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tokens: Snowball bifurcation less likely (Joshi Haberle 2012 ; Shields et al. 2013 , 2014 ). Planetary heat transport, if efficient enough, could destabilize partially glaciated states on a tidally locked planet and reintroduce a Snowball bifurcation as was discussed by Checlair et al. ( 2017 ). However, the authors did not explicitly model ocean heat transport, motivating further investigation into its role in possibly retrieving the Snowball bifurcation. Considering dynamic oceanic processes could increase the overall planetary heat transport, potentially making a bifurcation possible. Hu Yang ( 2014 ) previously modeled a tidally locked planet orbiting an M-star using an atmospheric GCM (CAM3) coupled to a dynamic ocean. They did not directly look for bifurcations or hysteresis, but found a smooth transition from a partially glaciated to a fully ice-covered planet, suggesting the potential lack of a bifurcation. In this Letter, we investigate the effects of including a dynamic ocean on the Snowball bifurcation for tidally locked planets in a coupled ocean–atmosphere GCM (ROCKE-3D). 2. Methods 2.1. Model Description We performed our calculations using ROCKE-3D (Way et al. 2017 ), a fully coupled ocean–atmosphere general circulation model (GCM) that is derived from NASA Goddard Institute for Space Studies ModelE2. ROCKE-3D has previously been used to examine the climate states of slowly rotating and tidally locked exoplanets, including Warm and Snowball climates (Del Genio et al. 2017 ; Way et al. 2018 ; Jansen et al. 2019 ). The reader is referred to Way et al. ( 2017 ) and references therein for details regarding ROCKE-3D and its parent model, but we note that ROCKE-3D includes a thermodynamic–advective sea ice model. In contrast with previous studies of Snowball bifurcations on tidally locked planets (Checlair et al. 2017 ), ROCKE-3D considers ice advection, allows for partial ice coverage within cells, and includes dynamical ocean heat transport. Our present investigation using ROCKE-3D thus represents a significant advance over prior work. 2.2. Model Configurations Our experiments consider an Earth-like planet with regard to mass, radius, surface gravity, and atmospheric mass and composition (although we exclude O 3 ). Our simulated planet nonetheless differs from the Earth in several ways. We simulate a tidally locked planet in a circular orbit with a 50 day period around an M-star (Kepler 1649, M5V). We chose the stellar spectrum from ROCKE-3D’s default spectra. It is based on the BT-Settl spectrum with T eff = 3200 K, log g = 5, Fe/H = 0 (Allard et al. 2012 ). The surface of the planet is covered exclusively by ocean. The ocean in our model consists of five layers and is considerably shallower than Earth’s, reaching a maximum depth of 189 m. In this configuration, running the model on 44 CPU cores requires on average 180 hr of wall time. A deeper ocean would increase the computational cost considerably. Our model configuration is summarized in