Exercises by decree


									                  Sea Surface Temperature Exercises

              The Leicester ATSR Global Analyser - LAGA V1.1
                     Introductory Notes and Exercises

1) Open the LAGA Application.
    Double-click on LAGA_V1.1.sav (IDL script)
    The IDL VM may ask you to continue
    A window will open asking you to identify the file containing the SST data to
     be analysed. Open the folder called „Data‟ and highlight the folder called
     „aatsr‟ , click OK (This folder contains global monthly averaged SSTs from
     AATSR, on a 0.5º degrees long-lat grid, for the entire AATSR mission)
    This file will take a short time to load and then another window will open
     asking you to identify a file with climatology data. Select „oi_clim-71 to00‟
     (This is NCEP SST climatology from 1971 to 2000, one file for each of the 12
    LAGA will now open, ready for action!

2) The Window
    In the window you can see the first of the series of consecutive monthly files
      of global SST. Temperatures are indicated by the colour scale and black
      areas indicate lack of SST data, generally because they are either over land
      or because there was persistent cloud over the grid-box for the entire month.
    Click or drag the horizontal slider from left to right to look at successive
      months through the 5-year period. You will notice changes as you progress
      through the data-set.
    You can zoom in by clicking on the map or by using the “Zoom IN2 and
      £Zoom Out” buttons, which also enable you to zoom out. Click on the “Whole
      Earth” button to return to the global view.

 The AATSR Global analyser will allow you to examine some of these
changes quantitatively throughout the annual cycles. You will see that
you now have ready access to a large data set which shows spatial and
          temporal variations in Global SST in much detail.

You will also realise that this type of analysis can only be done if there is
   a reliable and stable global observing system in place. This is why
 satellite data are essential for the study of global oceanography and its
                      application to climate research.

Activity 1 – Examine the Time-dependence of global and regional SST

1.1     Click on a position of your choice in mid-ocean Northern hemisphere. A good
starting-point is approximately 25-30˚latitude and, e.g. in the central North Atlantic at
about the same latitude as the Canary Islands. The image will zoom in and the long-lat
coordinates of the area you are looking at will be indicated below the image.
1.2     Reduce the dimensions of the box to about 5-10˚ (this will take approximately 5
or 6 clicks).
1.3     You should see only ocean in the window (not essential). LAGA can store the
average SSTs in two boxes, A and B. We will call this box „Box A‟: click on „Store Box
A, then click on „Show‟,
Now Zoom out or go to „Whole Earth‟ and you will see an outline of the rectangular box
you have selected.
1.4     Repeat the procedure to select a box in the Southern hemisphere, about the
same distance from the equator. Click „Box A=Box B‟ and store your new box as „Box
1.5     Click the „Averages‟ button and you will see a plot of the average temperatures of
the two boxes over the entire 5-year period of the data-set. Change the “Min Temp”
and “Max Temp” settings to improve the display (Note: You should change the
temperature value and press “Enter” for each setting) to expand the vertical scale and to
show the structure in the plot.
1.6     Select „Fit Sine‟ to analyse the periodicity of the plots.

The figures below show an example of one box

Question 1: Can you interpret the phases and periodicities of the time-variations of the
temperatures of your boxes?

At this point you can proceed to use the entire ERS-1, ERS-2 and ENVISAT SST
record, which started in 1991 and is nearly continuous to the present day. There still a
few gaps in this record, which have been filled, temporarily, with AVHRR „Pathfinder‟
data. Close LAGA by clicking the „Quit‟ box and restart as before, but selecting the SST
data folder called „Merged-V2‟.

You will see the complete global view in August 1991, with diamond gaps characteristic
of the 3-day repeat cycle used at the start of the ERS-1 mission.

Click the „averages‟ button. You will now see a plot of the average global SST as a
function of time throughout the 16-year data-set. Adjust the max and min temperature
settings to expand the vertical scale appropriately. You should find that a Min Temp of
around 287°K and Max Temp of around 293°K will show the variations on average
global SST. You will note that there is some periodicity over the five years of data so far
acquired by ENVISAT, but it is highly variable . . .

Question 1.1: Estimate the amplitudes of the annual cycle in the average global SST.
Identify the main similarities and differences you can see in the behaviour over the 16
years observed. Explain why there is any annual cycle in average global SST. Note
and comment, if you can, on the differences in behaviour between the early and late
halves of the record.

Activity 1.1 Compare the cyclical behaviour of average temperatures in the
Southern and Northern Hemispheres.

Set the display to show “Map” and “Whole Earth”.

