Abstract
Abstract. This paper, as a part of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP2b), assesses the impacts of different levels of global warming on the thermal structure of Lake Erken (Sweden). The General Ocean Turbulence Model (GOTM) one-dimensional hydrodynamic model was used to simulate water temperature when using ISIMIP2b bias-corrected climate model projections as input. These projections have a daily time step, while lake model simulations are often forced at hourly or shorter time steps. Therefore, it was necessary to first test the ability of GOTM to simulate Lake Erken water temperature using daily vs hourly meteorological forcing data. In order to do this, three data sets were used to force the model as follows: (1) hourly measured data, (2) daily average data derived from the first data set, and (3) synthetic hourly data created from the daily data set using generalised regression artificial neural network methods. This last data set is developed using a method that could also be applied to the daily time step ISIMIP scenarios to obtain hourly model input if needed. The lake model was shown to accurately simulate Lake Erken water temperature when forced with either daily or synthetic hourly data. Long-term simulations forced with daily or synthetic hourly meteorological data suggest that by the late 21st century the lake will undergo clear changes in thermal structure. For the representative concentration pathway (RCP) scenario, namely RCP2.6, surface water temperature was projected to increase by 1.79 and 1.36 ∘C when the lake model was forced at daily and hourly resolutions respectively, and for RCP6.0 these increases were projected to be 3.08 and 2.31 ∘C. Changes in lake stability were projected to increase, and the stratification duration was projected to be longer by 13 and 11 d under RCP2.6 scenario and 22 and 18 d under RCP6.0 scenario for daily and hourly resolutions. Model changes in thermal indices were very similar when using either the daily or synthetic hourly forcing, suggesting that the original ISIMIP climate model projections at a daily time step can be sufficient for the purpose of simulating lake water temperature.
Highlights
The thermal structure of lakes is controlled by heat and energy exchange across the air–water interface, which is in turn determined by meteorological forcing (Woolway et al, 2017)
When calibrating the General Ocean Turbulence Model (GOTM) model we found that model errors between simulated and measured water temperature were similar when GOTM was forced with either measured hourly or synthetic hourly meteorological data, and that the results obtained from the calibrations forced with mean daily metrological input were similar to those obtained from the calibrations based on hourly input
In this study, which is the first test simulating lake hydrothermal structure following ISIMIP2b protocols, ensemble simulations show that changes in Lake Erken’s surface temperature are projected to increase on average by 1.79 ◦C for RCP2.6 and by 3.08 ◦C for RCP6.0, and the length of the stratification is projected to be longer by 13 d for RCP2.6 and by 22 d for RCP6.0 by the end of the 21st century
Summary
The thermal structure of lakes is controlled by heat and energy exchange across the air–water interface, which is in turn determined by meteorological forcing (Woolway et al, 2017). Climate change will affect air–water energy exchanges and alter the temperature regime and mixing of lakes (Woolway and Merchant, 2019). Increases in air temperature result in a consequent warming of lake water temperature (Sahoo et al, 2015) causing shorter ice-cover periods (Kainz et al, 2017; Butcher et al, 2015), longer stratified periods (Ficker et al, 2017; Woolway et al, 2017; Magee and Wu, 2017), and increased lake stability (Rempfer et al, 2010; Hadley et al, 2014). The most direct effect of climate change on lakes is a warming of the lake surface temperature. Hypolimnetic temperature responds less clearly to warming and has been observed to be warming, cooling or not changing significantly with increasing air temperature (Shimoda et al, 2011; Butcher et al, 2015; Winslow et al, 2017).
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