Changes in Underwater Visibility Due to Turbidity Associated with Typhoon Soulik

Abstract

Changes of visibility in the water column off the southern coast of Korea due to the effects of Typhoon Soulik (Aug. 22-23, 2018; 955 hPa; maximum wind speed, 40 m/s) were investigated. The beam attenuation coefficient (BAC) was measured using a transmissometer (C-Star; Wet Labs, Inc.), and visibility range was calculated using the conversion method employed by the US Navy. For comparison and verification of the data, a conductivity-depth-temperature (CTD) probe (SBE19 Plus; Sea-Bird Scientific) equipped with optical backscatter and fluorescence sensors was deployed simultaneously with the transmissometer. The Secchi depth was also estimated during each cast to improve accuracy. The Secchi depth deviated by up to 20% from the calculated visibility range. Additionally, during continuous observation of BAC from the seafloor upward, a light source (color temperature: 6,500 K) was installed on a 5-m bar with length markings, and a video camera recorded changes in visibility. Daily Geostationary Ocean Color Imager (GOCI) satellite data were used to identify changes in surface visibility in the study area. Before the typhoon, the visibility in the water column was in the range 25-30 m. Near the seafloor, visibility approached 0 m due to a 3-6-m turbidity current. The visibility distance was significantly shorter in the bottom layers based on video images. Four days after Typhoon Soulik had passed over the Korean Peninsula, the visibility was in the range 2.6- 9.0 m in the upper layers of the water column, increasing gradually to 11 m in the middle layers. The size of the turbid bottom current did not differ significantly between observations before and after the typhoon. Surface visibility calculated from satellite data also decreased by 20-30% compared to that before the typhoon.