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Float no.1900919

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

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This PROVOR float was launched by French scientists soutwest of Cape Town on the southern tip of Africa at 37.1°S, 12.7°E on 25 October 2006. It has recorded a total of 164 profiles, and made its last report on 5 April 2011 from 26.1°S,  5.5°E.

The float was launched Sabrina Speich and Michel Arhan as part of the GOODHOPE project. The launch site was just west of the Agulhas Current Retroflection.

As you can see from figure 1(below left), this is a region of complex currents.

Schematic of currents

Figure 1. Schematic circulation. (Boebel et al., 1998).

It is also a key region for exchanges between oceans. Large, warm-core eddies (Agulhas rings) detach from the Agulhas Retroflection and move into the Atlantic. The eddies bring warm warm, salty subtropical water from the Indian ocean round Africa and into the South Atlantic.

The Agulhas rings move north, surrounded by the waters of the cold Benguela current - a slow, eastern boundary current that brings cold water along the coast of Southwest Africa towards the equator.

Float track

Float track for float 1900919 for the first three years of its life.

From the float track (right) you can see how this Argo float moved into just such an eddy of warm and salty Agulhas waters spinning northwards into the South Atlantic. These large eddies can be seen on maps from the Mercator analysis

Note: To see the maps choose any "Analysis date" you want to compare with the float data, then "Maps", "Analysis", and the parameter you want for each of the three proposed "Depths".

Temperature and salinity profiles

Temperature profiles Salinity profiles

Profiles of temperature (left) and salinity (right) from Argo float 1900919. 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.

Q1.
 
 

Look at the T and S-profiles above. At what depth range do you find the lowest salinity?
What is the variability like in the surface layer above this depth range?
What is the variability like below this depth?

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A1.

The lowest salinity is found from about 500m to 1000m depth. In the surface layer above 500m both temperature and salinity vary with time.

Below 1000m there is very little temperature change, but salinity changes quite a bit. In the low salinity region at 500-1000m the water is still slightly warmer than in the deep ocean, about 3-8°C, compared to the almost uniform temperature of 2-3°C below 1200m.

Schematic of water masses

Water masses in the Atlantic.

The low salinity is the signature of Antarctic Intermediate Water (AAIW), formed in the Southern Ocean. AAIW fills most of the southern hemisphere and the tropical oceans at about 800 to 1000 m depth, reaching as far north as 20° N.

There is still debate about the exact origin of AAIW. Does it come from specific regions of the sub-antarctic belt? Or does it spread northward from the entire circumpolar region?

This is one of the questions that Argo data may help to answer. This float, along with several others, was launched by the GOODHOPE project to provide uncprecedented information about the vertical and horizontal distribution of heat and salt in the Southern Ocean - a region where storms and high waves make ship measurements particularly difficult.

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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 1900919. 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.

Q2.
 
 

Look at the salinity section above (right). Can you identify the Antarctic Intermediate Water (AAIW) in this section? How does the depth of this layer vary with time? Can you find a relationship between the depth of the water and the salinity of the surface layer?

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A2.

Annotated salinity section

Annotated salinity section; click for larger figure.

The Antarctic Intermediate water can be seen as a low-salinity band (dark blue) of about 200m thickness, undulating around about 800-1000m depth. The depth of this layer is not constant, but varies with time. Sometimes there are dips or "bowls" in the isohalines (see figure on the right, marked with A and white arrows). These indicates that the float travelled several eddies with warm and saline cores (the biggest centered around cycle 18). These are Agulhas rings, contributing to transport of heat and salt from the Indian Ocean into the Atlantic. More about Agulhas rings

Twice the float drifted from deep water onto the continental slope (see figure, B), where it it encountered cooler, low salinity water near the surface. This may be explained in two ways:

  • Agulhas rings usually stay away from the continental shelf. Thus, when a float drifts closer to the coast it will be surrounded by the cold waters of the Benguela Current, which surrounds the warm Agulhas eddies. The Benguela water has a lower in salinity than the warm, subtropical Agulhas water.
  • The second explanation comes from the frequent upwelling events that occur in the Benguela region. Longshore winds push surface water away from the coast, and this is replaced by colder, deeper water which rises to the surface and flows onto the continental shelf. Such coastal upwelling is common along the southwest coast of Africa. The upwelling Antarctic Intermediate Water is high in plant nutrients, so the upwelling ecosystems support rich fisheries. More about the Benguela upwelling system.

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Q3.
 
 

Look at the temperature section and try to identify the seasonal cycles. Roughly how deep is the surface layer influenced by the seasonal heating and cooling. Can you explain why the second of the periods with summer warmth seems to be longer than the first warm period? (You may need to revisit the float track to see where the float was during its first and second summer).

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A3.

Annotated salinity section

Annotated temperature section; click for larger figure.

At surface, in the top 100m or so the temperature time series clearly shows the seasonal cycle of heating and cooling. Temperatures of 18-20°C or more can be seen:

  • between October 2006 and May 2007,
  • between December 2007 and ~ May 2008,
  • between December 2009 and April 2009.

These periods of warmer surface water correspond to summer and early atumn in the southern hemisphere. It is clear that summer heating has an impact on the surface ocean.

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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.

 

Link to the main Euro-Argo project website.