Transient Response of Geothermal Heating in the Southern Ocean in a Global Climate Model

Model and observational studies have concluded that geothermal heating significantly alters the global overturning circulation and the properties of the widely distributed Antarctic Bottom Waters. However global climate models continue to exclude this constant abyssal ocean buoyancy input. We present the first regional diagnosis of the impacts of geothermal heating on Southern Ocean temperature and circulation in a fully coupled climate model. By the end of the 500-year pre-industrial spin-up phase, temperature changes associated with geothermal heating are most prominent in the Southern Ocean (in some regions over 0.3^oC), and are spread over the entire water column. We describe how the background density structure and major circulation pathways act as drivers of these changes. Despite the impact of individual model mixing parameterisations on the meridional overturning circulation, geothermal heating enhances Southern Ocean deep overturning circulation by over 20%. The upwelling of warmer deep waters and cooling of upper ocean waters within the Antarctic Circumpolar Current (ACC) region decrease its transport by around 5 Sv- a similar magnitude to 21^st century climate projections. We focus on the large-scale dynamics transporting these Southern Ocean anomalies, including steeply sloping isopycnals, weak abyssal stratification, voluminous southward flowing deep waters and exported bottom waters, the ACC, and the polar gyres. We conclude that, given the Southern Ocean has been identified as a prime region for deep ocean warming, geothermal heating should be included in climate models to ensure accurate representation of abyssal temperature changes.