The Impacts of the Eastern Pacific and Central Pacific El Niño on North American Winter Climate
Recent studies have identified two distinct types of El Niño: the Eastern Pacific (EP) type and the Central Pacific (CP) type. Due to their different sea surface temperature anomaly centers, these two El Niño types can produce different global impacts. This dissertation aims to identify their different impacts on North American climate by analyzing reanalysis data, conducting numerical experiments using an ensemble forced atmospheric general circulation model, and examining outputs from the Coupled Model Intercomparison Project Phase 5 (CMIP5).
The El Niño impact on the US winter surface air temperature is found to rotate by 90-degrees between the two types, with warmer-than-normal produced over the northeastern (northwestern) United States and colder-than-normal anomalies over the southwestern (southwestern) United States during the EP (CP) El Niño. The different impacts result from the differing wavetrains each pattern excites in the extratropical atmosphere: the EP El Niño excites the negative phase of Tropical-North Hemisphere pattern, while the CP El Niño excites a Pacific North America pattern.
As for winter precipitation, both types of El Niño events cause drier than normal winter in the northern United States and wetter than normal winter in the southern United States. The CP El Niño increases the dry anomalies to the north and decreases the wet anomalies to the south caused by the EP El Niño over the United States, and, thus, has an enhanced drying effect over the continental United States. The southward displacement of the jet stream during the CP El Niño is responsible for the precipitation difference.
Climate models are more capable of simulating the EP El Niño's impacts than simulating the CP El Niño's impacts on United States winter temperatures. This is because the CP El Niño induces regional heating anomalies whose atmospheric response is more difficult to simulate by atmospheric models. The basin-wide heating response to the anomalies induced by the EP El Niño is easier to capture.