Why are there Rossby wave maxima in the Pacific at 10°S and 13°N?

Observations indicate the existence of two bands of maximum thermocline depth variability centered at similar to10degreesS and 13degreesN in the tropical Pacific Ocean. The analysis of a numerical integration performed with the National Center for Atmospheric Research ocean general circulation model (OGCM) forced with observed fluxes of momentum, heat, and freshwater over the period from 1958 to 1997 reveals that the tropical centers of thermocline variability at 10degreesS and 13degreesN are associated with first-mode baroclinic Rossby waves forced by anomalous Ekman pumping. In this study the factors that may be responsible for the Rossby wave maxima at 10degreesS and 13degreesN, including the amplitude and spatial coherency of the forcing at those latitudes, are systematically investigated. A simple Rossby wave model is used to interpret the OGCM variability and to help to discriminate between the different factors that may produce the tropical maxima. These results indicate that the dominant factor in producing the maximum variability at 10degreesS and 13degreesN is the zonal coherency of the Ekman pumping, a characteristic of the forcing that becomes increasingly more pronounced at low frequencies, maximizing at timescales in the decadal range. Local maxima in the amplitude of the forcing, while not explaining the origin of the centers of variability at 10degreesS and 13degreesN, appear to affect the sharpness of the variability maxima at low frequencies. Although the Rossby wave model gives an excellent fit to the OGCM, some discrepancies exist: the amplitude of the thermocline variance is generally underestimated by the simple model, and the variability along 13degreesN is westward intensified in the wave model but reaches a maximum in the central part of the basin in the OGCM. Short Rossby waves excited by small-scale Ekman pumping features, or the presence of higher-order Rossby wave modes may be responsible for the differences in the zonal variance distribution along 13degreesN.