On the use of coupled atmosphere-ocean-wave model for investigate air-sea interaction and ocean response to extreme Tropical-Like Cyclone “ROLF”

From 6 to 8 November 2011, a baroclinic wave moved from North Atlantic to Balearic Sea, produced a cut-off, at all altitudes, and developed into a Tropical-Like Cyclone (TLC) characterized by a deep-warm core. This TLC led to a mean sea level pressure minimum of about 987 hPa, 10 m wind speeds higher than 30 m/s around the eye, and very intense rainfall, especially in the Gulf of Lion and the surrounding areas, close to the mountain chains (floods in Genoa and Elba Island).

To explore in details the effect of the sea surface temperature on the TLC development, we employed the coupled modeling system COAWST, which consists of the following models: ROMS for the hydrodynamic part, WRF for the meteorological part and SWAN for the surface wave modeling, using a 5 km horizontal grid over all Mediterranean Sea.

COAWST was used with different configurations: Stand Alone (SA) approach using only the atmospheric part, atmosphere-ocean coupled mode (AO), and fully coupled version including also surface waves (AOW). Comparing the three runs, the effects of different simulations on the TLC trajectory are significant only in the later stage of the cyclone lifetime. On the other hand, wind intensity is higher in the SA case w.r.t. both coupled runs. When compared to case AO, winds are about 1 m/s larger, even though the spatial distribution is very similar (possibly because of the lower SST produced by case AO). Case AOW produces less intense winds than SA and AO case in the areas where the wave is most developed (differences are about 2-4 m/s), while winds are more intense nearby the cyclone’s eye. Moreover, the inclusion of the wave model (AOW) has implications in the water column, by increasing the mixing in the ocean, changing the depth of the ocean mixed layer along the track of the TLC, increasing the surface drag and the net heat fluxes from ocean to atmosphere, so that eventually SST in AOW run is colder than in AO.

It is observed that SST of the SA case is overestimated compared to the coupled cases, and in particular the best performances are observed using the fully coupled case, with the wave motion implementation. The best description of the SST impacts the cyclone intensity and the amount of precipitation at catchment scale. The vertical profiles show that wave induced mixing modifies the mixed layer structure and cools the water column, removing much of the SST (and Ocean Heat Content) anomaly present in the mixed layer.

The date chosen for the run initialization appears important: an earlier initial condition allows to properly simulate the evolution of the cyclone from the cyclogenesis between the inclusion and setting-up of air sea interaction effect, through the coupled models.

Warming SST in the Mediterranean Sea induced by climate change might increase TLC frequency and/or intensity, potentially becoming more harmful for coastal populations and infrastractures. Fully coupled AOW models might be better suited for studying such aspects. 

Funding from the STO Office of Chief Scientist (907EUR30) is gratefully acknowledged.