Response of the High‐Latitude Southern Hemisphere to Precessional Forcing: Implications for Pleistocene Ocean Circulation.

The cycling of climate into and out of glacial states in response to orbital forcing illustrates the important role of feedbacks in the Earth system. In this contribution, the Geophysical Fluid Dynamics Laboratory coupled ocean‐atmosphere model is used to qualitatively assess how changes in the Earth's precessional parameters may have impacted Atlantic Meridional Ocean Circulation and Southern Ocean dynamics. The Southern Ocean region responds to precessional forcing similar to that of the Last Glacial Maximum with a slight shift in maximum average annual wind stress, a reduction in austral winter wind stress, and an increase in austral winter sea ice. During austral summer, stratification and wind stress increase, Changes in the temperature and density of the North Atlantic lead to a decrease in Atlantic Meridional Overturning Circulation. Together these changes contribute to aging of the global deep ocean, except around Antarctica where enhanced local convection decreases the age of upwelling waters. These circulation changes may have implications for a precessionally driven feedback that contributes to enhanced storage of CO2 in the deep ocean and a reduction in atmospheric CO2. Plain Language Summary: The Earth has cycled into and out of ice ages over last 2.6 million years. The precise factors that cause these changes in climate are not fully understood. The correspondence between the Earth's climate and the Earth's position in space suggests that there is a connection between the two. In this work we use a fully coupled global climate model to explore whether the season during which Earth is closest to the Sun has an impact on ocean circulation. We use two model runs, one having the Earth in a northern hemisphere winter perihelion (close pass) position and another placing the Earth in a northern hemisphere summer perihelion. We find that the northern hemisphere winter perihelion position is associated with changes in Southern Ocean circulation that include an increase in upwelling and a decrease in the age of the upwelled water. We also find that deep ocean circulation in the Atlantic Ocean decreases and that global ocean deep waters increase in age. The observed changes are consistent with the idea that a Northern Hemisphere winter perihelion position may contribute to glacial interglacial climate change and a drawdown of atmospheric CO2. Key Points: Precessional forcing that places the Earth at a boreal winter perihelion position causes an increase in wind stress and Ekman upwelling during austral summer/fall and a decrease during austral winter/spring in the high‐latitude Southern OceanGlobal deepwater age responds to a boreal winter perihelion position by increasing ~200 yearsAtlantic Meridional Overturning Circulation decreases in response to a boreal winter perihelion position. Southern Ocean ventilation increases [ABSTRACT FROM AUTHOR]