A case study of impacts of the extreme weather system on ocean circulation features in the Mediterranean Sea: Medicane Apollo (2021)

In October 2021, the Sicily Channel and the central-western Ionian Sea were affected by the passage of the tropical-like cyclone, or MEDICANE, Apollo. The system reached its maximum intensity between 29 and 30 October 2021 producing several damages, intense precipitations and huge coastal floodings in Sicily and Calabria regions. The surface circulation in the MEDICANE impacted area was characterized by permanent cyclonic vortices, offering the chance to describe the impact of a tropical-like cyclone on a pre-existing cold circulation structure. Atmospheric and ocean reanalyses (ERA5 and Marine Copernicus Service), as well as in-situ data from Argo floats, were used to describe the temporal evolution of Apollo, the resulting air-sea interaction, the thermohaline and biological response to its passage in the upper layer (0-150 m) of the western Ionian Sea. During the event, the core of the marine cyclone was characterized by a dramatic drop in temperature, corresponding to a local maximum in the wind-stress curl, Ekman pumping and current field relative vorticity. The strengthening of the cyclonic circulation led by the wind stress curl produced a strong vertical mixing in the surface layer (from 0 m to the Mixed Layer Depth - MLD) and an upwelling in the subsurface layer below the thermocline (MLD-150 m). The combined effect of vertical mixing and upwelling resulted in a shoaling of MLD, deep chlorophyll-a maximum, nutricline, and halocline. Oxygen and chlorophyll-a concentrations increased at surface, due to the enhanced oxygen solubility in the cooler water and higher productivity due to the increase of nutrients upwelled to the surface layer. These results show that the pre-existing cyclonic vortex along Apollo's trajectory leads to a different physical response compared to the one observed during previous MEDICANEs, confirming the influence of the conditions in place in driving the ocean’s reply to the extreme weather systems.