Baroclinic stability of oceanic long Rossby wave in the North Pacific Subtropical Countercurrent

Based on the analysis of Sea Surface Height Anomaly(SSHA) from satellite observation,it is proved that the oscillation with period of 70～210 days is dominating in two branches of the eastward countercurrent in Subtropical North Pacific: Subtropical Countercurrent(STCC) and Hawaiian Lee Countercurrent(HLCC). It is found that in the STCC the amplitude of the Rossby wave,corresponding to the SSHA oscillation with period of 70～210 days,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 days doesn't increase when the wave propagates westward.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 days is unstable and the amplification mechanism of Rossby wave is baroclinic instability,because there is low potential vorticity water under STCC and the vertical gradient of density is smaller than that in HLCC.Since there is no low potential vorticity water the Rossby wave with period of 70～210 days 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 the unstable Rossby wave with period of 70～210 days.