Spatio-temporal patterns of Secchi depth in the waters around the Korean Peninsula using MODIS data

Abstract

Secchi depth (ZSD) is an important variable for the measurements of long-term changes in the water transparency of oceanic and coastal ecosystems. This study aims at analyzing the spatial and temporal variations of MODIS-derived ZSD time-series during 2000–2012 in the adjacent waters around the Korean Peninsula, highlighting the environmental variables affecting variations of ZSD with various water depths. A semi-analytical model is used to estimate the diffuse attenuation and beam attenuation coefficients from satellite data, and the reciprocal of the sum of the two parameters was compared with the in-situ ZSD to retrieve a satellite-based ZSD. In general, the open sea waters in the East Sea/Sea of Japan have higher ZSD than the coastal waters in the Yellow Sea. From the analyses, distinct seasonality was found in the monthly average images of MODIS-derived ZSD data and spatio-temporal patterns of the Empirical Orthogonal Function (EOF), and the seasonal variation of ZSD was associated with the variability in the ocean-atmosphere dynamics including the wind and current as well as chlorophyll-a. In the coastal and shallow waters in the Yellow Sea, prevailing north westerly winds produce turbid waters in winter, while in deep waters in the East Sea/Sea of Japan, water transparency is reduced by the spring bloom of chlorophyll-a. There are two clear ZSD peaks within a year in the East China Sea associated with the Kuroshio Current. A similar trend was observed in the South Sea, but less clear due to complex mixing processes of currents and effects of wind. Correlation of in-situ ZSD with water depth was also found suggesting the need for consideration in analyzing satellite ZSD data.