Vertical structure of an OGCM simulation of the equatorial Pacific ocean in 1985-1994
Boris Dewitte, Gilles Reverdin and Christophe Maes
We investigate the vertical structure of the variability in the equatorial Pacific in a high-resolution Ocean General Circulation simulation for 1985-1994. Near the equator the linear vertical modes are estimated at each grid point and time step of the OGCM simulation. The characteristics of the vertical modes are found to vary more in space than in time. The contribution of baroclinic modes to surface zonal current and sea level anomalies is analysed. The first two modes contribute with comparable amplitude but with different spatial distribution in the equatorial wave guide. The third and fourth modes exhibit peaks in variability in the east and in the westernmost part of the basin where the largest zonal gradients in the density field and in the vertical mode characteristics are found. Higher-order mode (sum of third to the eighth mode) variability is the largest near the dateline close to the maximum in zonal wind stress variability. Kelvin and first-meridional Rossby components are derived for each of the first three baroclinic contributions by projection onto the associated meridional structures. They are compared to equivalent ones in multi-mode linear simulations done with the projection coefficients and phase speeds derived from the OGCM simulation. The simulations reveal that in addition to the first mode forced equatorial Kelvin and Rossby waves earlier found in the data, forced waves higher vertical modes should also be observable. For the first two vertical modes, the anomalies in the linear and the OGCM simulations have a similar magnitude and usually present similar propagation characteristics. Phase speed characteristics are however different in the eastern Pacific with larger values for the OGCM. The effect of zonal changes in the stratification is tested in the linear model for a stratification change located either in the eastern or in the western Pacific. This results in a significant redistribution of energy to higher modes via modal dispersion. In particular the third mode increases to a magnitude closer to the one in the OGCM simulation. Gravest modes are also affected. This suggests that modal dispersion plays an important role and should be considered when interpreting data as a combination of linear long equatorial waves.
Acknowledgements. Computing supports for running the OGCM was provided by the Institut du Developpement et des Ressources en Informatique Scientifique (IDRIS). Corinne Michau (CNES) provided unvaluable technical assistance. The authors would like to thank Pascale Delecluse and Jean François Minster who were instrumentals in initiating this study. We particulary thank Rui Ponte for stimulating discussions during his stay at LEGOS and Claire Périgaud for her comments and suggestions during Boris Dewitte’s visits at JPL . We also thank Vincent Echevin for interesting discussions and Eric Guilyardi for presenting part of the results of this study at the IAPSO conference in Melbournes (July 1997). Thanks are adressed to Ken Clarke, Denise Kriesel, Eric Houplain and Markus Durstewitz for their support during the course of this study. Fruitful discussions with Yves DuPenhoat, Joël Picaut, Jean-Philippe Boulanger and Christophe Menkes were also appreciated. Finally, the author wish to thank Paul S. Schopf, Julian P. McCreary and an anonymous reviewer for their fruitful comments. Funding for B. Dewitte and G. Reverdin was provided by CNRS (Centre National de la Recherche Scientifique). Funding for C. Maes was provided by AGORA (Project of the environment program of the EEC).