Scale‐Dependent Temperature‐Salinity Compensation in Frontal Regions of the Taiwan Strait. Issue 2 (25th January 2023)

• Record Type: Journal Article
• Authors: Lin, Hongyang; Xu, Siyu; Liu, Zhiyu; Hu, Jianyu; Zhang, Fangtao; Cao, Zhiyong
• Is Part Of: Journal of geophysical research. Volume 128:Issue 2(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2023-01-25
• Subjects: Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2022JC019134
• Languages: English
• ISSNs: 2169-9275

Abstract

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. Plain Language Summary: Variations of seawater density are determined mainly by temperature variations (T‐dominated) in most of the global ocean, whereas salinity variations dominate in regions that are strongly influenced by river runoff or precipitation (S‐dominated). The phenomenon that temperature and salinity having opposing effect on density is referred to as temperature‐salinity (T‐S) compensation. Using high‐resolution field measurements, this study investigates the T‐S compensation and its dependence on spatial scales in frontal regions of the Taiwan Strait. A prominent T/S front was observed in the Taiwan Strait in winter and spring, with S‐dominated type of coastal waters on the mainland side and T‐dominated type of waters in the central strait. There was no density front co‐located with the T/S front due to T‐S compensation; instead, there were two slanted density fronts located at different sides of the T/S front. Further analysis suggests that the degree of T‐S compensation increases (decreases) with decreasing spatial scale in the western S‐dominated (eastern T‐dominated) front, which can be explained by the proposed restratification‐cooling mechanism. Specifically, submesoscale instabilities induce secondary circulations which generate a shallower mixed layer at the west (east) end of the western (eastern) front, leading to a larger temperature reduction under surface cooling conditions. Key Points: In winter and spring, two slanted density fronts are formed in the Taiwan Strait with a cone‐shaped isopycnal distribution Temperature and salinity are more (less) compensated from O (10 km) to O (1 km) scale in salinity‐dominated (temperature‐dominated) region The observed scale‐dependent density compensation features can both be interpreted by the restratification‐cooling mechanism