A new paradigm for the predominance of standing Central Pacific Warming after the late 1990s
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Canonical El Niño has a warming center in the eastern Pacific (EP), but in recent decades, El Niño warming center tends to occur more frequently in the central Pacific (CP). The definitions and names of this new type of El Niño, however, have been notoriously diverse, which makes it difficult to understand why the warming center shifts. Here, we show that the new type of El Niño events is characterized by: 1) the maximum warming standing and persisting in the CP and 2) the warming extending to the EP only briefly during its peak phase. For this reason, we refer to it as standing CP warming (CPW). Global warming has been blamed for the westward shift of maximum warming as well as more frequent occurrence of CPW. However, we find that since the late 1990s the standing CPW becomes a dominant mode in the Pacific; meanwhile, the epochal mean trade winds have strengthened and the equatorial thermocline slope has increased, contrary to the global warming-induced weakening trades and flattening thermocline. We propose that the recent predominance of standing CPW arises from a dramatic decadal change characterized by a grand La Niña-like background pattern and strong divergence in the CP atmospheric boundary layer. After the late 1990s, the anomalous mean CP wind divergence tends to weaken the anomalous convection and shift it westward from the underlying SST warming due to the suppressed low-level convergence feedback. This leads to a westward shift of anomalous westerly response and thus a zonally in-phase SST tendency, preventing eastward propagation of the SST anomaly. We anticipate more CPW events will occur in the coming decade provided the grand La Niña-like background state persists.
KeywordsCentral Pacific Warming La Niña-like mean state change Convection Low-level convergence feedback
We would like to thank Dr. Mark A Cane and Jong-Seong Kug for their comments and suggestions on this study. This work has been supported by the Climate Dynamics Program of the National Science Foundation under award No. AGS-1005599, and APEC Climate Center. The authors acknowledge partial support from International Pacific Research Center which is sponsored by the JAMSTEC, NASA (NNX07AG53G) and NOAA (NA09OAR4320075). TL is supported by ONR grant N000141210450. This is SOEST contribution number 8690 and IPRC contribution number 895.
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