Atmosphere-Ocean compound heat wave events in the Eastern Mediterranean

Mediterranean marine heat waves (MHW) can be defined as abrupt but prolonged, discrete and anomalously warm water events that last for five or more days and exceed temperatures warmer than the 99th percentile (Darmaraki et al. 2019). Like their atmospheric counterpart, Mediterranean MHW have already increased in intensity, frequency and duration - a trend projected to continue under anthropogenic climate change. Recent observations of MHW demonstrated a strong influence of these extreme climatic events on marine organisms, including mass mortalities and shifts in species ranges but also economic impacts on fisheries and aquaculture. MHW can be caused by a combination of atmospheric and oceanic processes and depend on the specific season and location of occurrence. However, the main triggers are generally still not well understood and the current knowledge is largely based on these reported regional impacts. This work focuses on historical (1985 – 2014) atmospheric and marine heat waves in a high resolution CMIP6 model as well as a fully three-dimensional oceanographic hindcast of the interconnected Eastern Mediterranean – Black Sea system. We detect the atmospheric and marine heatwaves and investigate the triggering, compound/concurrent effect of the atmosphere on marine heat waves in the Eastern Mediterranean. For the analysis of atmospheric heat waves, we follow the methodology of Kuglitsch et al. (2010). We use Eastern Mediterranean atmospheric model and ERA-Interim reanalysis to calculate daily maximum (TX) and minimum (TN) air temperatures as well as to set temperature thresholds to estimate the beginning and end of the heat wave events. We identify MHWs from daily sea surface temperatures, applying the approach of Darmaraki et al. (2019). Furthermore, we calculate the heat wave frequency, duration and intensity. The two pairs of datasets are then compared with respect to the spatio-temporal occurrence of heat waves in the atmosphere and ocean, in an effort to reveal feedbacks between the two spheres which would characterize the events as compound. Finally, we estimate a threshold at which an atmospheric heat wave triggers a marine heat wave, and thus a compound event.