Analysis of remotely forced oceanic Rossby waves off California

Abstract

Complex empirical orthogonal function (CEOF) analysis is used to investigate the coastal Kelvin wave driven Rossby wave response in the northeast Pacific. Using CEOF analysis, a spatial structure function is obtained from model upper layer thickness data. The model is a nonlinear, reduced gravity model of the northeast Pacific forced by coastal Kelvin waves originating in the equatorial Pacific. The spatial structure function is used to extract the interannual Rossby wave response from observed 300‐m‐depth temperature anomalies. The observed Rossby wave signal is termed the projection mode. Rossby wavelike features observed in the projection mode are order 1000 km long with most periods ranging between 2 and 4 years. The wave numbers and frequencies found are consistent with Rossby dynamics. The mean phase speed of the Rossby wavelike features within the projection mode is 1.3 cm s−1, in agreement with the theoretical Rossby wave phase speed at 40°N. Large amplitude nearshore and decreasing amplitude away from shore suggests nearshore generation of these waves. An important source of sea level variability along the east coast of North America at periods of 2–4 years was identified by Pares‐Sierra and O'Brien (1989) as poleward propagating Kelvin waves. Since the Rossby wavelike features observed in the projection mode have a majority of their periods ranging between 2 and 4 years, their forcing can be attributed to long‐period Kelvin waves. Spectral comparisons between the nearshore values in the projection mode and coastal sea level show greater than 90% coherence in the period band 3–4.4 years. The high coherence between coastal sea level variations and the projection mode shows that there is a strong correlation between the Rossby wavelike features within the projection mode and coastal Kelvin wave propagation. It is concluded that the Rossby waves within the projection mode are forced by coastal Kelvin wave propagation. The projection mode accounts for 47.5% of the variance in the 300‐m‐depth temperature anomalies. The implication of this result is that the physical mechanism of the numerical model, i.e., Rossby waves excited by coastal Kelvin wave propagation, accounts for 47.5% of the variance in the observed 300‐m depth temperature anomalies.