Characteristics of subsurface mesoscale eddies in the northwestern tropical Pacific Ocean from an eddy-resolving model
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Characteristics of subsurface mesoscale eddies in the northwestern tropical Pacific Ocean from an eddy-resolving model* XU Anqi1, 2, 3, YU Fei1, 2, 3, 4, 5, **, NAN Feng1, 3, 4, 5, REN Qiang1, 2, 3 1
Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
3
Key Laboratory of Ocean Circulation and Waves, Chinese Academy of Sciences, Qingdao 266071, China
4
Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China
5
Marine Dynamic Process and Climate Function Laboratory, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266237, China
Received Dec. 3, 2019; accepted in principle Feb. 4, 2020; accepted for publication Jul. 8, 2020 © Chinese Society for Oceanology and Limnology, Science Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Subsurface eddies (SSEs) are common features of the ocean interior. They are particularly abundant in oceanic basins and the vicinity of major intermediate water outflows. They are responsible for subsurface transport of mass, heat, and salt. Analysis of high-resolution general circulation model data has revealed the existence of subsurface anticyclonic eddies (SSAEs) and subsurface cyclonic eddies (SSCEs) in the northwestern tropical Pacific Ocean. SSEs are abundant east of the Philippines (0°–22°N, 120°E–140°E) and in latitude bands between 9°N–17°N east of 140°E. The composite structure of SSEs was investigated. SSEs had a core at about 400-m water depth and their maximum meridional velocity exceeded 10 cm/s. They exhibited two cores with different salinity polarities in the surface and subsurface. Additionally, spatial distributions of heat transport induced by SSEs in the northwestern tropical Pacific were presented for the first time. A net equatorward heat flux toward a temperature up-gradient was observed. The analysis of eddy-mean flow interactions revealed that the circulation is baroclinically and barotropically unstable at different depths and to differing degrees. The energy conversions suggest that both barotropic and baroclinic instabilities are responsible for SSE generation east of the Philippines, whereas baroclinic instability caused by a horizontal density gradient and vertical eddy heat flux are important between 9°N and 17°N east of 140°E. Meridional movement of the north equatorial current and the north equatorial undercurrent can contribute to SSE generation in our study region. Keyword: the northwestern tropical Pacific Ocean; subsurface eddies; composite structures; heat transport; energy source
1 INTRODUCTION The higher resolution of the merged dataset reveals that more than 50% of the variability over much of the World Ocean is accounted for by eddies with amplitudes of 5–25 cm and diameters of 100–200 km (Chelton et al., 2007). Mesoscale eddies can be simply classified into two distinct categories depending on their vertical position (surface or subsurface). There is evidence that surface-intensi
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