Analysis of mean and eddy energy of the Black Sea circulation derived under account the climatic and real atmospheric forcing

The aim of the work is to study the mechanisms of the Black Sea mesoscale variability based on an analysis of Lorenz energy cycles calculated from the density and currents velocity obtained by the results of three numerical experiments. An eddy-resolving z-model with a horizontal resolution of 1.6 km was used. Three experiments were carried out with different atmospheric forcing: 1) - climatic data; 2) - SKIRON data for 2011; 3) – SKIRON data for 2016. The mean current kinetic energy MKE, the eddy kinetic energy EKE, the mean available potential energy MPE, the eddy available potential energy EPE and the rates of energy conversion, generation and dissipation were considered in detail.

For all experiments the generation and dissipation rates of the MKE and EKE are close to each other, so the kinetic energy from wind dissipated inside the sea. A buoyancy work (described by the conversion between the MPE and MKE) increase the MKE. The EKE was increasing due to the energy transport from the mean current into eddies and the transport from the EPE to the EKE for all experiments. It is shown that these two energy fluxes were comparable in the experiment 1, while the ratio between of them has changed almost six times in the experiments 2 and 3. The c(MKE, EKE) prevailed in 2011, but the c(EPE, EKE) dominated in 2016.

The maps analysis of the EKE spatial distribution showed that its maximum in the climatic field was located above a continental slope and in areas of the biggest mesoscale eddies. The mesoscale variability of the climatic circulation was due to the influence of both baroclinic and barotropic instability. The zones of the EKE maximum were located in the abyssal part of the sea in the experiments 2 and 3. EKE was increasing in 2011 mainly due to the inflow from the mean current through barotropic instability. The growth of EKE in 2016 was due to conversion of EPE induced by baroclinic instability.

The difference in the EKE variability by the results of climatic and real forcing experiments is associated with the wind forcing. The contribution of the wind stress work to MKE was decreased for the experiments 2 and 3, so as a result, it was observed weakening in the mean current, intensive stream meandering and generation of mesoscale eddies not only in the coastal zones but also in the abyssal part of the sea. Thus, the Black Sea mesoscale variability is determined by barotropic instability or by the combined contribution of barotropic and baroclinic instability processes under intense wind action. The mesoscale variability is due to baroclinic instability under weak wind action.

The reported study was funded by RFBR and Government of the Sevastopol according to the research project No 18-45-920019 and the state task No. 0555-2021-0004.