Previous studies have indicated that the extratropics can influence ENSO via specific processes. However, it is still unclear to what extent ENSO is influenced by the extratropics in observation. Now we assess this issue by applying the regional data assimilation (RDA) approach in an advanced model, the GFDL CM2.1. Our study confirms a strong extratropical impact on observed ENSO. Quantitatively, the extratropical atmospheric variability poleward of 20° explains 56% of the observed variance of ENSO and greatly influences ∼67% of observed El Niño events during 1969–2008. This extratropical impact is still significant even as far as poleward of 30°. Furthermore, the impact from the southern extratropics is slightly stronger than that from the northern extratropics, partly caused by the Pacific ITCZ location north of the equator and different mixed-layer depth along the northern Pacific meridional mode (NPMM) and the southern Pacific meridional mode (SPMM). Our study further shows that all of three super El Niño events, those in 1972/73, 1982/83, and 1997/98, are influenced greatly by both hemispheric extratropics, with NPMM and SPMM interfering constructively, while most weak and moderate El Niño events are triggered by only one hemispheric extratropics, with NPMM and SPMM interfering destructively. Besides the extratropical Pacific influence on ENSO via NPMM/SPMM, the extratropics also has a potential impact on ENSO by influencing other tropical oceans and then by interbasin interactions.
North and South Pacific Meridional Modes (NPMM and SPMM) are known precursors of El Niño–Southern Oscillation (ENSO) and Tropical Pacific decadal variability (TPDV). However, the relative importance of these precursors and the timescale on which they impact the tropics remain unclear. Using a 30‐member ensemble of the Community Earth System Model as the control climate, we generate two additional members where the NPMM and SPMM are selectively suppressed. We find that both meridional modes energize the tropical variance independently on different timescales. The absence of NPMM leads to a significant reduction of the tropical interannual variability (~35%), while the absence of the SPMM has no appreciable impact on ENSO but significantly reduces the TPDV (~30%). While the relative importance of the NPMM and SPMM may be model dependent, the stochastic atmospheric variability in the extratropics that energizes the meridional modes emerges as a key source of TPDV.
more » « less- NSF-PAR ID:
- 10457260
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 46
- Issue:
- 2
- ISSN:
- 0094-8276
- Page Range / eLocation ID:
- p. 906-915
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Significance Statement The purpose of this study is to identify structures that lead to seasonal growth of diverse types of El Niño–Southern Oscillation (ENSO) events. An important contribution from this study is that it uses an observationally constrained, empirically derived seasonal model. We find that processes affecting the evolution of diverse ENSO events are strongly seasonally dependent. ENSO events with eastern equatorial Pacific sea surface temperature (SST) characteristics are closely related to a single “ENSO mode” that resembles theoretical models of ENSO variability. ENSO events that have central equatorial Pacific SST characteristics include contributions from additional “meridional mode” structures that evolve via different physical processes. These findings are an important step in evaluating the seasonal predictability of ENSO diversity.
