In process‐oriented studies, accurate representation of severe bora rotor dynamics in the northern Adriatic is known to require the use of model resolutions of the order of 100 m. In regional climate studies, computation time and numerical cost are, however, minimized with resolutions of the order of 10 km. The latter is not accurate enough to drive the coastal dense water formation and the long‐term Adriatic‐Ionian thermohaline circulation resulting from these events. This work leverages the capacity of kilometer‐scale atmospheric models to balance accuracy and efficiency in coupled atmosphere‐ocean climate studies in the Adriatic Sea. The sensitivity of severe bora dynamics and air‐sea interactions to atmospheric model resolution is thus tested within the Adriatic Sea and Coast (AdriSC) modeling suite as well as with the best available reanalysis. The Weather Research and Forecasting (WRF) model at 15‐km, 3‐km, and 1.5‐km resolution, and ERA5 at 30‐km resolution, are compared for an ensemble of 22 severe bora storms spanning between 1991 and 2019. It is found that (1) ERA5 reanalysis and WRF 15‐km model highly diverge (up to 43% for the wind speed) from WRF 3‐km results while (2) WRF 3‐km conditions converge toward the WRF 1.5‐km solution for both basic bora dynamics (differences below 6% for the wind speed) and air‐sea interactions (differences 5 times smaller than with WRF 15‐km results). Consequently, kilometer‐scale atmospheric models should be used to reproduce properly the dense water formation during severe bora events and the long‐term thermohaline circulation of the Adriatic‐Ionian basin.

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Abstract. In this evaluation study, the coupled atmosphere–ocean Adriatic Sea and Coast (AdriSC) climate model, which was implemented to carry out 31-year evaluation and climate projection simulations in the Adriatic and northern Ionian seas, is briefly presented. The kilometre-scale AdriSC atmospheric results, derived with the Weather Research and Forecasting (WRF) 3 km model for the 1987–2017 period, are then thoroughly compared to a comprehensive publicly and freely available observational dataset. The evaluation shows that overall, except for the summer surface temperatures, which are systematically underestimated, the AdriSC WRF 3 km model has a far better capacity to reproduce surface climate variables (and particularly the rain) than the WRF regional climate models at 0.11∘ resolution. In addition, several spurious data have been found in both gridded products and in situ measurements, which thus should be used with care in the Adriatic region for climate studies at local and regional scales. Long-term simulations with the AdriSC climate model, which couples the WRF 3 km model with a 1 km ocean model, might thus be a new avenue to substantially improve the reproduction, at the climate scale, of the Adriatic Sea dynamics driving the Eastern Mediterranean thermohaline circulation. As such it may also provide new standards for climate studies of orographically developed coastal regions in general.

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In this study the impact of the Adriatic-Ionian Bimodal Oscillating System (BiOS) on the interannual to decadal variability of the Adriatic Sea thermohaline circulation is quantified during the 1987-2017 period with the numerical results of the Adriatic Sea and Coast (AdriSC) historical kilometer-scale climate simulation. The time series associated with the first five Empirical Orthogonal Functions (EOFs) computed from the salinity, temperature and current speed monthly detrended anomalies at 1-km resolution are correlated to the BiOS signal. First, it is found that the AdriSC climate model is capable to reproduce the BiOS-driven phases derived from in-situ observations along a long-term monitoring transect in the middle Adriatic. Then, for the entire Adriatic basin, high correlations to the 2-year delayed BiOS signal are obtained for the salinity and current speed first two EOF time series at 100 m depth and the sea-bottom Finally, the physical interpretation of the EOF spatial patterns reveals that Adriatic bottom temperatures are more influenced by the dense water circulation than the BiOS. These findings confirmed and generalized the known dynamics derived previously from observations, and the AdriSC climate model can thus be used to better understand the past and future BiOS-driven physical processes in the Adriatic Sea.