Evidence of heatwaves in two deep Italian lakes in summer 2022

Lakes and their biodiversity are threatened by global warming, responding rapidly to climatic change and incorporating the effects occurring in the drained catchments. The thermal characteristics of lakes have undergone substantial alterations in response to the progressive increased air temperature. Surface waters have worldwide warmed, with a global average rate of 0.34°C per decade between 1985 and 2009 during summer. Over the last few years, increased attention is given to the response of lakes to extreme events, such as storms and heatwaves. There is evidence that climate change is leading to longer and more frequent marine heatwaves at the surface of the ocean, while much less is known about heatwaves in lakes, where field studies are generally lacking.

According to the C3S ERA5 dataset, August 2022 globally tied as the third warmest on record, 0.30°C warmer than the 1991-2020 average for August and 0.42°C warmer than the 1981-2010 average in this data record. The month of August was particularly warm over Europe and was also remarkable in terms of hydrological variables with drier than normal conditions over western Europe, sustained by heatwaves with significant impacts on economy and society. In this contribution, we show the thermal response of two deep Italian lakes (Lake Garda and Lake Iseo) under the extreme meteorological conditions occurred in summer 2022. In both these lakes, a floating station measured at a 1/min sampling rate the meteorological forcing at the lake surface as well as water temperature down to 100 m of depth with 0.01 °C accuracy. Thanks to the temperature data, we identified and characterized, in terms of intensity and duration, several long-lasting heatwaves and we compared their characteristics with past observations, showing evidence of an exceptional surface warming over the last 20 years. The overall high-resolution dataset allowed us to compute the thermal balance of the two lakes, comparing the relative importance of the different forcing and that of the morphological and hydrological characteristics of the two water bodies, and to discuss the impact of the internal hydrodynamics on the surface heating. This analysis underlined the fundamental role played by the wind, which affects both the thermal fluxes at the water surface and the vertical distribution of temperature. This result raises the issue about the degree of reliability of the prediction of extreme events in lakes under climate change due to the uncertainties of the wind predictions, especially in geographical context, such as pre-alpine or alpine regions, where the wind fields provided by large-scale resolution models are less accurate.