A Nonstationary ENSO–NAO Relationship Due to AMO Modulation

Many previous studies have demonstrated a high uncertainty in the relationship between El Niño–Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). In the present work, decadal modulation by the Atlantic multidecadal oscillation (AMO) is investigated as a possible cause of the nonst...

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Bibliographic Details
Published in:	Journal of climate 2019-01, Vol.32 (1), p.33-43
Main Authors:	Zhang, Wenjun, Mei, Xuebin, Geng, Xin, Turner, Andrew G., Jin, Fei-Fei
Format:	Article
Language:	eng
Subjects:	Anomalies Atmospheric circulation Atmospheric circulation models Atmospheric forcing Atmospheric models Collaboration Computer simulation Datasets Disasters Education El Nino El Nino phenomena El Nino-Southern Oscillation event General circulation models Influence Information science La Nina Laboratories Modulation North Atlantic Oscillation Ocean currents Ocean-atmosphere system Oscillations Phase transitions Science Sea level Sea surface Sea surface temperature Sea surface temperature anomalies Southern Oscillation Surface temperature Temperature Temperature anomalies Tropical climate Winter
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Summary:	Many previous studies have demonstrated a high uncertainty in the relationship between El Niño–Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). In the present work, decadal modulation by the Atlantic multidecadal oscillation (AMO) is investigated as a possible cause of the nonstationary ENSO–NAO relationship based on observed and reanalysis data. It is found that the negative ENSO–NAO correlation in late winter is significant only when ENSO and the AMO are in phase (AMO+/El Niño and AMO−/La Niña). However, no significant ENSO-driven atmospheric anomalies can be observed over the North Atlantic when ENSO and the AMO are out of phase (AMO−/El Niño and AMO+/La Niña). Further analysis indicates that the sea surface temperature anomaly (SSTA) in the tropical North Atlantic (TNA) plays an essential role in this modulating effect. Because of broadly analogous TNA SSTA responses to both ENSO and the AMO during late winter, a warm SSTA in the TNA is evident when El Niño occurs during a positive AMO phase, resulting in a significantly weakened NAO, and vice versa when La Niña occurs during a negative AMO phase. In contrast, neither the TNA SSTA nor the NAO shows a prominent change under out-of-phase combinations of ENSO and AMO. The AMO modulation and the associated effect of the TNA SSTA are shown to be well reproduced by historical simulations of the HadCM3 coupled model and further verified by forced experiments using an atmospheric circulation model. These offer hope that similar models will be able to make predictions for the NAO when appropriately initialized.
ISSN:	0894-8755 1520-0442