Asymmetric Impact of Atlantic Multidecadal Oscillation on El Nino-La Nina Characteristics

Abstract

It is known that the long-lasting cold surface condition of the North Atlantic, i.e., the negative phase of the Atlantic Multidecadal Oscillation (AMO), can lead to an intensification of El Nino-Southern Oscillation (ENSO) in the Paciifc. Here, we suggest that the intensification of El Nino is more pronounced than that of the La Nina during the negative phase of the AMO. We show that the asymmetric enhancement of ENSO is closely related to the warmer background state in the eastern tropical Pacific. A slight increase in the mean sea surface temperature (SST) over the central-to-eastern tropical Pacific leads to a significant enhancement of the sensitivity of atmospheric convection in its response to the local SST change, due to the nonlinearity in SST-atmospheric convection relationship over the cold and dry background state. Moreover, the enhanced atmospheric sensitivity over the central-to-eastern tropical Pacific acts to shift the atmospheric response to the SST anomaly to the east, leading to a stronger atmospheric feedback during El Nino years. On the other hand, the intensity of La Nina is not altered as much as that of El Nino, because the atmospheric response is less sensitive to the negative SST anomaly than to the positive SST anomaly.