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The latest research sheds light on the Western North Pacific's (WNP) significance as a crucial precursor to El Nino events. Future warming intensifies the conditions leading up to El Nino, with a focus on the Central Pacific. We found increased frequencies of stronger El Ninos and decreased weaker ones under WNP influence. Analysis of heat distribution reveals enhanced convergence and positive entrainment anomalies in the Central Pacific with warming. Additionally, a three-year same-signed sea surface temperature anomaly propagation from WNP to the equatorial Pacific precedes favorable conditions for El Nino.
The WNP is defined as a specific SSTA pattern depicting an opposite signed dipole located in the east of Taiwan and northeast of Papua New Guinea, which forms one year before a full-fledged El Nino/La Nina [1, 2]; a later analysis [3] found that the same-signed SSTA east of Taiwan can propagate southeastward to the Niño-3 region over a duration up to 4 years and, under favorable conditions, can connect to ENSO development.
The WNP precursor may have two modes, one involving the WES feedback-Kelvin wave forcing (top) and the other with North Pacific anticyclone's low frequency variability (bottom).
The former mode can trigger El Niño a year later and the latter could propagate across the subtropical Pacific and then connects to El Niño 2-3 years later.
The WNP index
The WNP index
Though the WNP index is defined as the 2nd mode of MCA with SSTA and surface wind anomalies over the western Pacific, it also can be represented by averaging de-trended SSTA over a small domain east of Taiwan and south of Japan.
Connecting the major ENSO precursors
Connecting the major ENSO precursors
By examining the longitude-time evolution of SSTA, a diagnostic study [3] identified a slow southeastward propagation from the WNP around 20°N to the Niño-3.4 region. The propagation is manifested as a narrow, southwest-northeast elongated SSTA band traveling across the subtropical North Pacific, and the trans-Pacific journey averages ~3 years.
While the WNP index is significantly correlated to the development of ENSO by the following winter, the analysis presented here further suggests that this relationship between the WNP and ENSO has significantly increased since the mid-20th century.
Wang, S.-Y., M. L’Heureux, and H-H Chia, 2012: ENSO prediction one year in advance using Western North Pacific sea surface temperatures. Geophysical Research Letters, 39, L05702 (pdf).
Wang, S.-Y., M. L’Heureux, and J.-H. Yoon, 2013: Are greenhouse gases changing ENSO precursors in the Western North Pacific? Journal of Climate, 26, 6409-6322. (PDF).
Wang, S.-Y., X. Jiang, and B. Fosu, 2015: Global eastward propagation signals associated with the 4-5-year ENSO cycle. Climate Dynamics, 44, 2825-2837 (PDF).
Fosu, B., J. He, and S.-Y. Wang, 2020: The influence of wintertime SST variability in the Western North Pacific on ENSO diversity. Climate Dynamics, in press. (PDF)
Chikamoto, Y., J. Johnson, S.-Y. Wang, M. J. McPhaden, and T. Mochizuki, 2020: El Niño Southern Oscillation evolution modulated by Atlantic forcing, Journal of Geophysical Research: Oceans, in press (PDF)
Borhara, K., B. Fosu, and S.-Y. Wang, et al., 2023: The Role of the Western North Pacific (WNP) as an El Niño–Southern Oscillation (ENSO) Precursor in a Warmer Future Climate. Climate Dynamics, in press (PDF)
Is Global Warming Changing the ENSO Precursor in the Western North Pacific? [ NOAA NWS Science and Technology Infusion Climate Bulletin ] - 2012
Presentation by Michelle L'Heureux (CPC):
Understanding Tropical Climate to Advance Sub-seasonal to Seasonal Prediction (link)
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