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

Abstract

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.

Optimum Multiparameter Analysis of Water Mass Structure off Western Guangdong during Spring Monsoon Transition

“…The T-S diagram clearly shows the existence of three types of water masses off western Guangdong: PR diluted water (PRDW) with the lowest salinity (<31 psu), SCS surface water (SCSSW) with high temperature and salinity, and SCS subsurface water (SCSSUW) with low temperature and high salinity. These characteristics are well-documented in previous studies [23,24,36]. Traditionally, the SWTs are defined by identifying prominent end members in T-S diagrams [37].…”

Spice Variables and Their Use in Physical Oceanography

“…No physical justification for the orthogonality property has been proposed by Veronis (1972), Huang (2011), and Huang et al. (2018), Gao et al (2020), Huang (2020a) or Huang (2020b). The fact that imposing the orthogonal property causes the need to choose an arbitrary relative scaling of the salinity and temperature axes provides a strong clue that there is no inherent physical meaning to the orthogonality property.…”

“…This misestimation, by a factor of order 100, of the strength of isopycnal water-mass variations is an unavoidable consequence of the imposition of orthogonality that is, built into HYZ18's potential spicity. No physical justification for the orthogonality property has been proposed by Veronis (1972), Huang (2011), andHuang et al (2018), Gao et al (2020), Huang (2020a) or Huang (2020b). The fact that imposing the orthogonal property causes the need to choose an arbitrary relative scaling of the salinity and temperature axes provides a strong clue that there is no inherent physical meaning to the orthogonality property.…”

“…(2018) (hereinafter HYZ18), and Gao et al. (2020) and Huang (2020a, 2020b) have reverted to pursuing the orthogonal property of Veronis (1972), claiming, without proof, that the “advantages of the orthogonal coordinates are fairly obvious.” Superficially, the perpendicular property is intuitively appealing, but we point out that that there is no physical oceanographic basis for adopting the orthogonal property on the salinity‐temperature diagram, and we draw attention to the following five downsides of enforcing the orthogonal constraint on the potential spicity variable. First, we demonstrate that the isopycnal variations of potential spicity are different to those of physically justified measures (such as spiciness, Absolute Salinity, and Conservative Temperature) by factors of more than 100.…”

“…Recently Huang (2011), Huang et al (2018) (hereinafter HYZ18), and Gao et al (2020) and Huang (2020aHuang ( , 2020b have reverted to pursuing the orthogonal property of Veronis (1972), claiming, without proof, that the "advantages of the orthogonal coordinates are fairly obvious." Superficially, the perpendicular property is intuitively appealing, but we point out that that there is no physical oceanographic basis for adopting the orthogonal property on the salinity-temperature diagram, and we draw attention to the following five downsides of enforcing the orthogonal constraint on the potential spicity variable.…”