Modeling surface low-salinity pools formed by heavy precipitation in the Yellow Sea

Abstract

Precipitation plays an important role in the dynamics and thermodynamics of the ocean surface layer. The effects of long duration (several weeks) precipitation events on the surface layer of the mid-latitude marginal sea are not well known. In this study, the influence of recurring heavy precipitation events on surface temperature, salinity, and currents of the Yellow Sea was examined using a regional three-dimensional ocean model. Heavy precipitation (100–500 mm) events for 8–20 days provided strong freshwater buoyancy forcing and created low-salinity pools (LSPs) in the surface layer during the slow northward progress of the East Asian summer monsoon front. The thickness and horizontal extent of the LSPs were dependent on wind stress and total precipitation. During the rainy days, salinity was reduced 1.1–2.0 in the surface layer of the LSP as it became thicker due to the accumulation of freshwater. After the heavy rainfall events, a salt-stratified pycnocline at the bottom of the LSP diminished the vertical exchange of heat and momentum. This made the surface layer 0.2–0.4 °C warmer and the thermocline layer 0.4 °C cooler. The freshwater in the LSP was then dispersed through vertical mixing by winds. A thin (∼2 m) LSP had stronger surface warming and its surface currents were promptly accelerated by the wind. When an LSP was relatively thick (∼10 m), a large-scale (120 km) surface clockwise geostrophic circulation was formed and persisted for three months. The long-lived surface LSP can modify the surface stratification, temperature, and surface currents in a mid-latitude marginal sea. The findings of this study suggest the importance of including precipitation and evaporation processes in ocean models when short (1–7 days) and medium (3–6 months) range ocean forecasts are made in the shelf seas with long duration intensive rainfalls.