The impact of consecutive typhoons on the hydrodynamic conditions in a small bay in the Taiwan Strait

Global warming is expected to increase the frequency and severity of compound weather, ocean and climate events. These can lead, due the interplay of multiple climate drivers and/or hazards, to far greater societal and environmental impacts than the sum of the isolated individual events. Multiple strong consecutive tropical cyclones occurring in quick succession can be classified as temporally compounding events. These events are associated with heavy rainfall, river flooding and storm surges. In the ocean, they have a combined and cumulative impact on the local hydrodynamic conditions, e.g. reduced salinity by the increased freshwater input, which in turn affects local ecosystems.

This study aims to evaluate the combined effect of strong winds and increased freshwater input during those compound events on the local salinity and circulation, while focusing on the area around Dongshan Bay, Fujian (China). The bay serves as an ideal case study, as the northern South China Sea has been increasingly hit by two or more strong consecutive typhoons in recent years.

For the investigation, the regional shelf ocean circulation model HAMSOM is used to downscale global climate scenarios to an appropriate regional scale through a nested, uncoupled modelling approach. The outer model setup covers the southern East China Sea, the Taiwan Strait and the northern South China Sea (SCS). It resolves the most important oceanic features for this study, including the circulation in the SCS, the influence of the Kuroshio and the throughflow in the Taiwan Strait. The outer model provides the lateral boundary conditions for the inner model, which has a high resolution of approximately 400m to adequately resolve the area around Dongshan Bay to the west coast of Taiwan. The atmospheric forcing and river discharges are provided by an hourly East-Asia Cordex dataset, which has proven to reproduce past typhoon tracks in the SCS quite realistically. The model setup allows to run control simulations with and without freshwater input to assess the effect of strong consecutive typhoon events on the local salinity. The results can then be used to assess the vulnerability of local ecosystems to these type of compound events.