Exploring the Potential for Tropical-Like Cyclone Formation in the Black Sea With Kilometer-Scale WRF Simulations

 

The Black Sea is a semi-enclosed basin with steep topographic features. During summer, it absorbs excessive heat, raising sea surface temperature (SST), which carries over into the next season. According to ECMWF Reanalysis 5th Generation (ERA5) data, SST in the Black Sea has been increasing at a rate of 1.3°C per decade in September, significantly increasing the likelihood of extreme weather events. In September 2005, an upper-level trough moved over the western Black Sea from Eastern Europe, evolving into a cut-off low. During this period, SST ranged from 20°C to 24°C, while the 500 hPa center of the low-pressure system dropped to -20°C, creating favorable conditions for deep convection. As the cut-off low remained over the region from September 25 to 29, a storm developed and made landfall in Istanbul. Under the SSP585 scenario, 25 Earth System Models (ESMs) project that Black Sea SST will increase by 1°C, 2°C, and 3°C compared to the 2005 event SST in 2040-2050, 2050-2060, and 2070-2080, respectively. This study quantifies the effects of future SST conditions on the 2005 Black Sea storm by forcing an increasing SST trend and assessing whether it could transition into a tropical-like cyclone. Sixteen numerical experiments were conducted using the Weather Research and Forecasting (WRF) Model with the simple Ocean Mixed Layer (OML) model. Simulations were forced with high-resolution (5.5 km) Copernicus European Regional Reanalysis (CERRA) data, SST from ERA5, and mixed layer depth (MLD) from Copernicus Marine Service (CMEMS). A 1.5 km horizontal resolution with 60 vertical levels was used for the WRF simulations, and two surface roughness parameterizations were applied, with and without the ocean model for SST control and future scenarios. In control simulations, minimum sea level pressure (SLP) dropped to 1000 hPa with the ocean model and 1002 hPa without it. In the SST+3 scenario, SLP ranged from 986 hPa to 994 hPa due to better representation of air-sea heat fluxes. Simulations with the ocean model showed reduced cyclone intensification due to sea surface cooling, while without it, higher SSTs led to stronger cyclones. In the SST+3 scenario, maximum wind speed neared CAT-2 hurricane strength (151 km/h), with a 142% increase in enthalpy flux. Other scenarios (SST+1 and SST+2) showed a similar trend, with the strongest signal in SST+3. Furthermore, landfall location shifted, cyclone duration increased, and precipitation extent expanded. This study suggests that future SST increases could lead to tropical-like cyclones in the Black Sea under favorable conditions.