Case study on the response of oceanic saddle-field to tropical cyclone

The atmospheric saddle-field is characterized by a distinct deformation field with relatively low horizontal velocity, and surrounded by two high-pressure systems and two low-pressure systems. The unique configuration of the atmospheric saddle-field often leads to the development of intense precipitation events. The oceanic saddlefield, defined as a deformation field and surrounded by four mesoscale eddies, i.e., two anticyclonic and two cyclonic eddies, is also popular in the ocean and expected to induce phytoplankton bloom. However, the physical properties and flow structures associated with oceanic saddle-field are still unclear. Based on in-situ and numerical model, we investigated the response of an oceanic saddle-field to tropical cyclone "Sinlaku". After tropical cyclone "Sinlaku", the deformation rate in the saddle-field decreased quickly; within mixed layer, the potential vorticity increased, and a submesoscale frontogenesis process associated with symmetric instability was triggered in the saddle-field. Based on four-boxes energy diagnostic analysis, we found that baroclinic instability term dominated the kinetic energy conversion for the mixed layer frontogenesis. This study proposed one special type of submesoscale process, i.e., oceanic saddle-field, which is expected to improve our understanding in submesoscale motions and probably contributes to the improvement of physical-chemical-ecology coupled model.