Numerical study on genesis of wintertime low sea level anomaly off northwestern Luzon and dominant factor in super-ENSO-related changes

Satellite remote sensing altimetry data has demonstrated that wintertime low sea level anomaly off northwestern Luzon (NWL-SLA) is a common feature. The genesis of the wintertime low NWL-SLA and its response to the most recent two super El Niño-Southern Oscillation (ENSO) in 1997-1998 and 2015-2016 are explored by conducting a series of numerical experiments based on a well-validated three-dimensional South China Sea (SCS) model. Our analysis emphasizes that the generation of low NWL-SLA and its super-ENSO-related variability are triggered by different dynamical processes. The wintertime local positive wind stress curl off northwestern Luzon serves as the primary driver of low NWL-SLA formation. However, remote signals originating in the tropical western Pacific Ocean represent the dominant source to regulate the super-ENSO-related changes in NWL-SLA, rather than the variabilities of local wind stress curl and ocean currents in the vicinity of Luzon Strait. These remote signals originate from the baroclinic component of sea level changes and propagate westward at speeds of the first-mode baroclinic Rossby wave; subsequently, they enter the SCS clockwise through the southernmost gap of Sibutu Passage and Mindoro Strait at speeds of the baroclinic coastal Kelvin wave. Furthermore, faster wave propagation speeds are observed during La Niña years than El Niño years because of the greater reduced gravity associated with stronger stratification during La Niña years.