Tropical Pacific Decadal Variability and Its Relation to Decadal Modulations of ENSO

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Abstract
A 1000-yr integration of a coupled ocean–atmosphere model (ECHO-G) has been analyzed to describe decadal to multidecadal variability in equatorial Pacific sea surface temperature (SST) and thermocline depth (Z20), and their relationship to decadal modulations of El Niño–Southern Oscillation (ENSO) behavior. Although the coupled model is characterized by an unrealistically regular 2-yr ENSO period, it exhibits significant modulations of ENSO amplitude on decadal to multidecadal time scales. The authors' main finding is that the structures in SST and Z20 characteristic of tropical Pacific decadal variability (TPDV) in the model are due to an asymmetry between the anomaly patterns associated with the model's El Niño and La Niña states, with this asymmetry reflecting a nonlinearity in ENSO variability. As a result, the residual (i.e., the sum) of the composite El Niño and La Niña patterns exhibits a nonzero dipole structure across the equatorial Pacific, with positive perturbation values in the east ... Abstract A 1000-yr integration of a coupled ocean–atmosphere model (ECHO-G) has been analyzed to describe decadal to multidecadal variability in equatorial Pacific sea surface temperature (SST) and thermocline depth (Z20), and their relationship to decadal modulations of El Niño–Southern Oscillation (ENSO) behavior. Although the coupled model is characterized by an unrealistically regular 2-yr ENSO period, it exhibits significant modulations of ENSO amplitude on decadal to multidecadal time scales. The authors' main finding is that the structures in SST and Z20 characteristic of tropical Pacific decadal variability (TPDV) in the model are due to an asymmetry between the anomaly patterns associated with the model's El Niño and La Niña states, with this asymmetry reflecting a nonlinearity in ENSO variability. As a result, the residual (i.e., the sum) of the composite El Niño and La Niña patterns exhibits a nonzero dipole structure across the equatorial Pacific, with positive perturbation values in the east ...