Sensitivity of freshwater dynamics to ocean model resolution and river discharge forcing in the Hudson Bay Complex
Introduction
Anthropogenic changes such as global warming, which is causing an intensification of the hydrological cycle in the Arctic region (Zhang et al., 2012; Déry et al., 2009), as well as hydroelectric development, are changing the river discharge in northern Canada (Déry et al., 2011, Déry et al., 2016, Déry et al., 2018; MacDonald et al., 2018). One such region undergoing these changes is the Hudson Bay Complex (HBC), which includes Hudson Bay, James Bay, and neighbouring basins, Foxe Basin, and Hudson Strait and Ungava Bay, shown in Fig. 1. The HBC receives about 900 km3/year of river runoff, equivalent to roughly three times the Mackenzie River (Shiklomanov and Shiklomanov, 2003; Holmes et al., 2012), causing this region to be quite fresh compared to the Arctic Ocean. The main pathway of heat, mass, and freshwater exchange between the HBC, the Arctic, and the North Atlantic is via Hudson Strait. This river water flows out of Hudson Strait and along the coast of Labrador in the Labrador Sea, where deep convection occurs (Aagaard and Carmack, 1989; Straneo, 2006; Lazier et al., 2002; Lozier et al., 2019). The role of the fresh Hudson Strait outflow in these processes, however, is still largely unknown. This study aims to provide multi-year estimates of the HBC freshwater budget, so as to understand the role of model resolution and river discharge on freshwater fluxes within the HBC as well as to the North Atlantic.
Isolated from large scale ocean circulation, the main sources of freshwater to the HBC are river discharge and sea ice melt (Prinsenberg, 1988). On time scales less than a year, sea ice melt/growth has a much larger role in the freshwater budget compared to river discharge (Prinsenberg, 1988). Freshwater sourced from river discharge is found mainly along the coast, while freshwater from sea ice melt is distributed more equally around the bay (Granskog et al., 2007, Granskog et al., 2011).
Spatially, the distribution of freshwater within Hudson Bay can be divided into two regions, the outer boundary region and the interior region. The exchange of freshwater between the interior and boundary regions is mainly driven by Ekman transport (St-Laurent et al., 2011). In summer, the freshwater is imported into the interior, and is released during the fall (St-Laurent et al., 2012).
In the context of climate change, the length of the ice free season in the HBC is increasing, with both earlier break up in spring (Gough et al., 2004a; Gagnon and Gough, 2005; Castro de la Guardia et al., 2017; Hochheim and Barber, 2014; Kowal et al., 2017) and later freeze up in the fall (Gagnon and Gough, 2005; Castro de la Guardia et al., 2017; Hochheim and Barber, 2014; Kowal et al., 2017). These changes have been found to be related to the region's air temperature (Hochheim and Barber, 2014; McGovern and Gough, 2015). Hochheim and Barber (2010) found, for every 1∘C increase in the region's mean air temperature, it can result in a decrease of 105,000–117,000 km2 in late November sea ice extent with concentrations >80%. Sea ice thickness, on the other hand, is weakly related to air temperatures (Gough et al., 2004b). Ice thickness derived from satellite altimetry (Landy et al., 2017) shows a significantly asymmetrical spatial pattern across Hudson Bay in spring due to the strong cyclonic ice drift in winter. Their study also estimated 742 ±10 km3 of freshwater is stored in sea ice within the bay in April.
Anthropocentric influences have also impacted the HBC river discharge. Discharge entering the HBC has increased (Déry et al., 2016), which is associated with the intensified hydrological cycle in the context of Arctic warming (Déry et al., 2009, Déry et al., 2011, Déry et al., 2016; Zhang et al., 2012; Rawlins et al., 2010). Seasonally, hydroelectric development has increased winter HBC streamflow (Déry et al., 2011). Increasing air temperature also leads to earlier spring peak runoff (Déry et al., 2005; Gagnon and Gough, 2002), however, this varies regionally (Gagnon and Gough, 2002). Under the 1.5∘ and 2∘C future warming scenarios, MacDonald et al. (2018) found discharge increased in all seasons, except summer, with the largest increases occurring in spring.
In light of these current trends, it is still unclear as to the impact that these changes will have on the freshwater dynamics in this region. The annual net freshwater flux of river discharge is large, thus, changes in river runoff, by seasonal and spatial redistribution, or long term trends, lead to changes in seawater density and stability. A high runoff year and regulated discharge have been shown to lead to a decrease in salinity, along with a general increase in sea ice thickness (Saucier and Dionne, 1998).
To date, there have been no multi-year evaluations of the freshwater budget in this region. Other questions remain regarding the freshwater budget, for instance, how important are small scale processes in HBC dynamics? Does the freshwater budget change with changes in river runoff? To determine the sensitivity of the HBC to runoff forcing as well as model resolution, we use a general circulation ocean model coupled with a sea ice model to evaluate the freshwater budgets, pathways, and boundary-interior exchange processes of each simulation. The following section contains a description of the model, as well as the various datasets used in the numerical experiments. In Section 3, an evaluation of the model and the freshwater budgets for each subregion in the HBC, as well as boundary-interior freshwater exchanges and riverine water residence time, are shown. Our analysis of the residence time is in Section 3.4, preceding the summary and conclusions. This work is part of the BaySys project, a bay-wide initiative to investigate effects of hydroelectric regulation and climate change on various aspects of the Hudson Bay environment, such as the biogeochemical, biological, and physical components of the system.
Section snippets
Numerical model
We use a general circulation ocean model, based on the Nucleus for European Modelling of the Ocean version 3.4 (NEMO; Madec and the NEMO team, 2008), which is coupled to the sea ice model, Louvain-la-neuve Ice Model version 2 (LIM2) with elastic-viscous-plastic (EVP) rheology (Hunke and Dukowicz, 1997), and includes both thermodynamic and dynamic processes (Fichefet and Maqueda, 1997), for our simulations. We use the Arctic and Northern Hemisphere Atlantic (ANHA) configuration, which has two
Model evaluation
To evaluate the model, we first show spatial SST for the model and observations in Fig. 4a–j for both summer and fall. During the winter (January, February, March) and spring (April, May, June), the simulated SSTs are close to the freezing point (not shown) since the bay is ice covered. In fall, simulated SSTs (Fig. 4a–d) agree very well with observations (Fig. 4e). The temperature gradient from north to south in Hudson Bay is captured well by all simulations. Most simulations are too cold
Discussion
In this study we investigated the sensitivity of freshwater in the HBC to model resolution and runoff forcing. The results obtained here also have implications for pathways and residence times of various nutrients or pollutants commonly found in river discharge.
In terms of the freshwater budget, our estimates of surface fluxes are comparable to Prinsenberg (1988), with our peak freshwater fluxes in the summer of 18 km3/day compared to their 12 km3/day (including areas of both Hudson Bay and
Acknowledgements
We would like to thank Environment and Climate Change Canada for the use of the CGRF forcing fields, as well as the producers of GLORYS for the reanalysis data that we use to initialize our model simulations as well as providing our model with open boundary conditions. Thank you to Dr. Gregory Smith who provided the CGRF atmospheric forcing to force our ocean model. This work is part of the BaySys project, thus we thank the Natural Sciences and Engineering Research Council of Canada and
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Now at Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada.