ENSO-forced Decadal Variability in the North Pacific

Variability of North Pacific SST, on both interannual and decadal timescales, is well modeled as the sum of direct forcing by El Ni�o-Southern Oscillation (ENSO), "re-emergence" of North Pacific SST anomalies in subsequent winters, and white noise. Thus, over a series of winter/springs, the North Pacific integrates the effect of ENSO, resulting in a power spectrum of the Pacific Decadal Oscillation (PDO) which has more power than ENSO itself for periods greater than about 10 years. An observational analysis yielding the above results is presented, and it is shown that a simple autoregressive model which includes some forcing by ENSO has a power spectrum consistent with the observed PDO. This simple model may be taken as a null hypothesis for the PDO, and may also be relevant for other climate integrators which have been previously related to the PDO.

This analysis is repeated for the output of ten different coupled models from the Coupled Model Intercomparison Project (CMIP2). In addition, a Linear Inverse Model (LIM; Penland and Sardeshmukh 1995) analysis is performed on three-month running mean tropical SST for each coupled model. Using the simple model formulation from above, differences in the simulated North Pacific decadal variability are directly related to spatiotemporal differences in the simulated ENSO variability, which are in turn well captured by the LIMs.