North Pacific decadal variability and climate change in the IPCC AR4 models

The dynamics of North Pacific decadal variability (NPDV) are explored in 10 coupled climate models used in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) for both the 20th century climate and under a greenhouse warming scenario. We examine the two dominant oceanic modes of NPDV, the Pacific Decadal Oscillation (PDO) and the North Pacific Gyre Oscillation (NPGO), along with their respective atmospheric forcing patterns associated with variability of the Aleutian Low (AL) and the North Pacific Oscillation (NPO).

We find that the decadal modes of ocean/atmosphere variability (e.g., PDO/AL, NPGO/NPO) are spatially consistent with modern observations in most models and show no change under climate change. The frequency content of the PDO and NPGO also shows no significant change in future climate. However, most climate models do not adequately reproduce the observed temporal variability in the 20th century, with some failing to capture the low frequency nature of the PDO and the broad frequency band of the NPGO. In the 20th century, while the dynamics of the PDO in the models are consistent with forcing by the variability in the AL, the NPGO of the models is not always driven by atmospheric variability associated with the NPO. Further analysis also reveals that the NPDV of the IPCC models is highly independent of the tropics contrary to the observed relationships between the PDO/AL and NPGO/NPO with the El Niņo-Southern Oscillation (ENSO). In fact, the PDO of most models appears as a mode unconnected with ENSO, and the seasonal footprinting mechanism, whereby the midlatitude NPGO/NPO winter expression leads the tropical ENSO peak in early winter, is captured in only 2 of the 10 climate models: the GFDL 2.1 and MIROC-HIRES. Hence, the inconsistencies between observed and modeled climate variability of the IPCC models motivates future climate studies to carefully consider the integrity of the model solutions used for decadal climate predictions of the North Pacific.