El Nino- and La Nina-Induced Sea Level Anomalies in The South China Sea

Abstract The South China Sea (SCS) is recognised as the largest marginal sea within the western Pacific Basin. The El Nino and La Nina phenomena alternately occur along the equatorial Pacific Ocean and have directly influenced the sea surface condition of the adjacent SCS region. Through the thermosteric effect mechanism, the relative location of the warm water pool along the affected region can impose additional sea level anomalies (SLA) observed. The El Nino- and La Nina-induced SLA is one of the understudied impacts, yet essential, reflecting additional deviation of projected sea level variation. This study aims to investigate the contribution of these phenomena to the additional SLA observed within the SCS region. The intense period of El Nino 2015 and La Nina 2010 were selected, and their corresponding SLA data were analysed. The study revealed that, in El Nino years, there is a discernible negative trend in the mean sea level anomalies, whereas, in La Nina years, a positive anomalies trend is observed within the study area. The analysis of sea surface temperature (SST) data indicates that the thermosteric effect plays a significant role in the fluctuations of sea level observed during these climatic phenomena. A decrease in sea surface temperature during the peak of El Nino over the SCS is associated with an average reduction of up to 15 cm in SLA. Conversely, an increase in SST during the peak of La Nina is linked to a 20 cm increase in the SLA. The highly affected areas of the Philippines and the northern area of Borneo demonstrate a 12-month average of 12 cm SLA range corresponding to both events. This phenomenon indicates that El Nino and La Nina events are crucial in propelling the region's climatic sea level fluctuations and offer valuable perspectives for enhancing disaster preparedness.