Water Exchange Through the Upper and Middle Luzon Strait Using the Sigma–Pi Diagram.

Water exchange in the Luzon Strait (LS) is critical for layered circulation in the South China Sea (SCS); however, observational evidence of sandwich‐like water exchange is scarce. This study presents a comprehensive analysis of the meridional and zonal spatial patterns of water exchange in the upper and middle LS, along with its seasonal variations in a sigma–pi diagram using Argo profiles. As observed viewed from the perspective of SCS, inflow and outflow occur in the upper and middle layers, respectively. Upper‐layer Kuroshio intrudes into the SCS primarily in the northern and central regions of the LS, extending along the continental shelf into the inner SCS. A significant middle‐layer eastward outflow is evident at 26.7–27.56 kg/m3 (500–1,500 m) in the northern part of the strait, extending to 123°E. The Kuroshio intrusion intensifies during the winter, whereas the middle‐layer outflow is most pronounced in the autumn. Plain Language Summary: Understanding the circulation in marginal and inland seas is closely tied to comprehending the exchange of water between straits connecting neighboring oceans. The Luzon Strait, as a semi‐enclosed marginal sea, holds a pivotal role in shaping the layered circulation in the South China Sea. This study utilizes an extensive data set of Argo profiles to provide observational evidence of a layered zonal water exchange in the upper and middle Luzon Strait, offering detailed insights into the meridional and zonal patterns of water exchange and how they vary with the seasons. It reveals that inflow and outflow processes occur in the upper and middle layers observed viewed from the perspective of South China Sea, respectively. The upper‐layer intrusion of the Kuroshio is more prominent during the winter, while the middle‐layer outflow is most noticeable in the autumn. This research enhances our understanding of water exchange in the Luzon Strait and contributes to our knowledge of processes like water renewal, heat and salt budgets, and biogeochemical cycles in the South China Sea. Key Points: A novel insight into water exchange in the upper and middle Luzon Strait is provided using the sigma–pi diagramMeridional distribution of inflow (outflow) zonal water exchange in the upper (middle) Luzon Strait are describedUpper‐layer inflow was enhanced in winter, whereas middle‐layer outflow was significant during the autumn [ABSTRACT FROM AUTHOR]