The Southern Oceans, Widnes SciBar 11th February 2015

The challenge of observing the deep ocean: who cares and why is it so difficult?

Dr Matt Donnelly, British Oceanonographic Data Centre (BODC)

On the 11th February, we welcomed Matt Donnelly a data scientist who works at the British Oceanographic Data Centre who outlined the key physical characteristics of the oceans:

• water density, largely determined by temperature and salt content,
• wind patterns at different latitudes that cause ocean currents,
• temperature,
• current speed and direction.

The Southern Oceans

These are significant features of the Thermohaline Circulation (thermo - heat; haline - salt). With a transit time of around 1000 years, currents cause an extensive mixing between the ocean basins, reducing differences between them and making the Earth's oceans a global system. On their journey, the currents transport both energy (in the form of heat) and matter (solids, dissolved substances and gases) around the globe. Such circulation has a large impact on the climate of the Earth.

Southern Ocean Matt’s particular interest is in the Southern Ocean, the only ocean that stretches all around the globe, linking the Atlantic, Pacific and Indian Oceans. Unlike other oceans there is no north / south barrier to current flow; as a result, there is a current around the Antarctic continent (the Antarctic Circumpolar Current) that carries more water than any other current in the world. The sea water can be cooled to near freezing at -1.8°C and become highly saline - as a result it sinks to the ocean floor and spreads out over the global oceans as part of the Thermohaline Circulation. Using food dyes Matt demonstrated the how pure water, slightly saline water and highly saline water when introduced into a tank do not readily mix, with the highly saline layer being at the bottom.

Impact of Oceans on Climate The oceans are a massive heat store - the top 5m of oceans have the same heat capacity as the entire atmosphere above them. The ocean currents move cold and warm water around the globe. Heat and moisture are exchanged between the atmosphere and the ocean, effectively acting as one. Climate change theorists had predicted a steady increase in the average temperature across the world. However, for the last 10 years or more this increase hasn’t been as significant, as observed by the conventional methods of measurement - as the heat input continues, where is it going?  One possible explanation is that the heat added by human activity has been absorbed by the oceans and distributed by ocean currents.

Gathering Information about the Oceans Matt described several methods of making measurement - here are a selection.

Ships. Dedicated research ships are the traditional way of studying the oceans & one experienced by Matt in the Southern Ocean. However, this is an expensive way of gathering information and is limited to the route that the ship takes. XBTs (Expendable Bathythermographs) can be dropped from ships not specialising in research (e.g. cargo ships); as it sinks the measurement probe sends data to the ship by wire until  the wire runs out and breaks, and the XBT then sinks to the ocean floor. Seal Tags. A measurement device & transmitter are glued to the rear of a seals head and measurements are received according to the route & depth the seal takes. When the seal moults, the device is lost. Matt showed the trail a seal took circumventing the Antarctic continent.

International Argo project. Matt described this as ‘a game changer’. With international collaboration, large numbers of Argo floats which have controllable buoyancy are dropped into the sea where they are moved by the ocean currents. Powered by batteries, they are programmed to:-

- sink to a specified depth to gather information, e.g. salinity, temperature,

- float with the current for a specified period, still gathering data

- return to the surface and transmit the data they have collected to a satellite,

- repeat this cycle until the battery is exhausted, usually after about 4 years. 

Matt said there are currently 3290 floats in operation, one of which has been operating for 8 years. In his role as a data scientist, he is involved in making use of the vast amount of data received (e.g. reports from 300 floats a day). Part of the international agreement is that the data are freely available to everyone, without restrictions.

Future Methods of Measurement. The Argo floats currently in use are limited to a depth of 1000m. The floats typically park there for ~9 days, descend to 2000m, then record a profile on the way to the surface.  Matt's role is to help ensure access to the data is maintained at all times for the scientific community. However, as the current Argo floats can only reach less than half the average depth of the oceans, the next stage is to deploy floats able to cope with the high pressure of 6km down - little is known about the oceans at such depths. One possible explanation of why the world average temperature has not increased in recent years is that the excess heat has been taken up in the deeper ocean water.

British Oceanographic Data Centre The BODC is a national facility for looking after and distributing data concerning the marine environment. It is hosted by the National Oceanography Centre, at its Joseph Proudman site in Liverpool.

Joseph Proudman.  Joseph Proudman (1888 - 1975) was ‘a distinguished British mathematician and oceanographer of international repute. His theoretical studies into the oceanic tides not only "solved practically all the remaining tidal problems which are soluble within the framework of classical hydrodynamics and analytical mathematics" but laid the basis of tidal prediction service (developed with A. T. Doodson) of great international importance’(Wikipedia). Incidentally, in his pre-university days Joseph lived in Bold and was a teaching assistant at Farnworth, both only a short distance from the Hillcrest Hotel where the SciBar meets. (We'll add a photo of the grammar school when we establish the date!)

Thanks to Matt for a very interesting and enjoyable evening.

Bob Roach

24th Feb 2015 (modified 8th March 2015)