Abstract
A 1-year-lagged relationship between the North Atlantic Oscillation (NAO) in winter and El Niño/Southern Oscillation (ENSO) in the following year was examined with a multi-model ensemble analysis using Coupled Model Intercomparison Project phase 3 models. A coherency index was developed as a measure of the coherence between the NAO and ENSO in each model and used as a weighting factor in the ensemble model. Weighted multi-model ensemble means of the regressed field on the maximum covariance analysis coefficients between the surface pressure field in the North Atlantic (NAO field) and the sea surface temperature field in the equatorial Pacific (ENSO field) were explored. The results indicated that when the wintertime NAO was in its negative (positive) phase, anomalous atmospheric circulation associated with a large (small) Eurasian snow mass anomaly intensified (weakened) the Asian cold surge and the westerly wind burst (WWB) in the warm pool region. Intensification of the WWB initiates El Niño. A sensitivity experiment using an idealized dry general circulation model revealed that the atmospheric response to anomalous, near-surface cooling associated with an anomalous Eurasian snow mass induced higher surface pressure near the Tibetan plateau and thus intensified the Asian cold surge and WWB. Linear regression analyses applied to the reanalysis data strongly supported the model results. Our results therefore suggest an influence of the NAO on the initiation of El Niño via a process that involves Eurasian snow anomalies associated with the NAO phase.
Highlights
In a companion paper, Oshika et al (2014) have shown observational evidence of relationships between the North Atlantic Oscillation (NAO), El Niño/ Southern Oscillation (ENSO), and the Western Pacific pattern (WP) through statistical analyses of a 51-year (1960–2010) reanalysis dataset
The resultant Z500 pattern one year after the NAO month is somewhat similar to the positive phase of the observed WP pattern, but its southern and northern center of action are shifted northward compared with the conventional WP pattern (Wallace and Gutzler 1981)
This study was carried out to complement the observational evidence presented in Part I that a winter NAO precedes an ENSO during the second half of the following year, and to identify a possible mechanism underlying the relationship between the NAO and ENSO by use of a multi-model analysis with a coupled atmosphere–ocean climate model
Summary
In a companion paper, Oshika et al (2014) (hereafter Part I) have shown observational evidence of relationships between the North Atlantic Oscillation (NAO), El Niño/ Southern Oscillation (ENSO), and the Western Pacific pattern (WP) through statistical analyses of a 51-year (1960–2010) reanalysis dataset. The analysis revealed a roughly 1-year-lagged negative correlation between the NAO, WP, and ENSO. When a negative (positive) phase of the NAO occurs in December, a warm (cold) ENSO phase occurs in the following year sometime in the summer-to-winter time interval, and a positive (negative) phase of the WP occurs in the following December. These results imply that the NAO can be used to predict monthly mean weather during the following winter in the Asian Far East. To confirm the proposed processes shown in Part I, we performed a multi-model ensemble analysis in Part II using the Coupled Global Climate Model (CGCM) from the Coupled Model Intercomparison Project phase 3 (CMIP3) of the World Climate Research Programme (Meehl et al 2007; Randall et al 2007) as a counterpart of Part I
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