Mesoscale variability in the northeastern tropical Pacific: Forcing mechanisms and eddy properties
Résumé
The generation mechanisms and properties of the mesoscale eddies in the northeastern tropical Pacific (NETP) are studied using a series of numerical solutions and satellite observations. The spatial and temporal resolution of mountain wind jets over the Gulfs of Tehuantepec and Papagayo are essential for an accurate modeling of the regional mesoscale variability. Three previously proposed eddy generation mechanisms are the local transient wind forcing, the combined low-frequency wind and boundary forcing, and the remote equatorial Kelvin wave forcing. In our model, the transient wind-forcing was represented by wind fields with synoptic variability, and the remote equatorial Kelvin wave forcing by 5-day open boundary conditions with interannual variability. Solutions with and without high-frequency forcings are contrasted to evaluate the contributions of the three mechanisms. The combined low-frequency wind and boundary forcing is the primary eddy forcing mechanism in the NETP. In the Tehuantepec region, the high-frequency wind-forcing contributes more to the mesoscale variability than the remote equatorial Kelvin wave forcing, while in the Papagayo region, their contributions are comparable. Eddies in the Tehuantepec and Papagayo regions are larger in both amplitude and size, and are more likely to deflect equatorward than eddies in the rest of the NETP. The maximum temperature anomaly of eddies is at around 50 m depth. Eddies with lifetimes >6 weeks are more likely anticyclonic, while eddies with lifetimes <6 weeks are more likely cyclonic. The anticyclone-dominance for large-amplitude long-lived eddies is the combined consequence of wind-driven asymmetric dipole spin-up and the evolutionary fragility of large-amplitude cyclones.