The coldest and most dense waters in the World Oceans are found on the Antarctic continental shelf. Here water is cooled due to interaction with floating glacial ice at great depths, and the resulting water mass is therefore termed Ice Shelf Water. This Ice Shelf Water sinks to the ocean floor, because cold water is more easily compressed with increasing pressure than warmer water with a similar density. The flow of Ice Shelf Water down the continental slope in the Weddell Sea (the Atlantic sector of the Antarctic Ocean) has been studied by oceanographers in Bergen for 30 years. The newly formed bottom water is of vital importance to the world oceans, as it is the only source of oxygen, and contributes to keeping the deep waters cold.
The new article in Tellus summarises almost all earlier profile-measurements of temperature and salinity, and documents a surprisingly large variability in the flow itself. The mean hydrography data, from more than 300 individual stations since the 1970’s, documents a cold and vertically well-defined layer of Ice Shelf Water. This Ice Shelf Water is steered downwards by two ridges crosscutting the continental slope, but also mixes with “warmer” water (above 0ºC). Close to the main ridge, at 2100 m depth, the temperature is still close to the surface freezing point. The minimum temperature here is -1.64ºC, but a nearby current meter reveals a minimum temperature of -1.92ºC. At this depth the cold layer has a mean thickness of about 100m.
The variability of the flow is to a large extent due to three distinct oscillations. These oscillations have periods of 35 hours, 3 days and 6 days, and are seen in both temperature and velocity records from moored current meters. The oscillations are episodic, cover the dense bottom layer as well as the water column above, and have a horizontal scale of ~20-40 km. Apart from these features the oscillations are a bit of a mystery, and no conclusion can be drawn as to the mechanism creating them. Three different theories are analysed to seek an explanation, theories that all explain seemingly similar oscillations observed elsewhere, but they all fail when compared to this unique set of data from the Antarctic continental slope.
 |
| Map showing mean temperature at the bottom of the Antarctic Continental slope in the Weddell Sea. |
This research is a part of the Bipolar Atlantic Thermohaline Circulation (BIAC) , which is one of several project within the Norwegian International Polar Year efforts.