Interannual Variability in the Eastern Subtropical North Pacific Ocean

Abstract

Interannual variability in the eastern subtropical North Pacific Ocean is investigated using the Naval Research Laboratory Layered Ocean Model (NLOM). Emphasis is placed on examining the nature of westward propagating Rossby waves and their interaction with the overlying atmosphere. Singular value decomposition (SVD), complex empirical orthogonal function (CEOF), and multichannel singular spectrum (M-SSA) analyses are used to isolate the standing and propagating response due to modeled Rossby waves and observed wind stress curl (WSC) anomalies as well as their dominant frequencies of oscillation, respectively. In addition to a large-scale interdecadal fluctuation, two distinct forms of Rossby waves are found to exist, one wind forced and the other freely propagating. SVD and CEOF analyses of NLOM upper layer thickness anomalies suggest a leading order source of variability stemming from a large-scale interdecadal fluctuation. M-SSA analysis also depicts this low frequency mode, but to a lesser extent and at higher order. The modeled temporal coverage limits the extent to which this feature can be studied further. Of primary interest in the eastern subtropical North Pacific Ocean are large-scale interannual wind-forced Rossby wave variations. Independent M-SSA analysis of modeled ULT and observed WSC anomalies suggests a low frequency (?51 month period) atmospherically forced ocean response in which westward atmospheric propagation leads forced oceanic Rossby wave propagation by roughly 3 months. In addition to this low frequency forced oceanic wave response with a phase speed of cr ? 12 cm s?1, a distinct freely propagating biennial oceanic Rossby wave is also found to exist, with a phase speed of cr ? 8 cm s?1, emanating from El Niño?Southern Oscillation related coastally trapped Kelvin waves.