Seasonality of the Pacific Decadal Oscillation

The seasonality of the Pacific decadal oscillation (PDO) is examined using North Pacific sea surface temperature (SST) in observations and in a 480-yr simulation with the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) coupled model. The PDO, both in observations and in the CFS, shows similar seasonality, with increasing SST variance during spring and a maximum in late spring and early summer. The vertical structure of the ocean temperature anomaly associated with the PDO in the CFS displays a significant transition from a deep to a shallow structure during late spring, consistent with the seasonal variation of the mean ocean mixed layer depth (MLD). An analysis of atmospheric surface wind and SST anomalies from the CFS simulation indicates that there is a 1-month delay in the PDO-related SST response to the atmospheric wind forcing. The results based on the CFS simulation are generally consistent with observations, including both atmospheric data from the NCEP/Department of Energy (DOE) Global Reanalysis 2 (GR-2) and ocean data from the NCEP Global Ocean Data Assimilation System (GODAS). The 1-month delay together with the seasonal variation of the mean MLD tends to amplify the PDO-related SST response to the atmospheric surface wind in late spring to early summer, and the combination leads to the maximum variability of the PDO, which is a 3-month delay from the peak phase of the surface wind in February and March.