16:15 〜 16:30
[PCG19-20] Climate system on snowball planets revealed by MIROC4m
キーワード：全球凍結惑星、気候モデル、気候システム、海洋大循環
Understanding the climate system is one of the biggest issues in Earth and Planetary science. Our Earth has experienced at least three snowball events where the planetary surface is covered with ice and snow. It is considered to be an important event for the terrestrial planet, as significant changes in the climate system and organisms before and after the snowball events. Quantifying the role of subsystems, such as the atmosphere, ocean, and glacier, during dramatic climate changes will help us understand the potentially habitable planets that will be the primary targets for next-generation exoplanet observations.
Previous studies have investigated the conditions for snowball planets using general circulation models under land planets, aqua planets, and modern Earth configurations. However, the global ocean circulations under snowball planets have not been simulated. The global ocean circulation is essential for understanding the conditions of the end of the snowball state, marine geochemical cycles, and biological activities for past snowball events of the Earth.
In this study, we conduct numerical experiments using a coupled atmosphere-ocean climate model MIROC4m, which has been used in simulations for past and future climates. We change the ocean component model of MIROC4m to separate sea surface height and sea ice thickness calculations, as the standard MIROC4m does not allow sea ice thicker than the surface mixed layer thickness (~40m). We use modern configuration of the Earth and conduct simulations with different solar constants. Each simulations is carried out for 2,000 years to reach respective steady states.
When the solar constant is set to 94% of the present-day, the sea ice covers all of the ocean within 1,040 years to reach a snowball state. At the end of the simulation, the sea ice thickness becomes more than 100 meters. As shown in previous studies, the transition to the full snowball state is fast (less than 100 years), associated with strong deep ocean circulation. As sea ice covers the entire global ocean, atmospheric cooling from the sea surface becomes weaker, and salinity stratification becomes stronger, leading to drastic weakening in the deep ocean circulation. In the final state of the snowball, the volume transport of the deep ocean circulation is less than 1 Sv, significantly smaller than the modern circulation of ~20 Sv.
Previous studies have investigated the conditions for snowball planets using general circulation models under land planets, aqua planets, and modern Earth configurations. However, the global ocean circulations under snowball planets have not been simulated. The global ocean circulation is essential for understanding the conditions of the end of the snowball state, marine geochemical cycles, and biological activities for past snowball events of the Earth.
In this study, we conduct numerical experiments using a coupled atmosphere-ocean climate model MIROC4m, which has been used in simulations for past and future climates. We change the ocean component model of MIROC4m to separate sea surface height and sea ice thickness calculations, as the standard MIROC4m does not allow sea ice thicker than the surface mixed layer thickness (~40m). We use modern configuration of the Earth and conduct simulations with different solar constants. Each simulations is carried out for 2,000 years to reach respective steady states.
When the solar constant is set to 94% of the present-day, the sea ice covers all of the ocean within 1,040 years to reach a snowball state. At the end of the simulation, the sea ice thickness becomes more than 100 meters. As shown in previous studies, the transition to the full snowball state is fast (less than 100 years), associated with strong deep ocean circulation. As sea ice covers the entire global ocean, atmospheric cooling from the sea surface becomes weaker, and salinity stratification becomes stronger, leading to drastic weakening in the deep ocean circulation. In the final state of the snowball, the volume transport of the deep ocean circulation is less than 1 Sv, significantly smaller than the modern circulation of ~20 Sv.