DIMES: Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean

The Earth's climate is changing, as it has done in the past. One of the great challenges faced by scientists today is to understand the causes and consequences of these changes. The way many scientists seek this understanding is by running computer-based climate simulations that mimic the complex interactions between the ocean, atmosphere, ice and living beings that are thought to be responsible for driving climate change. It is partly thanks to these simulations that we now know that ocean circulation plays a key role in modulating climate. One of the most important elements of ocean circulation is what scientists know as the 'meridional overturning circulation' (MOC). This term describes the cooling and resulting sinking of surface water masses in high-latitude regions, their journey through the deep ocean and their eventual warming and return to the surface, after many decades or centuries. The MOC is important to climate because the water masses involved in this long circuit through the ocean carry with them heat, CO2 and other significant substances such as plant nutrients, which in this way are distributed around the planet and locked away in the deep ocean for long periods of time. Perhaps the stage of the MOC that puzzles scientists the most, and one of the most serious challenges to the reliability of climate simulations, is the return of deep water masses to the surface. The reason is that the warming of the deep waters that allows this to happen is driven by currents occurring at the smallest spatial and temporal scales at which the ocean flows. Patchy measurements of these so-called 'mixing processes' in the ocean have led some scientists to believe that the surfacing of deep waters is mainly driven by the up-lifiting action of eddies (the weather systems of the ocean, which measure a few or a few tens of kilometres across) in the Southern Ocean. In turn, other scientists contend that the key driving process is small-scale turbulence arising from the breaking of waves (with crest-to-trough distances of tens of metres) travelling through the ocean interior. It is generally agreed amongst scientists that in order to resolve this 'ocean mixing conundrum' we must first understand how eddies and internal waves drive the surfacing of deep water masses in the Southern Ocean. This is so because the Southern Ocean is known to host a large proportion of the global upwelling, and offers the optimal conditions for both upwelling mechanisms to prosper. In order to achieve a breakthrough in this problem, we plan to conduct an experiment in which we will measure mixing processes in the Southern Ocean and their effect on ocean circulation. We will measure mixing directly by releasing a dye in the deep ocean west of Drake Passage and then observing how it spreads in space and time during a series of scientific cruises. In order to determine the extents to which eddies and internal waves are responsible for the observed mixing, we will obtain and analyze sophisticated measurements (many of which will be the first of their kind in the Southern Ocean) of their signatures in the temperature and salinity of the water and the velocity with which it flows. We will do this with a combination of instruments deployed from ships, freely flowing floats tracking water parcels, other floats profiling up and down between the surface and the deep ocean, instruments moored at great depth for 2 years, and satellites measuring sea level. With this unprecedented richness of information, we will be able to answer key questions such as 'What are the physics controlling the upwelling of deep water masses in the Southern Ocean?' and 'How should we represent the important mixing processes in climate simulations?'.