Internal-tide-induced fluctuations of hydrography and biogeochemical properties modulated by mesoscale eddies: Observation evidences

Mesoscale eddy plays an important role in physical and biochemical processes in the upper ocean, it could redistribute seawater masses, heat, and biogeochemical properties. The region of the Subtropical Countercurrent (STCC) within the northwestern subtropical Pacific Ocean (NWSTP) is characterized by an abundance of mesoscale eddies caused by baroclinic instability. The NWSTP serves a good place for studying variations of dissolved oxygen (DO) concentration and temperature induced by eddies. Observed by two metocean moored buoys deployed in the NWSTP, there was one cyclonic eddy (CE) and one anti-cyclonic eddy (ACE) during the summer of 2018 to 2020. Time series dataset of hydrography, DO concentration and chlorophyll-a in the upper subsurface layer provided precious in-situ observations, which helps us better understand physical dynamics under the influence of eddies and improve numerical model forecasts. The results of analysis showed that there was a significant difference between two types of eddies in the upper layer ocean. During the CE period, DO concentration and temperature had significant fluctuations up to 30 μM and 5 ℃, respectively, at depth of 50 meter. In response to diurnal and semi-diurnal tidal processes, their variations gradually decreased toward deep ocean; whereas their amplitude narrowed to 5 μM and 1 ℃, respectively, during the ACE period. During the non-eddy period, there was a small amplitude of fluctuations in the tidal bands except at 150 meter. The phenomenon could be associated with modulated thermal structure under the influence of CE and ACE through upwelling and downwelling, respectively. This led to changes in the vertical structure of the internal tide amplitude and concentration gradient. The combination of these two factors consequently resulted in different amplitude of fluctuations in DO concentration and temperature during two types of eddies passing through. These results will be presented herein in detail.