Baroclinic Stability of Oceanic Rossby Wave in the North Pacific Subtropical Eastwards Countercurrent

Abstract Based on the analysis of Sea Surface Height Anomaly (SSHA) from Satellite observation, it is proved hat the oscillation with period of 70~210 day is dominating in two branches of the Eastward Counter current in Subtropical North Pacific: Subtropical Countercurrent (STCC) and Hawaiian Lee Countercurrent (HLCC). It s found that in STCC the amplitude of the Rossby wave, corresponding to the SSHA oscillation with period of 0~210 day, increases when the wave propagates westward, and the amplitude is the largest in east of Taiwan; In HLCC, the amplitude of Rossby wave with period of 70~210 day doesn't increase when the wave propagates estward. Based on the relation of Rossby wave's growth rate with mean current and stratification from 2.5‐layer model, it is revealed that in STCC area the Rossby wave with period of 70~210 day is unstable and the mplification mechanism of Rossby wave is baroclinic instability, because there is low potential vorticity water under STCC and the vertical gradient of density is smaller in HLCC. Since there is no low potential vorticity water the Rossby wave with period of 70~210 day is stable in the west of HLCC, there is no amplification event when Rossby wave propagates westwards in HLCC. In STCC and HLCC, the Rossby wave with 1 year period is always stable, its amplitude is smaller than that of unstable Rossby wave with period of 70~210 day.