Seasonal and intra-seasonal variations of surface hydrological parameters over North Yellow Sea and its implications for applicability of quasi-synchronous ship-based multi-station hydrological investigation data

Abstract

The quasi-synchronous ship-based multi-station hydrological investigation is the most basic and classic oceanographic research method. Nevertheless, the temporal variability of regional hydrological parameters raises the uncertainty of interpreting quasi-synchronous multi-station datasets. There has been a lack of in-depth assessment of the accuracy of survey results. In this study, two hydrological investigations during the summer of 2016 and winter of 2017 in the North Yellow Sea (NYS), accompanied by time series buoy observations, are presented to elucidate its influencing factors and applicability evaluation of cruise hydrological investigation data. The results indicate that the solar radiation and tidal condition account for the diurnal and semi-diurnal cycle of sea surface temperature (SST) in the summer and winter seasons, respectively. The sea surface salinity shows no significant variation during the cruise survey period, while the seasonal characteristic is dominated by the regional ocean circulation. The vertical hydrological structures (stratified in summer or well-mixed in winter) limit or enhance the upward diffusion of bottom resuspended sediment in the offshore area, furtherly causing higher surface turbidity in winter. The upwelling phenomenon is underestimated in summer cruises due to the diurnal variation of SST, while the cold-water tongue may be overinterpreted in winter cruise because of the northerly wind event. The applicability of multi-station hydrological investigation data depends on the comparison of the spatio-temporal scale of the scientific question being studied with the time-varying of the data. The coefficient of variation (CV) of sea surface parameters ranges from 0.03 to 0.36 for SST, 0.02 to 0.04 for SSS, 0.01 to 1.37 for chl a, and 0.06 to 0.50 for Rrs_555. The sea surface parameters display higher variabilities during spring and autumn season than summer and winter season over NYS, and the coastal areas show larger temporal variation than central NYS. It is suggested that the multi-station investigation should be conducted at the time when the hydrological structure is relatively stable as far as possible, especially for the nearshore stations. This study provides a meaningful reference for oceanographic research that although the background hydrologic field can be well described through the ship-based investigation data, attention should also be paid to the influence of time-varying meteorological and hydrological conditions to achieve more objective data interpretation.