Prediction of nitrate concentration from two end member mixing in the Southern East China Sea

Abstract

Monthly hydrographic surveys were conducted along two transects across the Kuroshio frontal zone in the southern East China Sea from August 1990 to July 1991. The concentration of nitrate in the shelf water was linearly related to temperature. However, the relationship varied with time. This relationship could have resulted from two end member mixing between the nitratedevoid surface water and the nitrate-laden upwelled water. While the temperature of surface water was temporally variable, the temperature-nitrate properties of the upwelled water were almost invariant. The temperature of the surface end member (Ts) correlated well with the mean air temperature (Ta) at a weather station in the study area according to the equation: Ts = 15.3 + 0.45 × Ta The upwelled water, which was brought to the shelf by bottom intrusion of the Kuroshio, had properties similar to those of the Kuroshio subsurface water at σθ = 25.8 with a mean temperature of 14.8°C and a nitrate concentration of 13.2 μM. With the exception of the data obtained in September 1990, the concentration of nitrate in the shelf water can be predicted from the mean air temperature with a mean error of 0.9 μM from the observed values. The anomaly observed in September 1990 might have been the result of the unsteady condition during the reversal of the direction of the monsoon. There was also a slight deviation from the predicted trend in March 1991 when there was a seaward excursion of the Continent Coastal Water which may have constituted a third end member in the mixing process. In general, this simple mixing model is applicable to the middle and outer shelf north of Taiwan