A Nonstationary Relationship Between ENSO And Atlantic Climate Due To AMO Modulation In Winter

Based on observed and reanalysis data,we discuss the Atlantic Multidecadal Oscillation(AMO)is investigated as a possible cause of the nonstationary ENSO-NAO relationship and its possible physical mechanism.And further verified by CMIP5 model.Finally,The modulation effect of AMO on ENSO response in tropical Africa was studied.The major conclusions can be summarized as follows:(1)The negative ENSO-NAO correlation in late winter is significant only when ENSO and the AMO are in-phase(AMO+/El Ni?o and AMO-/La Ni?a).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 Ni?o and AMO+/La Ni?a).Further analysis indicates that the sea surface temperature anomaly(SSTA)in the tropical North Atlantic(TNA)plays an essential role in this modulating effect.Due to broadly analogous TNA SSTA responses to both ENSO and the AMO during late winter,a warm SSTA in the TNA is evident when El Ni?o occurs during a positive AMO phase,resulting in a significantly weakened NAO,and vice versa when La Ni?a occurs during a negative AMO phase.In contrast,neither the TNA SSTA nor the NAO show a prominent change under out-of-phase combinations of ENSO and AMO.The AMO modulation and associated effect of the TNA SSTA are verified by forced experiments using an atmospheric circulation model.(2)In the CMIP5 models,the modulation effect of AMO on the relationship between ENSO and NAO is poor.Among them,only the HadCM3 model performed well in simulating of the ENSO and AMO,which make it can reproduce the modulation effect of AMO on the relationship between ENSO and NAO.The other models can not reproduce the modulation effect of AMO on the relationship between ENSO and NAO because of their poor ability in simulating of the ENSO and AMO.(3)AMO also has a significant modulation effect on the response of ENSO in the tropical Atlantic-Africa region,and the SSTA in the tropical Atlantic region(TA)plays an important role.Due to broadly analogous TA SSTA responses to both ENSO and the AMO during late winter,a warm SSTA in the TA is evident when El Ni?o occurs during a positive AMO phase,which weakens the zonal circulation over the region and makes the response inthe tropical Atlantic-Africa region not significant.And vice versa when La Ni?a occurs during a negative AMO phase,a cold SSTA in the TA is evident which weakens the zonal circulation over the region and makes the response in the Africa region not significant,but the response in the TA region is significantly low pressure center.When El Ni?o occurs during a negative AMO phase,the SSTA in TA region is not significant,which does not affect the zonal circulation over the region and makes the response in the tropical Atlantic-African region a significant high pressure center and an easterly wind anomaly on the equator.And vice versa when La Ni?a occurs during a positive AMO phase,the SSTA in TA region is also not significant,which makes the response in the tropical Atlantic-African region a significant low pressure center and an westerly wind anomaly on the equator.