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Float no.6901632

             

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This ARVOR float was launched by French scientists during the OVIDE research cruise in the Atlantic northwest of Spain at 46.5°N, 19.7°W on 31 May 2014. It made its last report on 11 February 2015 from 45.2°N,  23°W.

Measuring the deep ocean

float in the water
Deploying the float

float tracks

Tracks of two Deep Argo floats. Blue: 6901631, red: 6901632.

This float measures temperature (T), salinity (S) and oxygen (O2) to a depth of more than 4000m, compared to a depth of 2000m for ordinary Argo floats.

This means it must be able to work under extreme pressure - more than 400 times higher than atmospheric pressure at the sea surface.

The float is part of the Deep Argo programme - an initiative to extend Argo to the deep ocean below the thermocline.

Understanding how temperature, salinity, oxygen and nutrients vary with time and space in the deep ocean is important for many reasons. It will, for example, improve our understanding of the ocean's role in Earth's climate system, the global carbon cycle and the cycles of plant nutrients such as nitrogen and their role in marine ecology.

Interpreting the data

Data from two deep floats released in May 2014 are shown below, and their float tracks on the above left. This region is influenced by very salty water flowing into the North Atlantic from the Mediterranean - known to oceanographers as Mediterranean outflow.

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Mediterranean outflow

MERIS image
The Strait of Gibraltar.

If the strait of Gibraltar were closed Mediterranean sea level would fall by about 1m each year! This is because evaporation is far greater than the input of fresh water from rain and river flow. To compensate, Atlantic water flows in through the Strait of Gibraltar, and continues eastwards in a surface layer about 150m thick.

Along its way the surface water becomes more and more salty, reaching over 38 psu south of Turkey. During winter this warm, salty water cools and sinks to become Mediterranean Intermediate Water (MIW), which is found at a depth of 150-600 m. MIW is still quite warm (about 14-15°C), and very salty (about 37-38 psu, compared to the 34-36 psu typical of Atlantic water). It flows slowly back towards the west, and eventually leaves the Mediterranean through the Strait of Gibraltar, underneath the Atlantic inflow.

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float tracks
T, S and O2 profiles from float 6901631 (blue on the map).

float tracks
T, S and O2 profiles from float 6901632 (red on the map).

Q1.
 
 

Look at the S-profiles above. At what depth range do you find the salty Mediterranean outflow water? How does the thickness of this Mediterranean water layer differ between the two floats? What about the maximum salinity? Why?

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A1.

The high-salinity Mediterranean outflow water is found between about 500m and 1500m in the profiles for float 6901631, and between about 650m and 1250m for float 6901632.

The layer of Mediterranean water is thicker (app. 1000m) in the profiles from float 6901631 west of Spain, than in those from the other float (app. 500m), which is further away from Gibraltar. The salinity of the Mediterranean outflow water peaks at 36.2 psu in the profile from float 6901631, but only reaches 35.8 psu in the profile from float 6901632.

As the Mediterranean water flows away from Gibraltar, it spreads out and mixes with Atlantic water in the layers above and below, so the layer of Mediterranean water becomes thinner and less salty.   CLOSE

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Oxygen concentrations

An increasing number of Argo floats are fitted with oxygen sensors, which measure the concentration of dissolved oxygen gas in micromoles per litre (or per kg) of seawater. The data allow us to build a picture of how oxygen varies with depth and from region to region over time scales from 10 days to years. This will help to document the ocean's loss of oxygen as a result of ocean warming - particularly important in low oxygen regions of the deep ocean. Oxygen data can also contribute to a better understanding of ecology in different regions.   More about this in oxygen in the ocean.

Q2.
 
 

How does the oxygen concentration of the Mediterranean water compare with that of the Atlantic water above and below?

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A2.

The Mediterranean water has a lower oxygen concentration than the Atlantic water above and below. In the profile from float 6901631 the lowest concentration is about 170 micromol at around 700-1200m depth.   CLOSE

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Link to the main Euro-Argo project website.