Equatorial forcing of interannual Rossby waves in the South eastern Pacific.
Andres Vega, Yves duPenhoat, Boris Dewitte and Oscar Pizarro
Long baroclinic Rossby waves generated at the western coast of south America are the main mechanism for the westward transfer of coastal perturbations. These waves are important in the adjustement of pycnocline at low frequencies and can affect weather patterns and climate in the surrounding regions. Whereas the variability along the coast is strongly modulated by coastal trapped waves (CTW) remotely forced at the equator, the off-shore region is characterized by westward Rossby wave propagations at time scales varying from semi-annual to interanual, with an amplification of the signal during warm ENSO events. To investigate the forcing of Rossby waves and their propagation characteristics in the southeastern Pacific (10°S to 40 °S), altimetric data and model simulation outputs were used for the 1992-2000 period. Rossby waves were simulated using a reduced-gravity linear model (RWM) forced by wind stress from ERS-1/2 and thermocline depth anomalies at the coast. The latter were provided by a high resolution Eastern Boundary linear Model initialized at the equator by a linear equatorial model forced with ERS-1/2 winds. A formulation for dissipative processes was included in the RWM in order to simulate the observed decrease of Rossby waves amplitude as they propagate westward. Model outputs were compared with observations from a composite altimetric product of satellites TOPEX/POSEIDON and ERS-1/2. Results north of 25 °S indicate that the Rossby waves forced by local wind stress represent less than 30 % of the observed variance. This suggests that equatorially forced thermocline fluctuations are the main forcing of Rossby waves in the northern part of south American coast. South of 30 °S, local forcing becomes dominant and variability associated with polar front forces the low-frequency propagative signals.