Present and future Laurentian Great Lakes hydroclimatic conditions as simulated by regional climate models with an emphasis on Lake Michigan-Huron

Present and future Laurentian Great Lakes hydroclimatic conditions as simulated by regional climate models with an emphasis on Lake Michigan-Huron Biljana Music Anne Frigon Brent Lofgren Richard Turcotte Jean-Franois Cyr Regional climate modelling represents an appealing approach to projecting Great Lakes water supplies under a changing climate. In this study, we investigate the response of the Great Lakes Basin to increasing greenhouse gas and aerosols emissions using an ensemble of sixteen climate change simulations generated by three different Regional Climate Models (RCMs): CRCM4, HadRM3 and WRFG. Annual and monthly means of simulated hydrometeorological variables that affect Great Lakes levels are first compared to observation-based estimates. The climate change signal is then assessed by computing differences between simulated future (2041-2070) and present (1971-1999) climates. Finally, an analysis of the annual minima and maxima of the Net Basin Supply (NBS), derived from the simulated NBS components, is conducted using Generalized Extreme Value distribution. Results reveal notable model differences in simulated water budget components throughout the year, especially for the lake evaporation component. These differences are reflected in the resulting NBS. Although uncertainties in observation-based estimates are quite large, our analysis indicates that all three RCMs tend to underestimate NBS in late summer and fall, which is related to biases in simulated runoff, lake evaporation, and over-lake precipitation. The climate change signal derived from the total ensemble mean indicates no change in future mean annual NBS. However, our analysis suggests an amplification of the NBS annual cycle and an intensification of the annual NBS minima in future climate. This emphasizes the need for an adaptive management of water to minimize potential negative implications associated with more severe and frequent NBS minima. 1 Introduction Over the last decades, there has been growing concern about the hydrological responses of the Great Lakes to global warming, stemming from potential changes in Great Lakes water level variability and the frequency of extremes under an evolving climate. Many of the historical studies addressing this issue are based on the so-called Bchange factor^ method, which is typically applied to a suite of models known as the Advanced Hydrologic Prediction System (AHPS; e.g., Croley 1990; Hartmann 1990; Lofgren et al. 2002; Angel and Kunkel 2010; Hayhoe et al. 2010). The change factors in climate variables, typically derived from different Global Climate Model (GCM) projections, serve to perturb observed historical time series. Perturbed time series are then used as input to the AHPS, which compute future time sequences of the Great Lakes Net Basin Supply (NBS). The NBS is the primary driver of the Great Lakes levels. These can be derived by applying the latest version of Great Lakes Channel Routing and Regulation model (Hartmann 1988) using the NBS time sequences as input. The NBS is defined as the sum of the net atmospheric supply of water (that is the difference between precipitation that falls into the lakes and evaporation from lakes) and the supply from the land through the Great Lakes tributary streams (i.e., runoff). A comprehensive overview of the AHPS modeling system is provided in Gronewold et al. (2011). It is important to highlight that AHPS is primarily intended for seasonal forecasting, and not necessarily for longer-term climate-scale projections, in which energy and water budget conservation play an important role. Climate change (CC) studies based on the above approach resulted in widespread belief that water supplies in the Great Lakes would inevitably decline. For example, early studies considering GCM projections under the 2xCO2 atmospheric scenarios report a 23 to 51 % reduction in NBS and a decrease in lake levels ranging from 0.5 to 2.5 m (Croley 1990; Hartmann 1990). Similarly, a study by Angel and Kunkel (2010), using more than 500 runs from 23 GCMs under the SRES B1, A1B, and A2 emissions scenarios (Nakicenovic and Swart 2000), also suggests decline in annual lake levels. The projected median changes in lake levels for Lake Michigan-Huron for 20802094 were 0.25, 0.28, and 0.41 m for the B1, A1B, and A2 scenarios, respectively. According to the projections of Hayhoe et al. (2010) under the SRES A1FI emissions scenarios, water level for Lake Michigan-Huron is expected to decline up to 0.60 m. Although various GCM runs under different emissions scenarios were considered in the Bchange factor^ based studies, projected changes in the runoff from the terrestrial part of the Great Lakes basin in all these studies come from a single hydrological model (AHPS Large Basin Runoff Model; Croley 1983), which uses air temperature as a proxy for potential evapotranspiration (PET). As discussed by Lofgren et al. (2011), traditional hydrological models using this approach project an exaggerated evapotranspiration increase for future climate, and consequently runoff reduction, because of the absence of surface energy budget constraints in these models. Another important issue is that the AHPS does not account for the two-way exchange of energy and water between the Great Lakes basin and the overlying atmosphere, and therefore fails to capture important feedback processes occurring at the landand lake-atmosphere interfaces. Finally, the Bchange factor^ method accounts only for changes in the mean (and eventually variance) and does not really grasp how the probability distribution function of climate variables will change in the future climate. An alternative to the Bchange factor^ method, not involving the AHPS or a temperature proxy for PET, is to use direct hydrological outputs of climate models that ensure energy and water conservation (Lofgren et al. 2013). Studies by Manabe et al. (2004) and Milly et al. (2005) based on GCM output suggest an increase in Great Lakes net outflow and consequently increasing water supply. Lofgren (2004) and MacKay and Seglenieks (2013), who promoted the use of Regional Climate Models (RCMs) in evaluating potential changes in the Great Lakes NBS, project less dramatic declines in water levels compared to Bchange factor^ based studies. This study is a contribution to regional climate modeling effort over the Great Lakes region. It is expected that detailed regional characteristics represented in RCMs, including better depiction of the land surface spatial variability and processes, operating at a smaller scale will improve the reliability of the hydrological regime simulated over the Great Lakes. Our analysis includes a total of sixteen pairs of simulations generated with three different RCMs. These involve several numerical climate change experiments belonging to the North American Climate Change Regional Assessment Program (NARCCAP; Mearns et al. 2012), as well as an ensemble of simulations generated b (...truncated)