Low-frequency pycnocline depth variability at Ocean Weather Station P in the northeast Pacific

Low-frequency variability of the depth of the main pycnocline at Ocean Weather Station P and over the northeast Pacific is examined in terms of the one-dimensional response to local Ekman pumping according to the Hasselmann stochastic climate model. The model is forced with monthly wind stress curl anomalies derived from the National Centers for Environmental Prediction reanalysis for the period 1948-2000. An empirical orthogonal function analysis shows that the leading mode of the response bears the signature of the Pacific (inter) Decadal Oscillation (PDO) and that the associated principal component captures the "regime shift'' of 1976/77. The correlation is 0.77 between annually averaged pycnocline displacement anomalies hindcast from the model and anomalies in the depth of the main pycnocline at station P (50degreesN, 215degreesE) observed over a 43-yr period. The comparison indicates that variability in the depth of the upper layer on interannual to interdecadal timescales at station P occurs largely as an integrated response to local Ekman pumping. In addition, the results suggest that the PDO mode dominates the observed variability.