Scale-Dependent Temperature-Salinity Compensation in Frontal Regions of the Taiwan Strait

Abstract

Based on high-resolution, cross-frontal towed measurements, this study investigates temperature-salinity (T-S) compensation and its scale dependence in the Taiwan Strait. In winter and spring when the northeasterly monsoon is prevailing, colder and fresher waters flow southward along the coast of the Chinese mainland, which encounter the northward-flowing, warmer and saltier waters in the Taiwan Strait. Two slanted density fronts are generated across the interfacing zone forming a cone-shaped isopycnal distribution in the cross-frontal direction. Analyses based on the density ratio and Turner angle suggest the expected S-dominated and T-dominated types of density variations at the western and eastern density fronts, respectively. Exact T-S compensation is observed within the interfacing zone. Analysis of the scale dependence indicates that temperature and salinity get more (less) compensated from O(10 km) to O(1 km) scale in the S-dominated (T-dominated) frontal zone. Although contrasting density compensating features are observed on the two flanks of the transition zone, they can both be interpreted by the restratification-cooling mechanism. Specifically, the overturning cells due to submesoscale instabilities in the upper mixed layer slump isopycnals in the frontal zone, inducing shoaling of the mixed layer at both ends of the sampled sections. Surface cooling results in larger temperature drops in the shallower mixed layers, and thus increases (decreases) the degree of T-S compensation in the S-dominated (T-dominated) frontal zone at the scale for submesoscale instabilities to develop.