Simulations of future changes in thermal structure of Lake Erken: proof of concept for ISIMIP2b lake sector local simulation strategy

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.

Phenological shifts in lake stratification under climate change

“…Due to its ecological importance, changes in stratification phenology have been studied in individual lakes 11 – 13 or a very few lakes within specific regions 14 , 15 . However, on a global scale, the influence of climate change on stratification phenology remains largely unexplored.…”

LakeEnsemblR: An R package that facilitates ensemble modelling of lakes

“…It was initially developed for modelling turbulence in the ocean (Burchard et al, 2006), but it has been recently adapted for use in hydrodynamic modelling in lakes (Sachse et al, 2014). GOTM has been used to model the dissolution of CO2 in lakes (Enstad et al, 2008), extreme events in a eutrophic marine system (Ciglenečki et al, 2015), impact of macrophytes on water quality (Sachse et al, 2014) and hindcasting and future climate change projections of the thermal structure of a lake (Ayala et al, 2020;Moras et al, 2019). LakeEnsemblR version 1.0.0 uses version 5.4.0 of the lake branch of GOTM.…”

“…Each of the three calibration methods can be run in parallel computation, where the models are distributed over the available cores. The parameters which are to be estimated, and their upper and lower bounds (if applicable) are specified in the master configuration file.Scaling factors of meteorological forcing are parameters that are often calibrated in models (e.g.,Ayala et al, 2020;Gaudard et al, 2019). Some models within LakeEnsemblR have internal parameters that scale the (meteorological) forcing, but not all.…”

Variability in epilimnion depth estimations in lakes

“…The Global Ocean Turbulence Model (GOTM), adapted for use in lakes, simulates small-scale turbulence and vertical mixing (Burchard et al, 1999;Sachse et al, 2014;Moras et al, 2019;Ayala et al, 2020) and was used to simulate daily profiles of water temperature ( • C) and vertical eddy diffusivity (m −2 s −1 ) for Erken lake and Lough Feeagh. GOTM was calibrated using 4 years of data (2006)(2007)(2008)(2009) for Erken lake and 2008-2011 for Lough Feeagh), including 1-year spin-up followed by 3 years of calibration.…”

“…In addition, the collation of these datasets globally through collaborative initiatives such as GLEON (http://gleon.org/, last access: 19 November 2020) and NETLAKE (https://www. dkit.ie/netlake, last access: 19 November 2020) (Weathers et al, 2013;Jennings et al, 2017) and modelling initiatives such as ISIMIP2b (Ayala et al, 2020) broaden the potential for long-term multi-lake studies. However, these datasets also introduce new challenges for estimating metrics such as the epilimnion depth.…”