. In contrast, the WWCE dipole is suppressed under an Eastern-Pacific (EP) type El Niño because the anticyclonic anomaly of the PNA⁺ dominates the whole North America. Moreover, the physical cause of why different types of El Niño influence PNA⁺ is further examined. It is found that different types of El Niño can significantly influence the midlatitude location of PNA⁺ through changing North Pacific midlatitude westerly winds due to changes in the Pacific Hadley cell. For the CP-type El Niño, the eastward migration of PNA⁺ is suppressed to favor its anticyclonic (cyclonic) anomaly appearing in the western (eastern) North America due to reduced midlatitude westerly winds via weakened Pacific Hadley cell. But for the EP-type El Niño, midlatitude westerly wind is intensified to cause the midlatitude anticyclonic anomaly of the PNA⁺ appearing over the whole North America due to enhanced Pacific Hadley cell.