Vigorous Forced Submesoscale Instability Within an Anticyclonic Eddy During Tropical Cyclone “Haitang” From Glider Array Observations

Abstract

In this study, we examine intensive observational measurements from a 12-glider array in the South China Sea, and reveal that tropical cyclone “Haitang” created the conditions for the development of several types of forced submesoscale instabilities within a mesoscale anticyclonic eddy. The anticyclonic eddy shed from the Kuroshio loop current in the Luzon Strait and propagated toward the South China Sea. Fine-scale temperature and salinity observation from gliders captured the complex mesoscale frontal structure induced by mesoscale strain around anticyclonic eddy (AE). Various favorable conditions for submesoscale instabilities show significantly different spatial distributions as well as temporal evolution characteristics in the AE. Analyses indicate that the occurrence probability of forced symmetric instability (SI) and gravitational instability (GI) during the tropical cyclone (TC) period (∼5 days) is found to be 2 times higher than that during the non-TC period (∼25 days). Heat loss creates conditions for GI in the upper part of the negative potential vorticity (PV) layer, and GIs tend to be distributed inside the eddy. Strong wind stress induced by the TC promotes the injection of negative PV through cross-front Ekman buoyancy flux, leading to the occurrence of SI at the edge of the eddy. During the TC, stable wind fields are more favorable for the development of submesoscale instability compared to rotating wind fields. The effect of strong winds breaks the normal diurnal cycle of SI, creating conditions for active submesoscale instabilities at midday. These findings help us to understand submesoscale air-sea interaction processes.