Link between the internal variability and the baroclinic instability in the Bohai and Yellow Sea

Abstract A regional ocean ensemble simulation with slightly different initial conditions demonstrates that internal variability is formed (not only) in the Bohai and Yellow Sea. In this paper, we analyze the relationship between the internal variability and the baroclinic instability, (represented by the Eady predicted theoretical diffusivity $${K}_{t}$$ K t ; the larger the $${K}_{t}$$ K t , the stronger the baroclinic instability level). In the ensemble, with tidal forcing, the spatial correlation between the Eady predicted theoretical diffusivity $${K}_{t}$$ K t and the internal variability amounts to 0.80. Also, the time evolution trends of baroclinic instability and internal variability are similar. Based on this evidence, baroclinic instability may be a significant driver for internal variability. This hypothesis is validated using an additional ensemble of simulations, which is identical to the first ensemble, but this time, the tides are inactivated. This modification leads to an increase in internal variability, combined with the strengthening of baroclinic instability. In addition, the baroclinic instability level and internal variability variation co-vary consistently when comparing summer and winter seasons, both with and without tides. Our interpretation is that a stronger baroclinic instability causes more potential energy to be transformed into kinetic energy, allowing the unforced disturbances to grow.