Aspect Ratio of Eddies Inferred From Argo Floats and Satellite Altimeter Data in the Ocean

Abstract

Mesoscale eddies dominate the majority of oceanic kinetic energy. The size of eddies is one of the basic properties; however, little is known about it because of lack of observations. Especially, their global vertical extent has still not been well investigated. Here, a ratio relationship of height to length scale for an eddy is derived based on dynamical constraints. Global satellite altimetry data combined with Argo float observations are used to calculate the aspect ratios of mesoscale eddies in the global ocean, and then their vertical scale is inferred. Approximately 25% of the total identified eddies with lifetimes ≥16 weeks from January 2002 to December 2017 trapped at least an Argo float near the eddy center. The result shows that the size of eddies has an obvious latitudinal dependence. Eddies at middle-low latitudes tend to be "large and shallow" with small aspect ratios, and eddies at high latitudes tend to be "small and deep-reaching" with large aspect ratios. Such results are coarsely considered as a competition between rotation and stratification. Eddy aspect ratios in middle-high-latitude regions show enhanced variance compared to those in low-latitude regions. More than half of eddies have a vertical extent smaller than 400 m. The majority of eddies extend over the mixed layer depth and are instead situated in the pycnocline. Some eddies at the middle-high latitudes penetrate through the entire pycnocline and reach deep ocean; as such, these eddies may play an important role in linking the deep-sea circulations and the surface circulations.Key Points:• The size (length and depth) of eddies has a distinct latitudinal dependence • More than half of eddies have a vertical extent smaller than 400 m • More than 70% of eddies penetrate through the entire pycnocline and reach deep ocean

Mesoscale Eddy-Induced Ocean Dynamic and Thermodynamic Anomalies in the North Pacific

“…The choose of the upper 400 m is in accordance with many literature works (Vivier et al, 2002;Doney et al, 2007;Song et al, 2014), based on the consideration that the upper 400 m is thermodynamically the most energetic layer of the ocean. It has been shown that about 52% of surface-intensified eddies extend to less than 400 m in the vertical (Liu et al, 2020). An alternative limit of 700 m does not change our findings qualitatively.…”

“…This algorithm provides eddy boundary information, which is of particular importance to this study in calculating eddy-induced anomalies. It should be noted that as in previous studies (e.g., Cheng et al, 2014;Faghmous et al, 2015;Liu et al, 2020), no strict distinction between mesoscale eddies and long baroclinic Rossby waves are made, since the two are both manifested as SSH extrema and they often coincide (Pingree and Sinha, 2001;Oliveira and Polito, 2013;Polito and Sato, 2015). Eddy trajectories are found by searching for similar eddies between the adjacent timesteps within a search radius determined by the maximum eddy propagation speed-the phase speed of long baroclinic Rossby waves.…”

“…To account for data complexity, here we allow the eddies to disappear for 1 day in its trajectory, thus not counting its re-appearance in the next day as a different eddy. This algorithm has been successfully used in the past to detect and track mesoscale eddies in the world oceans (Putrasahan et al, 2017;Ding et al, 2018;Liu et al, 2020).…”

“…Eddyinduced anomalies of other quantities are defined in the same way. It should be noted that in this study, in order to assess eddy-induced variability on the smallest possible scales, we do not apply any filtering criterion on e.g., eddy size or lifetime, as commonly did in literature studies (Frenger et al, 2013(Frenger et al, , 2015Cheng et al, 2014;Chen et al, 2017;Liu et al, 2020). This results in a tremendous number of weak eddies.…”

Ocean Mesoscale Variability: A Case Study on the Mediterranean Sea From a Re-Analysis Perspective

“…Conversely, the representation of ocean mesoscale dynamics is strongly limited by the spatial coverage and temporal sampling of a given satellite mission (e.g., Bonaduce et al, 2018;Chen et al, 2021). Moreover, the temporal and spatial co-location with the available Argo profiling floats in certain areas of the ocean is affected by uneven data coverage and sampling frequency (e.g., Liu et al, 2020), which can represent a shortcoming to investigate the properties of water masses induced by the occurrence of 3D eddies.…”