Linking the Antarctic Circumpolar Current with Weddell Sea convection and with the meridional overturning circulation in a hierarchy of GFDL models

Sergey Molodtsov, Texas A & M University College Station, Department of Oceanography, College Station, TX, United States, Irina Marinov, University of Pennsylvania, Department of Earth and Environmental Science, Philadelphia, United States, Behzad Asadieh, University of Pennsylvania, Philadelphia, PA, United States, John Edward San Soucie, University of Pennsylvania, Department of Earth and Environmental Sciences, Philadelphia, PA, United States, Anna Cabre, Institute of Marine Science, Department of Physical and Technological Oceanography, Barcelona, Spain and Ron Maor, University of Pennsylvania, Philadelphia, United States
Most climate models demonstrate strong natural variability of the Antarctic Circumpolar Current (ACC), which in turn depends on buoyancy conditions through the water column as well as wind forcing at the ocean surface. We hypothesize that the roles of buoyancy forcing and wind forcing will change with depth. Due to that ACC shows different dynamics in the upper part of the ocean and in the ocean interior. Here we analyze the relationship of surface and deep ACC to different processes of global ocean circulation and atmospheric conditions.

In the first part of the project we investigate potential teleconnections between the ACC and North Atlantic/Weddell Sea (NA/WS) convective areas, Atlantic Meridional Overturning Circulation (AMOC), Antarctic Bottom Water (AABW), Antarctic Intermediate Water (AAIW) transports and westerlies using output from the fifth generation of Earth System Models (ESM), part of the CMIP5 model intercomparison. ESM output from pre-industrial control run scenarios was used to calculate indices representing natural, multi-decadal modes of ocean/atmospheric variability for: ACC, AMOC, AABW, AAIW, NA/WS convection and westerlies. ACC shows clear correlation with calculated indices of water transport, convection and westerlies. Across many of the models, in decades with strong WS Convection, ACC and the AABW formation strength increase, while AAIW formation, and – with a delay - NA deep ocean convection and AMOC strength decrease.

In the second part of the project, ESM output from historical and RCP 8.5 runs was used to analyze jointly the impacts of climate change on ACC changes. ACC transport generally shows positive correlations with the The WS convective index: e.g. in the GFDL-ESM2G model both the ACC and the WS index show an ~50yr variability in the pre-industrial simulation. However, climate warming starting in the 20th century decouples the ACC and WS behavior. As the Southern Ocean freshens and warms it stratifies, and the WS convection eventually shuts down across models.

Curiously, the ACC appears to have a different behavior in different depths ranges, with a dynamical decoupling between the deep ACC below 3000m and the surface ACC above 3000 m both in pre-industrial times and under climate change. The most notable separation of the deep and surface ACC happens towards the end of the 21st century in the RCP 8.5 scenario, when surface ACC exhibits decreased variability in its signal while deep ACC undergoes drastic strengthening. We explore the ACC-WS convection connection across models and timescales.