Vertically propagating annual and interannual variability in a OGCM simulation of the Tropical Pacific in 1985-1994
Boris Dewitte and Gilles Reverdin
J. Phys. Oceanogr., 30, 1562-1581
The annual and interannual variability in the subthermocline equatorial Pacific is studied in a simulation of the tropical Pacific for the 1985-1994 decade using a primitive equation high-resolution model. The study focuses on temperature variability and vertical energy fluxes. Similarly to the observations in Kessler and McCreary (1993), the annual harmonic of vertical isotherm displacements presents phase lines sloping downward from east to west with maxima in amplitude in the zones on both sides of the equator. Estimates of zonal and vertical phase speeds and the location of maxima of isotherm displacements are consistent with those of the l=1 Rossby wave resulting both from the reflection of surface-forced Kelvin waves on the eastern boundary and direct forcing at the surface. The simulated field is somewhat more trapped towards the equator than the observations. A linear simulation is carried out for the vertical standing modes which caracteristics are derived from the OGCM simulation. In the linear solution, high-order meridional mode Rossby waves are more prominent than in the OGCM solution. However, the (l=1) Rossby wave in the linear model shares many characteristics with the one in the OGCM simulation.
On interanual timescales the simulation also presents vertical propagation of Rossby waves from the eastern boundary, but not as deep as for the annual cycle. The variability of vertical displacements exhibits an asymmetry with a larger amplitude north than south of the equator. The zone of large interannual variability originating at the eastern boundary extends westward following (l=1) or (l=2) WKB ray paths and reaches the western Pacific near 400m at 3-4° north but not south of the equator. The 3.3 year harmonic is prominent in isotherm vertical displacements for this particular simulation. The phase and amplitude of the (3.3 yr) harmonic suggests a prominent (l=1) Rossby wave, but the phase line characteristics are also indicative of the contribution of higher-order meridional modes. For this specific harmonic, the solution of the linear model for the (l=1) Rossby mode contribution agrees well with the OGCM solution.