Dynamics Of Low-Frequency Sea Level Variations In The Tropical And Subtropical Western North Pacific

Low-frequency sea level variations (including interannual, interdecadal and long-term trend variations) of the tropical and subtropical western North Pacific and its dynamic are studied based on the AVISO altimeter data, long-term tidal gauges observations and one-half-layer reduced-gravity (GR) model results. We research the following contents:the contribution of wind-driven Rossby wave to the low-frequency sea level variations in study region in a reduced-gravity model; the influence of the vertical velocity shear and baroclinic instability of Subtropical Countercurrent-North Equator Current(STCC-NEC) system on the sea level variation; the formation and extension of the North Pacific Subtropical Mode Water(STMW), the overlying atmospheric forcing at recirculation gyre and their relationship with the subtropical Pacific sea level variation; predictability of sea level low-frequency variation.Root-Mean-Square (RMS) of sea level at tropical, subtropical Pacific and Kuroshio Extension is intense, but low-frequency sea level RMS is week at subtropical region. South of 20°N low-frequency sea level variation is remarkable and has a meridionally coherent structrue, but conversely north of 20°N. Low-frequency sea level at study region is negative correlative to ENSO and PDO index, which indicate that low-frequency sea level variations are significant affected by the ENSO and PDO events.Based on tidal gauges data, wind-driven one-half-layer GR model very successfully reproduces the interannual sea level variation observed by tidal gauges and indicates the contribution of wind forcing to low frequency sea level variation in tropical North Pacific. The larger distance between simulating sites and boundary sites, there is smaller correlativity between simulating and observed data. Poor model predictability is the result of surface wind stress curl error accumulated along the baroclinic Rossby wave characteristics initiated from the boundary sites. Based on altimeter data the model well hindcasts the low-frequency sea level variations in the west date-line region 8°-20°N band, while predictive skill is poor in the region south of 8°N, west of 165°E and 20°-30°N band. In the well predictive region, low-frequency sea level variations can be explained by propagation of long baroclinic Rossby waves forced by wind stress curl anomalies. This area works as interannual information passage, and sea level abnormal (SLA) caused by wind stress curl anomalies accumulation is transported westward by Rossby wave. So western sea level variation this area is predictable by eastern SLAs and the wind stress curl anomalies. Remarkerble positive wind stress curl is found at this latitude in the almost all time. Prominent positive wind stress curl related to El Nino event cumulates at the tropical Pacific, which brings low sea level. This low anormality is propagated westward by Rossby wave and the whole region shows positive SLA. In the poor simulating region, it is possible that the model is hard to explain low-frequency sea level variation or low frequency sea level driven by wind of this region is not remarkable. Limitation of model itself and less low-frequency sea level amplitude are responsible for poor result. Poor predictive region is corresponding to surface wind stress curl negative anomaly and eastward geostrophic zonal flow. The correspondent of the location illuminates their internal connection. The predictive skill at different latitude is related to the Newtonian dissipation rate. The model result will be improved when the dissipation rate decreases with latitude increasing.Low-frequency signal propagating westward is significant in the subtropical west Pacific (zonal band 20-30°N). Eddy kinetic energy level is quite high at the STCC region, where low-frequency SLA is different to that of north 25°N. Overall, SLA is correlated with the EKE with few month lag. The SLA of the STCC western region can be predicted from the EKE, especially low-frequency variation. The SLA of this region is positive correlated with Ekman transport convergence term. Surface temperature gradient convergence within the STCC band brings large interannual-varying vertical shear in the STCC-NEC's background flow, and this large vertical shear induces enhanced baroclinic instability which causes increased eddy kinetic energy level. This progress affects low-frequency sea level variation of this region.Subtropical Pacific north to 25°N borders up North Pacific Subtropical Mode Water (STMW) formation region, so formation and extension southward of the STMW influence sea level low-frequency variation. Not only weak potential vorticity (PV) year but also rich PV year, STMW outcrops north of 28°N and south of the Kuroshio extension, and extends as south as to 25°N under 100m bellow. In 1993-97, corresponding deep winter mixed layer and large thickness of the 16°-18℃, low potential vorticity value of STMW and negative anomaly sea level are found. Whereas, in 1998-2001 shallow winter mixed layer and thin the 16°-18℃thickness are go with positive anomaly PV and sea level. Lagging the 16°-18℃thickness variation 6 months, SLA and the 16°-18℃thickness come to maximum negative correlation-0.45. And SLA is positive correlated to PV value with 6 months lagging.Winter surface net heat flux is negative with winter maximum mixed layer depth at STMW formation region. Intense surface momentum flux reduces losing heat, which brings low sea level in summer and autumn at subtropical region, vice versa. In the interannual time scale, winter maximum wind stress anomaly show positive correlation with maximum winter mixed layer depth. Surface net heat flux and momentum flux is lead-lag correlated to SLA of subtropical. When lager momentum flux and negative net heat flux bring low-PV value STMW formation at the KE recirculation gyre, propagating STMW southward to 25°N lead to low sea level. Subtropical SLA responds to the overlaying atmosphere forcing of the formation region of STMW with few months lagging.