New Insight Into the Formation Mechanism of Wintertime Thermal Front West of Luzon Island.

Wintertime thermal front west of Luzon Island (WTFWLI) plays a vital role in local marine biogeochemical environment, geostrophic thermal advection of Luzon cold eddy (LCE) was once recognized as the causation of WTFWLI. However, current investigation based on multi‐source data revealed poor spatial correspondence between WTFWL and LCE over an interannual scale during 1993–2022, highlighting the necessity of revisiting the mechanism of WTFWLI. Our analysis shows that formation of WTFWLI was mainly caused by a northward warm tongue caused by geostrophic thermal advection of a northward current west of Luzon Island. The northward warm tongue encounters cold water west of Luzon Island and forms the WTFWLI at the boundary of cold and warm water. Our study provides a new explanation for the formation mechanism of WTFWLI, which will help deepen people's understanding of thermal front dynamics. Plain Language Summary: Wintertime thermal front west of Luzon Island (WTFWLI) plays an important role in sea‐air interaction and marine ecosystem, previous studies pointed out that the formation of WTFWLI was related with the Luzon cold eddy (LCE). Warm (cold) water was located in the southeast (northwest) part of the LCE due to geostrophic thermal advection, then leading to the formation of WTFWL at the boundary of cold and warm water. However, current investigation shows that there was poor spatial correspondence between WTFWLI and LCE in winter from 1993 to 2022, which is inconsistent with previous conclusions. Further analysis shows that formation of WTFWLI was mainly caused by a northward warm tongue caused by geostrophic thermal advection of a northward current west of Luzon Island. The northward warm tongue encounters cold water west of Luzon Island and forms the WTFWLI at their boundary. This study provides a new insight into the formation mechanism of WTFWLI, which will raise people's awareness of thermal front dynamics. Key Points: A new insight was provided into the formation mechanism of thermal front west of Luzon Island in winterThe thermal front west of Luzon Island was mainly caused by the encounter of a northward warm tongue and cold waterThe northward warm tongue was caused by a northward current west of Luzon Island [ABSTRACT FROM AUTHOR]