Impact of realistic mean flow on equatorial waves in the Pacific Ocean

An ocean general circulation model (OGCM) is used to produce a temporally and spatially dense data set for the tropical Pacific from 1992 to 1997. The model incorporates routines which assimilate temperature and altimetry data. Model currents are compared with TOGA-TAO current observations at 110°W, 140°W and 165°E. This comparison shows that the model accurately represents zonal currents over a broad range of time scales. A one-dimensional numerical model is formulated to derive the structure and phase speed of equatorial free waves based on the OGCM depth dependent equatorial density structure. Using a finite difference method this model is solved as a generalized eigenvalue system. Analyses of the model output indicate the presence of equatorial Kelvin waves with velocity and structure similar to the calculated values. These waves are excited by westerly wind bursts in the western tropical Pacific during late 1996 and early 1997. These waves propagate across the Pacific, and their arrival in the eastern Pacific coincides with the onset IV of the 1997–1998 El Niño. The phase speed predicted by a linear model in the absence of mean currents for these Kelvin waves is slower than the waves detected in the OGCM. A two-dimensional numerical model is developed which allows for the calculation of equatorial wave phase speed and structure in the presence of a realistic mean flow. This two-dimensional model uses the density structure and zonal current field on a latitude-depth grid from the OGCM as input. It solves for long, low frequency, nondispersive equatorial Kelvin and Rossby waves. A spectral method is used incorporating Chebychev polymonials to reduce the problem to a generalized eigenvalue system. Experiments are run with and without the OGCM derived zonal current field. The ouput is compared to the actual wave velocity and structure seen in the OGCM data allowing for the determination of the importance of wave-mean flow interactions in the real ocean.