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Understanding Argo data

  The T and S profiles   The T and S time series
  Seasonal variation   Regional differences      

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This page explains how to interpret Argo plots, using example data from some of the floats in our selection. Clicking on a figure will bring up a larger plot with an explanation of what you see.
To display data from a specific float, follow this link.

Float trajectory Trajectory of float 6900211.

The T and S profiles

The T and S profiles show how water temperature (T) or salinity (S) change with depth. Interpreting the profiles means thinking about the float trajectory, the climate of the region the float has passed through, and the currents that have moved the float from place to place.

It is often useful to look at the temperature and salinity profiles together. This is because density is a function of both T and S, and the density of a water mass determines its depth in the ocean.

Temperature profiles
Figure 1. Temperature profiles from Argo float 6900211.
 

Salinity profile
Figure 2. Salinity profiles from Argo float 6900211.
 

Things to look out for in the temperature profiles:

  • How does the recent surface temperature compare with the temperature at 2000m depth? Can you explain this difference? (Think about the location of the float and the time of year).
  • Is there a 'thermocline' - a depth interval where temperature changes markedly with depth?
Argo float
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This example is from the Atlantic west of Gibraltar in May. Here the surface water, heated by the sun, is much warmer than in the deep ocean, where no sunlight can reach.

A rapid decrease in temperature to about 600m is followed by a depth interval (about 600-1400m) where there is little change with depth. The water in this layer comes from the Mediterranean through the Strait of Gibraltar. Below this Mediterranean Outflow lies another thermocline, a mixing zone between Mediterranean water and Atlantic intermediate water, where temperatures decrease to just over 4°C at 2000m depth.   CLOSE

Latest temperature profile.

Float 6900211: last three temperature profiles

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  • Looking at all the profiles together, what is the temperature range near the surface? How does this range compare with the range at 2000m depth?
  • Is there any part of the plot where the differences between earlier and later profiles is particularly great? Think about the float trajectory (track) and relate this to information you have about currents, water masses and climate in this area.
Argo float
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The surface temperature range is 10 °C, over five times the range at 2000m. This variability is due to seasonal heating and cooling of surface waters. In the deep ocean there is no such seasonal change.

Float trajectory

The 600-1400m depth interval shows a marked difference beetween early and late profiles. To understand why, we need to look at the float track. This float started west of the Bay of Biscay, at first moving west and northwards in the North Atlantic. In this region temperature gradually decreases with depth from the surface to about 1500m. The decrease is particularly marked in summer. Later the float drifted southward into the region of the Mediterranean Outflow, where the temperature remains high (about 11°C) to a depth of 14-1500m.   CLOSE

Temperature profiles

Float 6900211: all profiles

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Things to look out for in the salinity profiles:

  • How does the surface salinity in the most recent profiles compare with the salinity at 2000m depth? Can you explain this difference? (Think about the float location, the time of year and the climate (temperature, rain/snow, solar heating, evaporation, freezing or melting of ice).
  • Is there a 'halocline' - a part of the profile where salinity changes markedly with depth?
Argo float
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The surface salinity in the last three profiles is relatively high, 36 psu or more. In May the sun is already strong, and evaporation from the surface increases the salinity of surface waters in this region. The salinity at 2000m is lower, 35.1-35.2; a salinity typical of North Atlantic water at mid depths.

However, the salinity profile in this region does not decrease steadily. Instead there are three haloclines; a decrease down to 600m, followed by an increase to peak salinities between 600 and 1400m. These high salinities are typical of the Mediterranean water flowing into the Atlantic through the Straits of Gibraltar.   CLOSE

Latest salinity profiles.

Float 6900211: last three salinity profiles

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  • Looking at all the profiles together, what is the salinity range near the surface? How does this range compare with the range at 2000m depth?
  • Is there any part of the plot where the differences between earlier and later profiles is particularly great?
Argo float
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The salinity range near the surface is about 0.8 psu, about three times the range at 2000m. The difference between the early profiles, obtained in the central North-East Atlantic (blue-turquoise) and the latest profiles from the region of the Mediterranean outflow is substantial. The earliest profiles (dark blue) measured near the launch site west of the Bay of Biscay show some influence of Mediterranean outflow. This disappears as the float moves further west and north (turquoise). In the later profiles (red/brown) the high salinity Mediterranean water can be identified between 600m and 1600m depth.   CLOSE

Salinity profile

Float 6900211: all salinity profiles

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Taking both profiles together

density diagram

Density diagram for float 6900211.

The Mediterranean water flows out through Gibraltar at a 300m depth. So why does it end up at 1000m in the Atlantic? To understand this you need to think about the density of Mediterranean and Atlantic water.

In general water massess in the ocean will sink (or rise) to a depth where it is surrounded by water of the same density. If water near the surface changes density, for example by cooling, or by loosing fresh-water through evaporation, it may become dense enough to sink.

Density can be calculated from the temperature and salinity of the water. Density diagrams such as the one on the right are a quick way to find the density of a body of water.

The T and S time series Up arrow

The temperature (T) and salinity (S) time series of data collected by a single float. Each measurement cycle is represented by a very thin column coloured with dark red for the highest values and dark blue for the lowest values. These are packed together along the x-axis, which represents time or cycle number. The colour bars relate the colours to actual data values.

Seasonal variation

Regional differences

 

Link to the main Euro-Argo project website.