Cooling by Cyprus Lows of Surface and Epilimnion Water in Subtropical Lake Kinneret in Rainy Seasons

Comparison between high-precipitation (HP) years and low-precipitation (LP) years led to our main findings which are as follows: Cyprus lows are instrumental in the cooling of surface and epilimnion water in subtropical Lake Kinneret and in the cooling of eastern Mediterranean surface water. Cyprus lows are responsible for cold weather, rainfall, and for an increase in cloudiness causing a decrease in solar radiation over the eastern Mediterranean and north Israel (including Lake Kinneret). In the daytime, comparison between HP and LP years of Kinneret surface water temperature (SWT) and epilimnion water temperature (WT) showed water cooling of up to 2 °C in HP years. This study was carried out using the 21-year period of satellite and in-situ data: (1) MODIS 1 km × 1 km resolution records of SWT, in (2) shipboard measurements of WT vertical profiles down to a depth of ~40 m (2000–2020). We found that a decrease in solar radiation caused by Cyprus lows (due to an increase in cloudiness) was the main factor contributing to Kinneret water cooling. In winter (December–January) when solar radiation (SR) was minimal, no water cooling was observed: the WT difference between HP and LP years was insignificant. However, in spring (March–April) when SR increased and became the main factor contributing to water heating, water cooling was observed: SWT and epilimnion WT, averaged over the HP years, was lower by ~2 °C and ~1.4 °C, respectively, than SWT and epilimnion WT, averaged over the LP years. Not only was water cooling observed in Lake Kinneret, but also in eastern Mediterranean surface water. Comparison of SWT over the eastern Mediterranean between the same HP and LP years in spring showed SWT cooling by ~1.2 °C. This is evidence of the regional character of the daytime water-cooling phenomenon caused by Cyprus lows.