The Atlantic as a Driver of the Multidecadal ENSO-Tropical Pacific Annual Cycle Relationship

Many previous studies have shown through observations and models that the annual cycle of equatorial Pacific and the ENSO amplitude are inversely related. Using sea surface temperature reconstructions and proxy data, the impact of the annual cycle of the equatorial Pacific on ENSO can be estimated through the strength of the spring persistence barrier, allowing us to reconstructed the general properties of the annual cycle over longer records. Recent studies have suggested that reduction of ENSO activity over the first decade and a half of the 2000s has been driven by changes in the Atlantic-Pacific differences. Using the 125 year long observed sea level pressure record from the Azores Islands and equally long independent sea surface temperature reconstructions, we find that the Azores sea level pressure is strongly related to the ENSO amplitude on multi-decadal periods. The multi-decadal Atlantic sea level pressure variability impacts ENSO amplitude by modulating the strength of the trade winds in the tropical Pacific. Further, the impact on ENSO amplitude is seasonally dependent with differing effects between boreal winter and summer. The seasonally dependent forcing of ENSO by the multi-decadal Atlantic sea level pressure signal leads to the inverse relationship on multi-decadal timescales of annual cycle and ENSO amplitude, which has been previously noticed in proxy data, observations, and models.