17:15 〜 19:15
[MIS14-P24] Miocene climate simulations using the MIROC4m AOVGCM
キーワード：Miocene, Climate modeling
Multiproxy reconstructions show that the Miocene epoch, 23.03-5.33 million years ago (Ma), was significantly warmer than present day and experienced atmospheric CO2 levels of about 300-600ppm, with possibly much higher levels during the Miocene Climatic Optimum (MCO, 16.75-14.5 Ma). As such, the Miocene epoch, in particular the MCO, has been suggested as a possible analogue for future projected climates corresponding to emission scenarios such as RCP 4.5 to RCP 6.0. The Miocene Model Intercomparison Project (MioMIP1) was a collection of climate simulations carried out with several models using related, but not the same, Miocene boundary conditions and forcings. To initiate a more systematic approach to assessing and comparing how models simulate Miocene warmth and other climate features, a more formal MioMIP2 is currently being organised.
In the present study, we use the latest formal Miocene boundary conditions (MioMIP2.1), apply them to the MIROC4m AOGCM coupled to the LPJ dynamical vegetation model, and run numerical experiments for several CO2 levels. The paleogeography is based on the MCO. Geographical features include the complete closure of the Bering Strait, and the opening of the Central American Seaway and Tethys Gateway. Ice sheets are fixed and confined mostly to small parts of Greenland and West Antarctica, and to half of East Antarctica. The model is integrated for 4,000 years until the ocean reaches a near-equilibrium state. With pre-industrial (PI) CO2 levels, tundra covers the extreme northern high latitudes, but is gradually replaced by boreal forests up to 3 x PI CO2 levels. Similarly, tundra across ice-free parts of Antarctica is gradually replaced by boreal forests in coastal areas. The Atlantic Meridional Overturning Circulation collapses while the Antarctic Bottom Water in the Atlantic strengthens and expands both northwards and upwards as CO2 levels increase. On the other hand, a weak Pacific Meridional Overturning Circulation develops in the northern hemisphere. The Miocene paleogeography alone leads to warming in most regions, in particular over Antarctica, including ice-free regions, where precipitation also increases. Comparison of model zonal temperatures with terrestrial and marine proxy data show generally good agreement, especially at higher CO2 levels, with notable exceptions at northern high latitudes where model data do not replicate the degree of sea surface warming shown by proxy data.
In the present study, we use the latest formal Miocene boundary conditions (MioMIP2.1), apply them to the MIROC4m AOGCM coupled to the LPJ dynamical vegetation model, and run numerical experiments for several CO2 levels. The paleogeography is based on the MCO. Geographical features include the complete closure of the Bering Strait, and the opening of the Central American Seaway and Tethys Gateway. Ice sheets are fixed and confined mostly to small parts of Greenland and West Antarctica, and to half of East Antarctica. The model is integrated for 4,000 years until the ocean reaches a near-equilibrium state. With pre-industrial (PI) CO2 levels, tundra covers the extreme northern high latitudes, but is gradually replaced by boreal forests up to 3 x PI CO2 levels. Similarly, tundra across ice-free parts of Antarctica is gradually replaced by boreal forests in coastal areas. The Atlantic Meridional Overturning Circulation collapses while the Antarctic Bottom Water in the Atlantic strengthens and expands both northwards and upwards as CO2 levels increase. On the other hand, a weak Pacific Meridional Overturning Circulation develops in the northern hemisphere. The Miocene paleogeography alone leads to warming in most regions, in particular over Antarctica, including ice-free regions, where precipitation also increases. Comparison of model zonal temperatures with terrestrial and marine proxy data show generally good agreement, especially at higher CO2 levels, with notable exceptions at northern high latitudes where model data do not replicate the degree of sea surface warming shown by proxy data.