A multi-faceted, long-term analysis of Mediterranean heatwaves using in-situ temperature observations, remote sensing imagery and large-scale circulation types.

The Mediterranean basin is one of the most thermally vulnerable regions of the planet, already affected by frequent extreme heat events, which are projected to increase in the future. Using multiple extreme climate indices and daily temperature observations from over 50 meteorological stations in the region (NOAA Global Historical Climatology Network daily database) the dates of past heatwave events were identified from 1961 to 2020. While derived heatwave metrics from threshold-based indices generally differed from more complex, calendar day percentile-based indices (e.g., CTX90PCT), findings confirmed a statistically significant positive trend for the frequency, intensity, and duration of heatwave events for the majority of stations (Mann-Kendall test). Next, long-term (years: 2002-2020) daily satellite-derived Land Surface Temperature (LST) from the MODIS Aqua platform was used to compute surface thermal anomalies at the station locations. The dates and the summary metrics of the maximum anomalies were compared to the previously derived air temperature heatwave events to assess the feasibility of using LST estimates as a heatwave proxy for areas with a scarce station network. Finally, a physical understanding of the mechanisms behind heatwave trends for Athens, Greece was attempted through the use of large-scale synoptic data. Daily 500 hPa geopotential height anomalies were computed from ERA5 reanalysis data (1979-2020); then, an automated classification scheme was modified and applied to detect the circulation types for all past heatwave events for this period in Athens. Results indicated an evolving mixture of circulation types through the decades, associated with different intensities and durations of heatwave events.