| 16 Jun 2025
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.
This manuscript integrates multiple observations from satellite altimeters, Argo floats, gliders, drifters, and survey vessels to investigate a case of an asymmetric eddy dipole in the South China Sea, with a particular focus on the evolution of its vertical structure. However, two reasons prevent me from recommending it for further consideration in Earth System Science Data.
(1) I cannot identify sufficient novel aspects of this work to match the high impact expected for ESSD. Although the authors provide a dataset collected from multiple observation platforms, it is limited to a single local case of an asymmetric eddy dipole. Therefore, I do not believe this dataset has the potential to attract sufficient interest from the international research community. As noted in the authors’ review in the introduction, many previous studies have revealed the vertical structure of asymmetric eddy dipoles using observational data (Line 50-63). In addition, the results of this manuscript mainly offer a simple descriptive analysis of the observations, without presenting any new findings that distinguish this study from previous work. The “gear-like” process has already been proposed in their previous studies based on global analyses. The vertical structures of temperature, salinity, dissolved oxygen, chlorophyll, and zooplankton have already been well documented in numerous previous studies. Although I acknowledge that most of those studies focused on anticyclonic and cyclonic eddies rather than asymmetric eddy dipoles, it remains unclear what new findings this work provides that distinguish it from previous research using the same type of dataset. The authors claim that their observations provide evidence of the impacts of asymmetric eddy dipoles on the vertical transport of water. However, this point has already been well demonstrated by previous numerical and observational studies, such as Guidi et al. (2012).
(2) As a manuscript submitted as a data description paper, I believe the current style and format do not provide sufficient details about the dataset. I recommend that the authors pay more attention to thoroughly describing the data collection methods, sensors used, processing procedures, temporal resolution, instructions, and application prospect, especially for the new subsurface observations. In addition, as this is a data description paper, I recommend that the authors share their code for data processing. At present, the manuscript reads more like a research article than a data paper.
Other comments:
Line 101 The half-power wavelength cutoffs of 20° in longitude and 10° in latitude appear excessively large. For comparison, Pegliasco et al. (2022) set this value to 700 km.
Line 171 T/S anomalies are calculated using monthly WOA dataset?
In Figures 8, 9, and 10, it would be better to change the color of the missing values, since white is already used in the colormap.
In Figure 12, I am surprised that diel vertical migration of zooplankton was not observed. It may be more appropriate to plot the results on a logarithmic scale.
Guidi, L., Calil, P. H., Duhamel, S., Björkman, K. M., Doney, S. C., Jackson, G. A., ... & Karl, D. M. (2012). Does eddy-eddy interaction control surface phytoplankton distribution and carbon export in the North Pacific Subtropical Gyre?. Journal of Geophysical Research: Biogeosciences, 117(G2).