Dust-related heatwaves in the hypersaline Dead Sea and freshwater Lake Kinneret which were missed by satellite observations

Observations and model data showed a gradual increase in desert dust intrusions into the Eastern Mediterranean region. The role of dust intrusions in the formation of lake heatwaves has not been investigated in previous studies. In our study we focused on this point. In-situ buoy measurements showed that, in the two lakes located in the Eastern Mediterranean: freshwater Lake Kinneret and the hypersaline Dead Sea, - a severe dust intrusion (AOD of over 2.5 and surface dust concentration of over 4000 µg/m³) caused the formation of lake heatwaves (LHWs), as appeared in September 2015. This was because desert dust absorbed both shortwave solar radiation and longwave terrestrial radiation contributing to air heating in the near-ground atmospheric layer and water heating at the lake surface.

  At the water surface, for 10 days in a row (7 – 17 September), the LHWs were represented by abnormally high daily maximal and minimal surface water temperature (SWT) in comparison with their seasonally varied 90^th percentile thresholds. The intensity of surface LHWs was as high as 3 ^oC. We compared satellite (METEOSAT and MODIS-Terra) SWT data with actual SWT based on buoy measurements. First, spatial distribution of METEOSAT and MODIS-Terra SWT showed that, over any part of the Dead Sea, SWT on dusty days was lower than SWT on clear-sky September 6. This contradicted the increase in actual SWT in the presence of the dust intrusion. Next, we conducted quantitative comparison between satellite SWT data and actual SWT. Our quantitative comparison showed that, in the presence of the dust intrusion, both orbital (MODIS-Terra) and geostationary (METEOSAT) satellites were incapable of representing the surface LHWs. Unexpectedly, in the two lakes, the satellite SWT retrievals underestimated actual SWT by more than 10 °C. This indicates the satellites’ inability to represent the observed LHW phenomenon. The obtained significant difference between the satellite-derived SWT and actual SWT can be explained by the impact of the dust-caused infrared (IR) perturbations on satellite IR measurements. This should be considered when using satellite data to analyze heatwaves in the presence of dust pollution.

  As for the subsurface LHWs in the two lakes, our findings imply the following significant point: the physical nature of subsurface LHWs in the hypersaline Dead Sea is essentially different from that of subsurface LHWs in fresh-water lakes. This is because double-diffusive processes are thought to be essential to the formation of abnormal vertical temperature distribution at a depth from 5 m to 20 m causing the development of subsurface LHWs.

   Reference: Kishcha et al., Remote Sensing 2024, https://doi.org/10.3390/rs16132314