Impacts of two types of La Niña on the NAO during boreal winter

The present work identifies two types of La Nina based on the spatial distribution of sea surface temperature (SST) anomaly. In contrast to the eastern Pacific (EP) La Nina event, a new type of La Nina (central Pacific, or CP La Nina) is featured by the SST cooling center over the CP. These two types of La Nina exhibit a fundamental difference in SST anomaly evolution: the EP La Nina shows a westward propagation feature while the CP La Nina exhibits a standing feature over the CP. The two types of La Nina can give rise to a significantly different teleconnection around the globe. As a response to the EP La Nina, the North Atlantic (NA)–Western European (WE) region experiences the atmospheric anomaly resembling a negative North Atlantic Oscillation (NAO) pattern accompanied by a weakening Atlantic jet. It leads to a cooler and drier than normal winter over Western Europe. However, the CP La Nina has a roughly opposing impact on the NA–WE climate. A positive NAO-like climate anomaly is observed with a strengthening Atlantic jet, and there appears a warmer and wetter than normal winter over Western Europe. Modeling experiments indicate that the above contrasting atmospheric anomalies are mainly attributed to the different SST cooling patterns for the two types of La Nina. Mixing up their signals would lead to difficulty in seasonal prediction of regional climate. Since the La Nina-related SST anomaly is clearly observed during the developing autumn, the associated winter climate anomalies over Western Europe could be predicted a season in advance.