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Float no.6900701

    Temperature and salinity profiles   Time series plots
  Download the data   Using the float data with ocean analysis maps   Comparisons  

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This ARVOR float was launched by French scientists west of Portugal at 40.5°N, 11.1°W on 1 November 2011. It has recorded a total of 121 profiles, and made its last report on 6 February 2015 from 42.4°N,  11.9°W.

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.

Temperature and salinity profiles

Temperature profiles Salinity profiles

Profiles of temperature (left) and salinity (right) from Argo float 6900701. The profiles show how temperature (T) and salinity (S) change with depth from the surface to 2000m. Early profiles are dark blue, the latest profiles are deep red or brown. Click on the images for larger plots.     Source of plots: IFREMER/Coriolis.

The temperature and particularly the salinity profiles give us some interesting clues about how water moves below the sea surface in the North Atlantic west of Spain.

Q1.
 
 

Look at the T-profiles above. At what depth range do you find the greatest variability?
How does this compare with variability at the surface? Why is this surprising?
How does the variability in the salinity at this depth compare with the variability in temperature?

Argo float
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A1.

The greatest temperature variability is found between 600m and 1600m depth. This is also the depth with the greatest variation in salinity.

Surprisingly the temperature variability is greater at 600-1600m than at the surface, where heating in the summer and cooling in winter has the greatest impact. Such seasonal effects decrease with depth, as you can see if you look at other floats from similar latitudes.

Flow through Gibraltar

Flow across the sill at Gibraltar.

The variation in salinity is even more marked than the variation in temperature. Why is this?

The Mediterranean Sea is a warm, dry, area. Each year more water is lost from the sea surface by evaporation than is gained by rainfall or the flow into the Mediterranean from rivers like the Nile. As a result Mediterranean water is saltier and denser than the water from the Atlantic.

In the Strait of Gibraltar the Atlantic water flows into the Mediterranean on the surface, while the denser Mediterranean water flows out into the Atlantic below this.     CLOSE

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Q2.
 
 

The Mediterranean outflow across the sill at Gibraltar into the Atlantic explains the higher temperatures and salinities found at depths of 800 - 1300m in these profiles. Yet the Mediterranean water enters the Atlantic across a sill that is only 300m deep. Looking at the T-S-density diagram on the left, can you explain this?

How to use T-S-density diagrams

Click on the orange float icons to reveal the answers!

Argo float
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A2.

T-S-density diagram

T-S-density diagram of the Med. outflow

A is Atlantic water near the surface (T ≈ 10-15 °C, S ≈ 35.3-36.0 psu).
B is Atlantic water at depths over 1500m (T ≈ 3-4.5 °C, S ≈ 34.9-35.2 psu).
C is Mediterranean ouflow water mixed with some Atlantic water (T ≈ 11-12 °C, S ≈ 36.2 psu).

Although the Mediterranean outflow is warmer than most of the Atlantic surface water, its high salinity makes it denser, even after some mixing with Atlantic water. You can see by following the dotted red lines of equal density on the diagram (right). As a result the Mediterranean water flows downhill from the 300m deep sill at Gibraltar.

Salinity map

Ocean salinity at 1000m.
Source: Mercator ocean analyses

Subsurface water in the Mediterranean is around 12° C and 38 psu. As it flows out through Gibraltar and down the slope of the sill, it mixes with the Atlantic water above until it reaches a depth of about 1000m or more, where its density equals that of the water around it.

We can see the spread of the Mediterranean water from an analysis of all the Argo data carried out by Mercator Ocean (left). This shows that the saltiest water is found mostly northwest of Gibraltar.

The rotation of the earth makes the current veer to their right, so the deep salty outflow is concentrated in an underwater river running along the sea floor parallel with the south coast of Spain. When it reaches Cape St Vincent it turns to the right and heads northwards.

CLOSE

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Time series of temperature and salinity

Temperature section Salinity section

Time series of temperature (left) and salinity (right) from Argo float 6900701. The sections show all the temperature (T) and salinity (S) profiles measured by the float during its life-time side by side. Each profile is represented by a very thin column where deep red is the highest values and deep blue the lowest. The colour bars on the right relate the colours to actual data values. Profile numbers are given along the top of the plot, with corresponding measurement dates along the bottom. Click on the images for larger plots.     Source of plots: IFREMER/Coriolis.

Salinity map

Ocean salinity at 1000m.
Source: Mercator ocean analyses

The Mercator analysis map shows that the Mediterranean water does not spread evenly out across the Atlantic, but is patchy with changes in salinity even over a short distance. This patchiness is also evident in the salinity section above (right).

Meddies

As the warm, salty Mediterranean water flows out across the Atlantic, eddies pinch off and drift southwestward. These lenses of warm, solty water rotate in a clockwise (=anticyclonic) direction and are called 'Meddies'.

Typical Meddies are around 800 metres thick and 100 kilometers in diameter. Their salinity is about 0.8 psu higher than the surrounding ocean water. They are quite long lived because they rotate rapidly as they move slowly through the calm waters around them.

Using the float trajectories with ocean analysis maps

Look at the float trajectory in Google Earth to see where the float has been. (If in doubt about how to reveal the float tracks, see our Google Earth screenshot for help.) Compare this to the maps of temperature and salinity for different depths available for example from Mercator ocean analyses.

Downloading the float data

The Argo Information Centre has more information about this float. You can also download the data from one of the Data Centres - just select Data > Data Downloads.

There are many different formats available. ASCII data can be viewed in spreadsheets such as Excel. The other data types may require more specialist software.

Comparisons

This float may be compared with 6900708, and 6900277. Look out for similarities, but bear in mind there may also be small, but important differences.

 

Link to the main Euro-Argo project website.