Abstract
Sea surface temperature anomalies (SSTa) in the Western North Pacific (WNP) have been linked to the development of El Niño Southern Oscillation (ENSO) events a full year in advance. However, the contribution of the WNP precursor to the temporal evolution and spatial complexity of ENSO remains unclear. Using the preindustrial experiment of the Community Earth System Model as the control climate, a partially coupled experiment is conducted in which WNP SSTa are restored to the model climatology. By comparing the perturbed experiment to the control, we are able to clearly characterize ENSO’s response to WNP SST variability. We find that the WNP is predominantly linked to eastern Pacific (EP) ENSO events. Without SST variability in the WNP, central Pacific (CP) ENSO events are more likely to develop. This variation in ENSO flavor is controlled by how the WNP projects onto wind stress anomalies in the western equatorial Pacific, which in turn impacts the discharge and recharge of ocean heat during the ENSO cycle. Specifically, the removal of SSTa in the WNP weakens the buildup of ocean heat in the western equatorial Pacific, which then hinders the development of EP-type events.
Highlights
Several studies have linked the onset of El Niño Southern Oscillation (ENSO) events to extratropical atmospheric and oceanic variability
Are there specific precursors or triggers for EP and central Pacific (CP) events, or are these differences happening by chance, and how would the existence of these precursors impact our ability to predict these different events (Kirtman 2019)? While the Pacific Meridional Mode (PMM) has emerged as a key driver in the generation of the CP type ENSO (Yu and Kim 2011; Kim et al 2012; Vimont et al 2014; Stuecker 2018), the South Pacific Meridional Mode (SPMM) has been linked to amplified SST anomalies (SSTa) in the eastern Pacific (Zhang et al 2014)
The literature is replete with examples of the Niño-3.4 index being used to identify ENSO episodes, it may not be optimal for describing the various ENSO flavors
Summary
Several studies have linked the onset of El Niño Southern Oscillation (ENSO) events to extratropical atmospheric and oceanic variability. While the PMM has emerged as a key driver in the generation of the CP type ENSO (Yu and Kim 2011; Kim et al 2012; Vimont et al 2014; Stuecker 2018), the SPMM has been linked to amplified SSTa in the eastern Pacific (Zhang et al 2014) Unlike these meridional modes, the mechanisms by which the more distantly located WNP SSTa influences the various types of ENSO are still poorly understood. Apart from augmenting our understanding of the range of ENSO variations, extratropical and subtropical precursors to ENSO can directly be utilized to improve long-lead ENSO predictions Against this backdrop, this study is designed to determine the contribution of SST variability in the WNP to the temporal evolution and spatial diversity of ENSO using the fully coupled global climate Community Earth System Model (CESM) (Gent et al 2011). Product (Kalnay et al 1996) and consist of monthly mean values from 1948 to present on a 2.5° × 2.5° horizontal grid globally
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