Friday, 14 March 2014

An introduction to DIMES

DIMES is an exciting oceanography project which has absolutely nothing to do with American pennies. DIMES stands for 'Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean'. It is a bit of a terminology mouthful, so lets break it down. First of all, you have to understand a little bit about the structure of the ocean. The ocean is stratified which means that it is made up of many layers of water – less dense at the top and heavier as you go down. Density is dependent on the temperature and salinity of the water. Warm, fresh water is less dense or lighter than cold, salty water. Isopycnals are a bit like isobars on a weather chart, they map areas of water that are the same density. So isopycnal mixing refers to water motions along density surfaces and diapycnal processes happen across density layers. 

 Water moves easily along horizontal or isopycnal layers, but mixes only slowly across the layers (diapycnal mixing). This vertical mixing is predominantly driven by tiny turbulent and stirring motions – a bit like the way you might stir up the layers in a latte to get the coffee to mix with the milk on top. Essentially it is a combination of the diapycnal and isopycnal mixing which drives the upwelling of deep waters back to the surface around Antarctica. Climate models show that the level small scale mixing in the ocean has a profound affect on the global ocean circulation. However we still don’t really know how best to represent these small-scale processes in our models, which is why these observations are so important.

Acoustic data from the ocean – the red bands follow ocean density layers and track an underwater wave, or internal wave. As the wave breaks, it mixes up the water column forming fantastic swirling patterns and overturns, just like waves on a beach.

Measuring tiny mixing processes across a vast ocean is a very tricky task. In DIMES we use several methods. One method, involves releasing a blob – ‘or drop’ - of chemical dye tracer into a targeted ocean density layer. In DIMES the tracer, a compound called (take a deep breath) trifluoromethyl sulfurpentafloride was released about a mile below the sea surface. Each year, the horizontal and vertical spread of the tracer is mapped out by measuring its concentration in hundreds of seawater samples. The techniques used are so accurate that they can identify one milligram of tracer in a cubic kilometer of seawater – that really is a drop in the ocean! This way we are able to identify how quickly the Southern Ocean moves water particles around – both in the horizontal and vertical.

A model of the tracer molecule – SF5CF3

Supplementing the tracer experiment are measurements of temperature, salinity, current strength and turbulent motions. We also have moored-instruments that are left in one spot in the ocean for several years, enabling us to monitor how things are changing from day to day. The moorings capture other features like eddies and underwater waves, which play a crucial role in Southern Ocean mixing. Together, these data will help us to understand the processes that drive the ocean circulation and mixing so that we can predict how our Earth system will respond to the increased levels of carbon dioxide that we have pumped into the atmosphere.

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