Using the Sigma-Pi Diagram to Analyze Water Masses in the Northern South China Sea in Spring

The temperature-salinity (T-S) diagram is widely used in water mass analysis, but the boundaries between water masses are vaguely distinguished by conventional T-S-based methods. Herein, we propose a new method based on the potential density-potential spicity (sigma-pi) diagram. The new method has been applied to the conductivity-temperature-depth data collected in the northern South China Sea during a spring cruise in 2011. The water masses in the study region are classified into 13 types according to both the standard deviation of potential spicity in each potential density layer and the water volumetric distribution in the sigma-pi space. The results suggest that this new method is reasonable and robust for classifying water masses in the sigma-pi space as compared to previous methods based on the traditional T-S space. In addition, the westward intrusion of the West Pacific Ocean water to the northern South China Sea can be clearly detected by the tongue-like potential spicity structure and relatively high potential spicity patches on potential density layers, further verifying the robustness and efficiency of our method in the water mass analysis.