Timothy Merlis and Tapio Schneider, 2010
To explore climate dynamics of tidally locked Earth-like exoplanets and to illustrate how the climate would adjust if Earth abruptly entered a tidally locked state, we conducted simulations with the GFDL CM2.1 coupled climate model (Delworth et al. 2006). Starting from a typical January 1 initial condition in the present-day climate, we set the planetary rotation rate to 1/365th of the present value (so that 1 day is equal to 1 current Earth year). We fixed the insolation so that there is a perpetual subsolar point on the equator at 88°W (near the coast of Ecuador). The simulation was run for 50 years. This simulation with a comprehensive coupled climate model illustrates and expands upon the dynamics discussed in the context of an aquaplanet atmosphere-only model in Merlis and Schneider (2010).
The surface temperature changes rapidly over land masses to day-side values of about 290 K and to night-side values of about 240 K. The ocean surface temperature changes more slowly; the night-side ocean remains near the freezing point for the length of the simulation.
Precipitation patterns change rapidly (within a few months) from the zonally-elongated intertropical convergence zone that is typical of Earth today to a configuration in which there is substantial precipitation near the subsolar point and little precipitation on the night-side of the planet. The regions of large precipitation are determined by the atmospheric circulation: the near-surface atmospheric flow converges near the subsolar point, leading to strong ascending motion and condensation (see Merlis and Schneider (2010) for a detailed discussion). The topography (e.g., the Andes) modulate the precipitation patterns so that they are less concentric about the subsolar point than in the aquaplanet simulations in Merlis and Schneider (2010).
The net evaporation field (evaporation minus precipitation) shows that atmospheric water vapor is transported from the night side to the day side. Regions on the day side of the planet away from the subsolar point, such as Canada, experience net drying. They become relatively warm because of the loss of evaporation as a cooling mechanism for the surface. Sufficiently far from the subsolar point, there is net evaporation, which eventually would lead to the formation of deserts and complicating habitability of those regions.
Because the ocean is actively convecting where it is near freezing, no sea ice
forms during this simulation, except near the coasts. However, the ocean
surface will eventually freeze (on the much longer timescales needed to cool the entire water column to freezing).
It may seem surprising that no widespread sea ice forms on the night side of the planet within 50 years; after all, new sea ice forms every winter in Earth's high latitudes. However, Earth's polar regions currently experience net precipitation, and the fresh water effect on the ocean density allows the surface to freeze without the need for the entire column of ocean water to reach the freezing point. In contrast, the night side of the tidally locked Earth experiences net evaporation, so the ocean surface is becoming cooler and saltier, so convection penetrates deeper into the interior ocean as the simulation progresses and prevents the surface from freezing.
Delworth et al. 2006: GFDL's CM2 Global Coupled Climate Models. Part I: Formulation and Simulation Characteristics. Journal of Climate, 19, 643-674. Official version
Zonal wind animations: slow rotation, rapid rotation