Clim. Past 9, 2687–2702 (2013)

A period of relative climate warmth ended about 14 million years ago during the middle Miocene climate transition. The closure of the Tethys Seaway that connected the Indian and Mediterranean basins could have contributed to this cooling, numerical simulations suggest.

Noémie Hamon of IPHEP and LSCE, France, and colleagues used a coupled ocean–atmosphere general circulation model to assess the impacts of the closure of the Tethys Seaway on the global ocean circulation. In simulations with a deep seaway, a warm and saline water mass formed at intermediate depths, and carried heat from the northern Indian Ocean to the Southern Ocean. However, if the seaway shoaled or closed entirely, intermediate water formation ceased, and the Indian Ocean cooled and freshened as far south as 60° S. The closure of the eastern Tethys did, however, promote the flow of water from the Mediterranean to the Atlantic down to 400 m depth, strengthening the southern limb of meridional overturning circulation in the Atlantic.

The absence of Tethys intermediate water and the strengthening of Atlantic overturning should have strengthened the ocean current that circles Antarctica, but were probably not the only causes of cooling at this time.