Reproducing the Variability of the Thermal State of Lake Ladoga in 1980--2020 Based on Two Models of Thermohydrodynamics

The results of reproducing the thermal state of Lake Ladoga in the period 1980–2020 using two models, the St. Petersburg Baltic Eutrophication Model (SPBEM) of the St. Petersburg Branch of the Shirshov Institute of Oceanology of the RAS, and the Inland Sea Hydrodynamics Model (ISHM) of the Institute of Limnology of the RAS, are compared with available data from contact and satellite measurements, as well as with each other. In general, both models adequately reproduce the main features of the average state and intra-annual variability of the lake. The long-term annual averages of the surface temperature of Lake Ladoga based on the MODIS Aqua (2002–2020), MODIS Terra (2000–2020) and Suomi NPP VIIRS (2012–2020) data are 7.75, 7.66 and 7.90 °C, respectively, while the SPBEM estimates are underestimated by 1.00, 0.96 and 1.00 °C, respectively, and the ISHM estimates are underestimated by 0.66, 0.57 and 0.54 °C. The interannual trends in the surface temperature calculated using satellite data were statistically insignificant at a 95 % confidence level. The average annual modeled surface temperature in the period 1980–2020, according to statistically significant linear trends, increased by 1.2 (1.7) °C, respectively, according to SPBEM and ISHM. The temperature increase was 1.6 (2.6) °C during the summer period and 0.8 (1.0) °C in the winter. The average annual temperature of the entire lake changed by 0.5 (0.8) °C. SPBEM and ISHM overestimate the observed average ice area of 7816 km² for the period 1980–2020 by 15 and 14 %, respectively; its annual averages decreased by 85, 68 and 108 km² per year according to SPBEM, ISHM and observations.