Analysis of extreme Winter Warm Spell and Sea Surface Temperature Anomaly during 2021 and 2022 over Western Mediterranean Sea.

From December 23rd 2021 to January 4th 2022 and from December 20th to January 8th 2023, two “Extreme Warm Spell” (EWS) episodes have occurred in the Mediterranean Basin and central-western Europe. These phenomena led to several and long-lasting record-breaking atmospheric temperatures in the impacted areas. Combining both intensity and duration, these two episodes can be identified as extremely warm winter events. In the first one (2021-2022), 850hpa temperatures over 15°C were recorded near Spanish, Italian and French coasts and the Western Mediterranean (WM) in general, and a SST anomaly of 1°C in December over the WM sea. The second event (2022-2023) was characterized by a zonal development, affecting the Balearic Sea, Northern Africa coasts and Italy, with 850hPa temperatures that, in different impulse, reached 12-16°C and a strong SST anomalies, affecting both previous months (showing peaks between +6.8 and +5 °C in September and November) and December (+2/3°C). In this work, the two EWSs are statistically analyzed by comparison with the climatology data set from both atmospheric and oceanic point of view using EOF analysis. Then, the last 40-year trend is assessed. Using satellite, Argo float, buoy and model dataset we characterize the 6 months preceding the EWS, exploring the Surface Temperature, Mixed Layer Depth (MLD) and its dynamic, estimating the Ocean Heat Content (OHC). Moreover, we investigated the impact of SST anomalies and Mixed Layer Depth on the atmospheric structure, and vice-versa, using WRF-ARW (Weather Research and Forecasting System) numerical model at 5km resolution horizontal grid, covering all the Mediterranean and central Europe, and SST provided by CMEMS-GOS 1 km resolution daily dataset and ECMWF-IFS boundary conditions. WRF-ARW model is coupled with a 1D ocean model (SLAB ocean model), computing the evolution of the MLD and SST in function of heat fluxes at the air-sea interface, to estimate how much heat was exchanged between the two environments. We also performed a sensitivity run (for each event), in which we removed the SST anomaly and the MLD anomaly, with the aim of isolating the contribution of the OHC in the atmospheric dynamics, and the impact of the atmosphere on the SST and OHC anomaly. Results show two distinct conditions, despite the EWSs are driven by similar large-scale forcing and atmospheric patterns. In particular, the preconditioning features of SST and OHC are very different (the 2022-2023 event derives from 6 month of strong positive anomaly if compared to 2021-2022 event). In general, the SST anomaly impacts especially humidity and ground temperature, up to 850 hPa pressure level. Nevertheless, the main driver is the synoptic atmospheric circulation, precisely a deep trough in the central Atlantic which advects warm air from the tropics and interacts with the mild Mediterranean Sea. On the other hand, the sea does not show high heat fluxes during December–January (usually period of strong cooling in the WM) retaining a large part of OHC between the surface and the thermocline, with possible role on subsequent events, both in thermohaline circulation and in atmospheric dynamics.