A Method for Calculating Thermocline Characteristic Elements in Shelf Sea Area--Quasi-step Function Approximation Method

It is shown from the CTD data collected in Chinese shelf sea area that most of vertical structures of hydrographic elements can be idealized as a 3-layer structure pattern, namely, upper homogeneous layer, thermocline and lower homogeneous layer. In this paper, the quasi-step function approximation method is propsed, i.e., a quasi-step function composed of 3 line segments is used to fit the vertical profile of hydrographic element, the least square method is used to get the two inflection point positions, the upper and lower thermocline bounds, and the slope of the line connecting the two inflection points is taken as the thermocline intensity. The vertical temperature structures in the shelf sea area can be summarized as 4 patterns, positive thermocline, reverse thermocline, unidirectional multi-thermocline and mixed positive and reverse thermocline patterns, and the characteristic elements of different thermocline patterns can be calculated by the least squares fit of one or several quasi-step functions.