Set the “Min Latitude” box to 0 degrees, i.e. the equator, so that you are viewing the
Northern hemisphere only. Click the “Store Box A” button, which will save the Northern
hemisphere data on display. (You can also define the box by clicking and dragging).
Do the same for the Southern hemisphere and store as Box B.

Display the averages, and discuss any features you see.

Activity 2: Examine the annual cycle in SST using transects and Hoffmueller

Note: It is advisable to restart LAGA when changing the type of activity.

Click on the „select transect‟ and „view transect‟ boxes at the bottom left-hand side of
the window. Draw, by clicking and dragging, a north-south line covering the maximum
distance possible in the mid-Pacific. Near to the left-hand edge of the image is a
suitable place. When you release the click button the image will refresh itself showing
he the transect you have selected. The coordinates of the transect will also be given
next to the „select transect‟ box.

To view the SST values along the transect, click on the transect button. Use the
horizontal shift buttons and slider to see changes in the transect from month to month.

Try some other transects, for example a North-South transect in the East Pacific, as
illustrated below:

Question 1.2: The screen images above show a typical transect in the Pacific and two
plots from April and December 2005. Can you see significant change in the transect
SST values during the annual cycle? If so, what is going on?

Activity 3: Use a Hovmuller diagram to view the annual cycle.
In the previous activity you got only an impression of the cyclical behaviour. A
Hoffmueller diagram is a powerful way of representing information so that a temperature
at all distances along the transect can be displayed throughout the time-series on a
single plot. To view the Hovmuller diagram for the transect you have defined (or a new
north-south transect) simply click on the „Hovmuller Diagram‟ button and the cyclical
behaviour which you previously identified can now be very clearly and precisely seen.
The screen below shows a Hoffmueller plot from the same Pacific N-S transect used in
the previous activity.

Activity 4: Use anomaly plots to examine the El Niño region.

Return to a whole Earth map view and click on „Show Anomalies.‟ This will now show
you the difference between ATSR SST and climatology. This is a very powerful way to
display oceanic behaviour.
This exercise involves drawing a transect along the equator and therefore the minimum
temperatures will be fairly high. Therefore type in a temperature of 290 in the „Min
temp‟ box and press „Enter‟. When you enter the new temperature value the image will
refresh showing the new colour scale. Cold water upwelling along the coast of Peru
should be apparent. As you progress through the months you should also see the well-
known „cold tongue‟ of water being wind-driven along the equator westward during the
February-July period. The absence of this tongue or a reduction in its intensity is the
principal sign that an El Niňo event is developing.

Draw a transect from west to east, along the equator, finishing at the coast of Peru.
Display the Hoffmueller diagram and you should clearly see the annual upwelling and
development of the cold tongue over he annual cycles of data.

Question: Comment on the relative intensity of the Cold Tongue in successive years of
the data-set. Do you see anything like an El Niño?

You can get another view by taking a north-south transect through the cold tongue.
Draw a transect at a longitude of about 122 degrees west, from about 5 degrees north
to 10 degrees south. Look at the Hoffmueller diagram optimise the temperature scale to
improve the information content.

You can also look at box differences: zoom into the equatorial Pacific. Set the minimum
temp to 296. Draw a box about 5degrees square on the equator at the western edge
and store as Box A. Select a Box B on the equator fairly close to the Peruvian coast,
equalise the boxes.

Use the averages button to show the values in the boxes and, much more informative,
use the differences button to indicate the actual differential along the equatorial Pacific.

Using this facility you can develop very sensitive quantitative indicators of the intensity
of major oceanographic features as a function of time, demonstrating the considerable
power of global, continuous data-sets obtained from satellites.

Further activities:

Similar investigations can be carried out and, particularly, the great El Niño event of
1997 can be seen very dramatically by comparison with climatological fields. It is
interesting to compare the magnitude of this event with the relatively minor El Niño
events that have been observed by AATSR on ENVISAT.

You are now well-equipped to examine the cyclical behaviour of the major oceanic
features and processes - GO FOR IT!!

Suggested areas for examination include:

Gulf Stream – Labrador Current

Kuroshio - Okashio
(Look for similarities and difference in behaviour between these two Western Boundary
current systems)
Malvinas current
(East side of southern tip of S America)
Aghoulas Current
(Around southern cape of Africa)
Somali Upwelling
(Near the Horn of Africa)
etc etc.

The example below shows an example of a Kur0-Shio break-out current in June 2006.
What else can you find from the AATSR Global data-set?

Note: LAGA is intended as „Quick-Look‟ facility rather than a fully-developed research
tool. You will find that you can carry out most of these activities, and take your
investigations much further with them by using Bilko, or with other visualization
packages such as IDL or ENVI.


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