Characterization of Ocean Mixing and Dynamics during the 2017 Maud Rise Polynya Event

Abstract. During 2017 Austral winter, an open ocean polynya appeared in the Lazarev Sea, centered over Maud Rise. The vertical structure of the water column presented temporal and spatial variability with a weak stratification during the period of observations from January 2015 to January 2019. While over the Northern Maud Rise area, a highly stratified layer was identified between 80–180 m depth. This layer works as a thermal barrier where the energy from summer months is stored, preventing the warm sub-surface waters from mixing with the shallow waters. So far, a complete description of the polynya formation and maintenance processes is still lacking. To characterize the internal structure of the ocean during the 2017 Maud Rise polynya event we use in situ observations and ocean model reanalysis data. The obtained results revealed that the incidence of thermobaric convection, diapycnal and isopycnal mixing processes over the Maud Rise drives the exchange of energy in the water column. We highlight three relevant factors that contribute to the energy flux for the open-ocean polynya preconditioning: level of instability, pycnocline fluctuation, and bathymetric influence. Another remarkable feature is the warmer summer surface layer over the Maud Rise, which transfers heat to intermediate layers accumulating energy for almost four months. Energy storage at the thermal barrier is evaluated based on heat flux calculations to quantify the exchange of energy between the different water layers. These processes together operate as an ocean preconditioning to the formation and maintenance of an open-ocean polynya event.