Understanding the Emerging Central-Pacific ENSO and Its Impacts on North American Climate

It is being increasingly recognized that there are two distinct types of El Niño-Southern Oscillation (ENSO): an Eastern-Pacific (EP) type that has its sea surface temperature (SST) anomalies centered near the South America coast and a Central-Pacific (CP) type that has its SST anomalies centered near the international dateline. IPCC AR4 simulations project that the CP type may become the prevailing type of ENSO in a future warmer world, which is consistent with the fact that CP ENSO events have occurred more frequently in the past three decades than in earlier decades. There is a need to better prepare for the emergence of this mode of tropical climate variability, and to revise existing modeling and prediction strategies developed primarily with the conventional EP type of ENSO in mind. One source of uncertainty in the prediction and projection of North American climate may have to do with whether or not modern climate models can produce both types of ENSO, simulate the alternation between them, and capture their different impacts. This project proposes data analyses and model experiments to better understand the evolution of the CP ENSO and its regional impacts on the Pacific-North America sector and to identify the key atmospheric and oceanic processes for differing the impacts of the CP and EP ENSO’s on North American climate.

Specifically, this project will make use of the existing Coupled Model Intercomparison Project Phase 3 (CMIP3) simulations and the upcoming CMIP5 simulations to understand the relative importance of the extratropical forcing and tropical coupling in controlling the evolution of the CP ENSO and to identify the concurrent and extended impacts of CP and EP ENSOs on North American Climate. The different impacts produced by the EP and CP ENSOs will be translated into uncertainties in the prediction and projection of the North American climate variability and will be assessed. Partial-coupling and forced experiments will then be conducted to further understand how the ocean and atmosphere in the Pacific-North American sector respond to CP and EP ENSO forcing, how the responses are projected onto the Pacific-North American (PNA) and North Pacific Oscillation (NPO) modes of variability, and how they are manifested as variations in the Aleutian Low, Subtropical High, and tropospheric jestreams. Special attention will be given to understanding the ENSO-induced SST anomalies in the North Pacific, which are hypothesized to extend ENSO’s influence on North America after the demise of the ENSO events. The possibility of using statistical models, such as the Markov model, to perform CP ENSO predictions using both extratropical and tropical information will also be explored.

This project is expected to quantify the sensitivity of North American climate to the alternation of the ENSO type and to make suggestions on how it can be better captured in modern climate models by laying out the specific atmospheric, oceanic, and coupled processes that establish the sensitivity. New metrics that gauge not only tropical but also extratropical atmospheric and ocean fields will also be developed to help further improve model simulations of the two types of ENSO. These efforts are relevant to (a) “support the development of next-generation global climate models” and (b) “evaluate uncertainties in regional-scale climate predictions and projections”, both of which are priority areas specified by the FY2011 MAPP program for the research theme of Advance in Regional-Scale Climate Predictions and Projections.