Integrated Observation of an Asymmetric Eddy Dipole in the South China Sea

Abstract. Mesoscale dipoles consist of mesoscale eddies with opposite signs. They are globally discovered and play a significant role in eddy-eddy interactions. Due to the complexities of the marine environment, many dipoles are asymmetric, characterized by unequal strengths between the dipole eddies. The strong interaction has been observed between asymmetric dipole eddies, a phenomenon referred to as the “gear-like” process. Specifically, stronger dipole eddies generally drive weaker ones to move around, resulting in a reduction of discrepancies in their kinematic properties, such as rotational speed, amplitude, and eddy kinetic energy. An integrated observation of an asymmetric eddy dipole was conducted in the South China Sea in April 2023. The general characteristics of the dipole eddies were derived from satellite altimeter data, and the evolution of their vertical structures was studied based on the joint data from Argo floats, gliders, drifters, and a survey vessel. Employing rigorous criteria, a 10−day successive coupling process of an asymmetric dipole was identified between a weaker anticyclonic eddy (AE) and a stronger cyclonic eddy (CE) from 13 to 22 April 2023. AE was initially weaker than CE in early April and strengthened when it coupled with CE, which is similar to the “gear-like” process. In addition, the drifting speed of the drifters further confirmed the “gear-like” process between the target asymmetric dipole. The vertical temperature anomalies were surprisingly positive (~0.5 °C at 50−300 m) on the CE periphery and reveal a distinct conical AE structure at 60−350 m. AE induced a significant sinking of dissolved oxygen saturation during the coupling process. Furthermore, the coupling interaction increased CE’s positive temperature anomaly near the contact zone and deepened AE’s temperature and dissolved oxygen saturation structures. Both thermohaline and biological responses provide evidence that the interaction between the asymmetric dipole eddies impacted the vertical transport of water. These findings, on the one hand, support the observations that weaker dipole eddies strengthen after coupling with stronger ones. On the other hand, the results offer valuable vertical structure information of the asymmetric eddy dipole.