Absence of surface water temperature trends in the presence of atmospheric warming as evidence of increasing evaporation in fresh-water Lake Kinneret

Abstract

<p>In the summer months, characterized by the absence of precipitation and by limited cloud cover, subtropical lakes are particularly sensitive to atmospheric warming which causes increasing heating of surface water. Therefore, these lakes are best suited to the investigation of this phenomenon.</p><p>Within the Jordan Rift valley there are two lakes: the fresh-water Lake Kinneret (Sea of Galilee) (a surface area of 106 km<sup>2</sup> and a maximal depth of 40 m) and the hypersaline Dead Sea (a surface area of 605 km<sup>2</sup> and a maximal depth of 300 m). We investigated water surface temperature (WST) and its trends in the two lakes. This was carried out using MODIS 1 km x 1 km resolution records on board Terra and Aqua satellites together with in-situ measurements, during the period (2003 – 2019). In fresh-water Lake Kinneret, we found that, in summer when evaporation is maximal, despite the presence of increasing atmospheric warming, satellite data revealed the absence of WST trends (Kishcha et al., 2021). The absence of WST trends in the presence of increasing atmospheric warming is an indication of the influence of steadily increasing evaporation on WST. Increasing water cooling, due to steadily increasing evaporation, compensated for increasing heating of surface water by regional atmospheric warming. This resulted in the obtained statistically-insignificant WST trends. During the study period (2003 – 2019), in summer, in contrast to satellite data, in-situ measurements of near-surface water temperature (at a depth of 0.1 m) in Lake Kinneret showed an increasing trend of 0.7 <sup>o</sup>C  decade<sup>-1</sup>. This trend in near-surface water temperature reflected the presence of increasing atmospheric warming in the absence of evaporation.</p><p>In contrast to fresh-water Lake Kinneret, in the hypersaline Dead Sea (located only 100 km apart), MODIS showed an increasing statistically-significant trend of 0.8 <sup>o</sup>C decade<sup>-1</sup> in summer WST. This fact was obtained during the same study period (Kishcha et al., 2021). The increasing WST trend, in the presence of atmospheric warming, is evidence of the absence of increasing evaporation in the Dead Sea. This fact is supported by a constant rate of ~1 m/year of Dead Sea water level drop during the last 25-year period (1995 – 2020). The absence of increasing evaporation could be explained by surface water salinity in the Dead Sea skin layer. Increasing surface water salinity suppresses further increases in evaporation. As a result, there was no acceleration in Dead Sea water level drop in the presence of an increasing SWT trend of 0.8 <sup>o</sup>C decade<sup>-1</sup>. We consider that this is a characteristic feature of the hypersaline Dead Sea, which is not present in the fresh-water Lake Kinneret.</p><p>Reference:</p><p>Kishcha et al. (2021). Absence of surface water temperature trends in Lake Kinneret despite present atmospheric warming: Comparisons with Dead Sea trends. <em>Remote Sensing</em>, 13, 3461. https://doi.org/10.3390/rs13173461</p>