Abstract
Oceanic mesoscale eddies with typical sizes of 30–200 km contain more than half of the kinetic energy of the ocean. With an average lifespan of several months, they are major contributors to the transport of heat, nutrients, plankton, dissolved oxygen and carbon in the ocean. Mesoscale eddies have been observed and studied over the past 50 years, nonetheless our understanding of the details of their structure remains incomplete due to lack of systematic high-resolution measurements. To bridge this gap, a survey of a mesoscale anticyclone was conducted in early 2014 in the South China Sea capturing its structure at submesoscale resolution. By modeling an anticyclone of comparable size and position at three horizontal resolutions the authors verify the resolution requirements for capturing the observed variability in dynamical quantities, and quantify the role of ageostrophic motions on the vertical transport associated with the anticyclone. Results indicate that different submesoscale processes contribute to the vertical transport depending on depth and distance from the eddy center, with frontogenesis playing a key role. Vertical transport by anticyclones cannot be reliably estimated by coarse-resolution or even mesoscale-resolving models, with important implications for global estimates of the eddy-driven vertical pumping of biophysical and chemical tracers.
Highlights
Quantifying the contribution of submesoscale dynamics at the edge and within the core of eddies to the overall eddy transport remains an important but challenging problem, and is key to our understanding of biogeochemical interactions, primary productivity and export production[14,21]
The survey focused on a large anticyclonic eddy, about 200 km in diameter, and the measurement was carried out approximately along 116.5°E (+/−0.01°) from 18.5°N to 21°N (Fig. 1a)
Current velocities were measured with an Acoustic Doppler Current Profiler (ADCP), and hydrographic quantities with a Conductivity-Temperature-Depth (CTD), a microstructure MSS90 profiler and a number of Expendable Bathythermographs (XBT) at submesoscale-resolving sampling intervals
Summary
Quantifying the contribution of submesoscale dynamics at the edge and within the core of eddies to the overall eddy transport remains an important but challenging problem, and is key to our understanding of biogeochemical interactions, primary productivity and export production[14,21]. For the first time we explore the submesoscale structure of an anticyclone using in-situ data from a high-resolution, submesoscale permitting survey conducted in the winter of 2014 in the South China Sea (SCS), a basin populated by energetic mesoscale eddies as revealed by both observational and modeling studies[32,33,34,35,36]. Once the resolution requirement is established, the vertical transport within the eddy is quantified in the model using three-dimensional Lagrangian particles. Such comparison highlights the contribution of ageostrophic, small-scale circulations to the vertical transport within the eddy and its periphery while explaining the mechanisms responsible for patterns of upwelling/downwelling inside anticyclones at different water depths
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