Historical modelling of changes in Lake Erken thermal conditions
Authors: Moras Simone , Ayala Ana I. , Pierson Don C. 
Published: December 2019

Abstract. Historical lake water temperature records are a valuable source of
information to assess the influence of climate change on lake thermal
structure. However, in most cases such records span a short period of time
and/or are incomplete, providing a less credible assessment of change. In
this study, the hydrodynamic GOTM (General Ocean Turbulence Model, a
hydrodynamic model configured in lake mode) was used to reconstruct daily
profiles of water temperature in Lake Erken (Sweden) over the period
1961–2017 using seven climatic parameters as forcing data: wind speed (WS),
air temperature (Air T), atmospheric pressure (Air P), relative humidity
(RH), cloud cover (CC), precipitation (DP), and shortwave radiation (SWR).
The model was calibrated against observed water temperature data collected
during the study interval, and the calibrated model revealed a good match
between modelled and observed temperature (RMSE =1.089 ∘C). From
the long-term simulations of water temperature, this study focused on
detecting possible trends in water temperature over the entire study
interval 1961–2017 and in the sub-intervals 1961–1988 and 1989–2017, since
an abrupt change in air temperature was detected in 1988. The analysis of
the simulated temperature showed that epilimnetic temperature increased
on average by 0.444 and 0.792 ∘C per decade in
spring and autumn in the sub-interval 1989–2017. Summer epilimnetic
temperature increased by 0.351 ∘C per decade over the entire
interval 1961–2017. Hypolimnetic temperature increased significantly in
spring over the entire interval 1961–2017, by 0.148 and
by 0.816 ∘C per decade in autumn in the sub-interval 1989–2016.
Whole-lake temperature showed a significant increasing trend in the
sub-interval 1989–2017 during spring (0.404 ∘C per decade) and
autumn (0.789 ∘C per decade, interval 1989–2016), while a significant
trend was detected in summer over the entire study interval 1961–2017 (0.239 ∘C per decade). Moreover, this study showed that changes in the
phenology of thermal stratification have occurred over the 57-year period
of study. Since 1961, the stability of stratification (Schmidt stability)
has increased by 5.365 J m−2 per decade. The duration of thermal
stratification has increased by 7.297 d per decade, corresponding to an
earlier onset of stratification of ∼16 d and to a delay of
stratification termination of ∼26 d. The average
thermocline depth during stratification became shallower by ∼1.345 m, and surface-bottom temperature difference increased over time by
0.249 ∘C per decade. The creation of a daily time step water
temperature dataset not only provided evidence of changes in Erken thermal
structure over the last decades, but is also a valuable resource of
information that can help in future research on the ecology of Lake Erken.
The use of readily available meteorological data to reconstruct Lake Erken's
past water temperature is shown to be a useful method to evaluate long-term
changes in lake thermal structure, and it is a method that can be extended
to other lakes.