Persistent upwelling and front over the Sulu Ridge and their variations

The persistent upwelling and front isolated over the Sulu Ridge are newly detected by a suite of satellite measurements and confirmed by historical cruise observations. A three‐dimensional tide‐circulation coupled model is employed to investigate the dynamic mechanisms. The intense vertical velocity shear induced by the vigorous internal tidal mixing is identified as the vital dynamic factor to trigger the upwelling and nutrient pumping, through the strong tide‐topography interactions. Satellite observations indicate that the pronounced surface cooling and phytoplankton bloom are isolated over the Sulu Ridge throughout the year. The steady front characterizes 10–20 km width and strength over 0.3°C/km near the southern edge. The feature is further confirmed by the numerical simulation and in situ hydrological observation. Modeling experiment suggests that a quasi‐symmetrical vertical circulation bifurcation across the Sulu Ridge due to the asymmetry of tidal currents is the main mechanism to maintain the persistent upwelling and front. Moreover, the local wind forcing plays an important regulating role in the seasonal and interannual variations. The surface cooling and thermal front intensity reach the maximum in winter under the forcing of strong northeasterly monsoons. Due to prolonged El Niño–Southern Oscillation (ENSO) effects, the surface cooling was significantly weakened during 1998 and 2010 summer. Further analysis indicates the fact that the southwesterly monsoon is weakened dramatically as part of the anticyclonic atmospheric circulation anomalies over the northwest Pacific. The latter results from the significant influence of tropical Indian Ocean capacitor effect associated with El Niño–Southern Oscillation.