Image credit: Hengkai YaoAbstract
As a ubiquitous motion phenomenon in the ocean, mesoscale eddies play a key role in the transport and distribution of global heat, salinity, energy and marine biochemical processes. The Kuroshio-Oyashio extension region is one of the regions with most abundant eddy activities in the global ocean, and it is also a hot spot for air-sea interaction in the mid-latitudes. As the intersection of the subtropical circulation and the subpolar circulation in the North Pacific, this sea area has complex ocean fronts and large-scale circulation systems, resulting in rich and diverse mesoscale eddy phenomena. The study of mesoscale eddies in the Kuroshio-Oyashio extension region is of great significance for understanding the contribution of oceanic mesoscale phenomena in mid-latitude air-sea interactions and global multi-scale energy balance. Based on satellite observations, in situ observations of Argo profiles, and reanalysis data, this dissertation systematically studies the surface features, vertical structures and sources of the mesoscale eddies in the Kuroshio-Oyashio extension ocean.
Based on two different spatial resolutions of CMEMS satellite remote sensing data for more than 20 years and FORA-WNP30 reanalysis data for more than 30 years, the TOEddies eddy identification algorithm was used to generate tree-like eddy datasets with parent-child relationship. Comprehensive and detailed statistics of the morphological, kinematic and dynamic characteristics of mesoscale eddies in multiple sub-regions in the Kuroshio-Oyashio extension region were carried out. Statistically, it is found that the eddy characteristics under each sub-region show significant differences, which further refines the previous understanding of the eddy statistical characteristics in this region. The evolution of the eddy features in the normalized life cycle reveals significant time-reversal properties and quintile evolution.
Based on the above two sets of eddy datasets with different resolutions, the study found that the high-resolution datasets identified and tracked more eddies than the low-resolution datasets, and the eddies in the high-resolution datasets were significantly different in lifetime, amplitude, Radius, eddy kinetic energy (EKE), azimuthal velocity, propagation distance, nonlinear parameters and other characteristic quantities are smaller, while the characteristic quantities such as propagation velocity, eddy strength (EI), average vorticity and average deformation rate are bigger. It reveals the possible overestimation problem of eddy characteristic quantity which is limited by the resolution of eddy observation in the current global eddy statistics.
Based on the above two sets of vortex datasets and combined with Argo profile data, a composite eddy 3D structure was constructed in multiple sub-regions of the study area, and the spatial variation of the eddy 3D structure in the study area was further refined on the basis of previous research. Based on the mathematical similarity of the temperature-salt density profiles in the eddy, this paper uses the hierarchical clustering method to perform cluster analysis on the eddy for the first time, and obtains four typical vertical eddy structures (surface type 1, surface type 2) in this area, subsurface type and middle-layer type, compared with the traditional sub-region method, the typical eddy vertical structure obtained in this study is more representative, eliminating the vertical structure hybrid phenomenon that is difficult to remove in the traditional method.
Based on the eddy parent-child relationship and water mass analysis in the above two sets of eddy datasets, a traceability analysis of the above typical vertical vortex structures was carried out to find the source of each type of eddies. Surface type 1 eddies are generated in the Oyashio extension (OE) region north of the Subarctic Boundary (SAB), where anticyclonic eddies (AE) are mainly generated near the Subarctic Front (SAF), while Generated near the Oyashio Intrusion (OI); surface type 2 eddies are generated in the Recirculation Gyre (RG) region and the central North Pacific; subsurface eddies are generated south of the Subarctic Boundary (SAB) and the Kuroshio extension (KE) North of the Kuroshio axis; middle-layer type eddies are generated over a broad area south of KE and in the central North Pacific. The different types of vortices can be divided into different unstable structures through linear baroclinic instability analysis.
Based on the results of the above cluster analysis, this paper discusses several main factors affecting the vertical structure of the eddies in this region, including the flow system and instability mechanism in the eddy generation stage, and the vertical displacement of the water column in the eddies caused by the eddy itself, the effect of background stratification changes in the pathway region during eddy propagation, the interaction of air-sea interaction on the vertical structure of surface-type eddies, and the possibility of diapycnal mixing.
Based on the TOEddies algorithm, a dataset of eddy events (splitting and merging) was generated for this study, and the vertical structure of eddies during their life cycle was discussed for both non-eddy-event eddies and eddy-event eddies. It was found that the eddy structure in non-eddy events remained relatively stable, while in eddy events, the eddy core underwent a significant vertical displacement of over 70m before and after the event."
Hengkai YAO
Ocean Scientist
My research interests include Mesoscale Eddies, AI Oceanography and Underwater Communication.
Jian Lan
Professor of Physical Oceanography
Ping Chang
Professor, Louis & Elizabeth Scherck Chair in Oceanography
Dr. Chang’s expertise is on climate dynamics and climate prediction, as well as global and regional climate modeling.
