New insights into the seasonal cycle and interannual variability of water, heat and salt budgets over the South China Sea from a high-resolution numerical study.

With 70% of the planet covered by water, oceans and seas are important drivers of global climate. The effects of global warming and human pressure are unevenly distributed across ocean regions, and semi-enclosed seas and shallow coastal areas are particularly vulnerable to global changes. Located in the Western Pacific ocean-atmosphere system, characterized by a large and complex topography and subject to many variability factors such as typhoons, tropical monsoons, ENSO, etc., the South China Sea (SCS) plays an important role in the exchange of water, heat and salt between the Pacific and Indian Oceans, Southeast Asian seas and the atmosphere, and is an ideal location for studying ocean-atmosphere interactions and the effects of climate variability and change.

This study provides new insights into the water, heat and salinity budgets over the SCS from annual to interannual time scales using numerical methods. A high-resolution (4 km) configuration of the SYMPHONIE ocean model developed by the SIROCCO group (sirocco.obs-mip.fr), including an explicit representation of tides, has been implemented over this region within the framework of the Vietnamese-French International Joint Laboratory LOTUS (lotus.usth.edu.vn). The simulation is analysed over 9 years (2010 - 2018) to investigate the contribution of lateral exchanges through different straits (the South China Sea Throughflow SCSTF) and through air-sea and land-sea interactions to the water, heat and salt budgets over the area.

On a climatological scale, the SCS receives an average annual water input of 4.5 Sv, mainly from the Luzon Strait. This water is then discharged to neighbouring seas through the Mindoro Strait (49%), Taiwan Strait (28%) and Karimata Strait (22%). 70% of this Pacific water input is transferred to the Indian Ocean through the SCSTF. The seasonal variability of water volume and salinity budget is driven by the lateral interoceanic exchange, while the annual cycle of the heat budget is controlled by atmospheric fluxes. On interannual time scales, the variability of the water volume budget is controlled by the annual atmospheric water flux. Similarly, the interannual variability of salinity is mainly driven by the lateral interoceanic exchange of water fluxes, which in turn is driven by the variability of the atmospheric water input. In particular, we show that the saltening of the SCS between 2012 and 2016 is mainly due to the ENSO-induced decrease in rainfall freshwater input, which was compensated by an increase in (saline) seawater import across Luzon from the western Pacific. The salinity of the inflowing seawater slightly enhanced this effect, but played a minor role. The interannual variability of the heat content is driven by the variability of the total lateral heat flux through the interoceanic straits, which in turn is driven by the variability of the temperature of the outgoing and incoming waters.