The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) programme aims to enhance our understanding of Southern Ocean mixing processes in the context of the Meridional Overturning Circulation (MOC), which is a critical regulator of Earth's climatic processes.
Climate models are highly sensitive to the representation of mixing processes in the southern limb of the MOC, within the Southern Ocean. Theories and models of the Southern Ocean circulation have been built on the premise that flow in the ocean interior occurs without any change in heat, with processes involving heat transfer being confined to the upper-ocean mixed layer. It is also often assumed that interior mixing across density surfaces is small.
Evaluation of mixing in the Southern Ocean has proven difficult due to the limited availability of suitable data but a few recent studies have suggested that mixing across density surfaces might be large in some locations, particularly over rough bathymetry. This may have implications for the accuracy of climate models.
The primary objective of DIMES is to assess eddy-driven mixing along, and interior mixing across, density surfaces in the Southern Ocean. The project aims to investigate the manner in which these mixing processes affect the overall energetics and property balances for the Southern Ocean and, in turn, the global ocean.
Fieldwork comprises the collection of hydrographic and bathymetric data within the Antarctic Circumpolar Current (ACC) to the southwest of South America and through the Drake Passage into the Scotia Sea. The programme also includes a Lagrangian experiment comprising the release of an inert chemical tracer at the upstream edge of the study area in late 2009 and the deployment of floats and drifters in 2009 and 2010.
DIMES involves the collaboration of both UK and US research institutions, with the US undertaking scientific cruises in 2009 (RV Revelle 09) and 2010 (RV Thompson 10) and the UK undertaking cruises in 2009 (RRS James Cook JC041), 2010 (RRS James Cook JC054), 2011 (RRS James Clark Ross JR276), 2012 (RRS James Cook JC069), 2013 (RSS James Clark Ross JR281) and 2014 (RSS James Clark Ross JR299).
The programme is funded by the Natural Environment Research Council (NERC), the British Antarctic Survey (BAS) and the National Science Foundation (NSF), and is co-ordinated by the National Oceanography Centre (NOC), and the Woods Hole Oceanographic Institution (WHOI).