The influence of merger and convection on an anticyclonic eddy trapped in a bowl
Introduction
Mesoscale eddies are a prominent feature of the ocean circulation. They have a strong influence on biological activity (Chelton et al., 2011), tracer transport (Zhang et al., 2014), and physical and chemical properties of the water column (Dong et al., 2014). In some regions, semi-permanent eddies can be seen throughout the year, at a nearly constant position. Among other examples, two particular cases are the Lofoten Vortex (LV), and the Rockall Trough eddy (RT eddy). These two semi-permanent eddies have the peculiarity to be anticyclonic, and located above a topographic depression – a bowl. The formation of such vortices has recently been examined by Solodoch et al. (2021). Authors showed using idealized simulations that successive merging events form a permanent anticyclone lying in the topographic depression. The dynamics of the resulting vortex depends on the ratio of eddy’s vorticity to topography’s potential vorticity. However, the mechanisms that sustain semi-permanent anticyclones in bowl-like topography such as the LV and the RT eddy are not yet fully understood.
The LV can be found in the Lofoten Basin in the Nordic Seas. It appears as a large anticyclone at the center of the basin. It was first detected by in situ data between 1970 and 1990 (Ivanov and Korablev, 1995). The LV is intensified between 700 and 900 m depth and has a radius of about 30 km (Yu et al., 2017). Two processes are candidate to explain the long lifetime of the LV. First, from observational data, Ivanov and Korablev (1995) and Bosse et al. (2019) argued that wintertime intensification resulting from convection plays a determinant role in sustaining the LV. Second, model studies showed that the LV is sustained by the merger and alignment with smaller vortices generated by unstable boundary currents (Köhl, 2007, Trodahl et al., 2020). In the current state of knowledge, the relative importance of each process is not clear. One of the aims of the present study is to give new answers to this question.
The RT eddy is located in the Rockall Trough, off Ireland in the North Atlantic. It has a clear signature at the sea surface (Heywood et al., 1994, White and Heywood, 1995, Volkov, 2005, Xu et al., 2015), but also at depth with high values of eddy available potential energy (Roullet et al., 2014). This eddy is less sampled than the LV and less known. However, thanks to recent in situ deployments, it has been shown that it is intensified at depth, with a maximum azimuthal velocity of near 500 m depth (Smilenova et al., 2020). It has a radius of approximately 40 km and can reach down to 1500 m. Its lifecycle, as well as the mechanisms that sustain it are yet poorly documented. However, recent model studies by Le Corre et al. (2019) and Smilenova et al. (2020) have shown some evidences that (1) the RT eddy formation is the result of successive mergers of deeply generated submesoscale vortices along the Porcupine Bank, (2) the merger of the RT eddy with these small vortices as well as wintertime convection sustain the RT eddy, and allow it to remain semi-permanent in the Rockall Trough.
In this paper, we investigate the impact of several parameters on the lifecycle of an anticyclonic eddy lying in a topographic depression. In particular, we discuss the impact of merger and convection on the lifetime and shape of the anticyclone. To explore the parameter space, we use an idealized approach based on the Rockall Trough Eddy case. This allows to (1) discuss on the general behavior of anticyclonic eddies in a bowl, and (2) give insights in the particular case of the Rockall Trough Eddy that is yet poorly documented. In Section 2 we present the methods, the numerical simulation setup and the diagnostics performed on outputs. In Section 3 we present the results of our study, the impact of the different parameters on the vortex dynamics. In Section 4 we summarize and discuss the results.
Section snippets
The numerical simulations
In this section, we present the idealized simulations performed for this study. The aim of these simulations is to simulate schematically the dynamics occurring in the Rockall Trough area: a semi-permanent anticyclone (the RT eddy) lying in a bowl-like topography, fed by anticyclonic Submesoscale Coherent Vortices (SCVs) generated hundreds of kilometers away from the main eddy (hereafter, the main eddy designates the eddy that lies approximately in the center of the bowl-like topography, and
Results
In this section, we describe the results of our study. We first explain qualitatively the course of a simulation representative of the RT, i.e., the mRTD simulation. Then we discuss the impact of the different parameters on the evolution of the main eddy.
Summary and discussion
We studied the lifecycle of an anticyclonic eddy trapped in a bowl-like topography, which is subject to the interaction with like-signed SCVs and/or convection. From the analysis of 16 simulations with varying parameters, we show that the balance between merger and bottom drag allows the eddy to have a roughly constant 3D shape throughout several years. On the one hand the vortex merger with small SCVs allows the eddy to grow in size, and intensify at depth. As merger events occur at the SCV
CRediT authorship contribution statement
Charly de Marez: Conceptualization, Methodology, Software, Writing – original draft. Mathieu Le Corre: Software, Writing – review & editing. Jonathan Gula: Writing – review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was funded by the Direction Générale de l’Armement (DGA), France via a full grant for Charly de Marez’s Ph.D. J.G. gratefully acknowledges support from the French National Agency for Research (ANR) through the project DEEPER (ANR-19-CE01-0002-01). Simulations were performed using the HPC facilities DATARMOR of ‘Pôle de Calcul Intensif pour la Mer’ at Ifremer, Brest, France. The authors thank X. Carton for helpful discussions.
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