Lagrangian analysis of surface transport patterns in the northern south China sea

The present work investigated the surface transport structures in the Northern South China Sea (NSCS) by estimating Lagrangian Coherent Structures (LCSs) in the altimetry-based Finite-Size Lyanpunov Exponents (FSLE) fields. The investigations covered the period of the two LEDS (Living-resource and Ecosystem Dynamics on the Slope) surveys: LEDS2014 (10–30 October 2014) and LEDS2015 (10–30 June 2015), with special focus on the survey area. FSLE analysis allowed us to describe the sampling area as divided regions characterized by different dynamical patterns. Backward longitude and latitude advection experiments were performed in addition to mapped LCSs to depict the pathway and origin of water parcels. The water masses of the sampling area were further categorized by their estimated origins to provide essential insight into the interpretation of the field measurements. The in situ measurements by cruises and satellite images were compared in order to validate the FSLE-derived estimation. The FSLE analysis results revealed completely different surface transport patterns between the two surveys. Cross-slope transport was only observed in LEDS2015 summer survey and was considered as a result of eddy entrainment. FSLE revealed the pathway of the Pearl River plume water from the shelf to the basin along the hyperbolic structure lines. No such cross-slope exchange was detected in LEDS2014 autumn survey while attractive LCS appeared at the inner edge of the survey area and no eddy structures were present.