5.6 Relationships between SST and chlorophyll

In the last section you looked at composites from two different sensors over the same time period. In this section you will be looking at how temperature and chlorophyll, from the same platform and time period, compare.

You can do this by comparing MERIS data to Envisat AATSR data from the same period. However, to do this you would first need to create a composite of 8 or 9 AATSR image. If you wish to do so, the relevant AATSR images have been made available. However, to save time, we will use the two MODIS Level 3 data sets here. The principles behind the comparison are the same for both MODIS and the two Envisat sensors.

Start by opening the file l5_mod_a20040332004040.L3m_8D_SST_4.hdf.

1. Look in the left hand pane and select (click on) scientific global attributes.
2. In the right hand pane click on scientific global attributes and right click, select open properties.
3. Explore the information contained in the scientific global attributes.
4. Once you are familiar with the information select Scientific Data » l3m_data and double click the image file in the right hand pane to open it.

Subsampling the image

The file is a global image, so just like when you compared the different sensors you need to extract the study area, to do this:

Hint: you may need to setup the geographical grid before doing this, if you cannot remember how see section 5.

1. Select Edit » Goto or [CTRL-G] to open the GoTo dialogue.
2. Make sure that the Coords option is ticked, enter the position as x: 0°E and y: 20°S.
3. Set the selection size to DX: 30°E and DY: 20°S
4. Click ok or press Enter.
5. Right click in the image and select New » IMAGE document.
6. Click OK or press Enter
7. Save this file as SST_l3m_8d_extract.dat.

Open the MODIS composite modis_8day_chla.dat that you prepared in section 5. Set the stretch to logarithmic, the null value to zero and the upper data value to 4 and have a look at the two files. You should be able to instantly see the similarities and pick out the same features in each image.

Converting the data

Unfortunately the data in the SST file is not in an appropriate format for comparison, so we need to convert the data to °C

Open the formula modis_convert_algal.fm and familiarise yourself with its contents.

1. Make the changes you suggested in your answer to question one and save the new formula as SST_convert_deg.fm.
2. From the Options! menu make sure that the output image type is set to 32 bit float.
3. Right click on the image and select connect adding the correct number of blank images and arranging them in the right order.
4. Copy and paste the formula into the image stack, tab to the new image and save the resulting file as SST_l3m_8d_extract_deg.dat.

You can now close all files except Close all the files except SST_l3m_8d_extract.dat and modis_8day_chla.dat.

Apply the stretches you decided on in your answer question two before continuing.

Looking at the data

The data is now ready for analysis, you are going to use similar tools to those you used in section 5.1 to compare the sensors to look at correlations between temperature and chlorophyll.

Start by stacking the images. Right click on one of the images and select connect. Select the two images in the connect dialogue and make sure that stacked is ticked. Click OK.

Visual analysis

You are going to start by simply looking at the two images, in the sst image a darker colour represents a cooler sea, in the chlorophyll image a lighter colour higher chlorophyll.

You may want to experiment with different stretches to pull out different features of the image. For example, using the MODIS chlorophyll image set the stretch to a maximum of 2. This will expose detail in the ocean further from the upwelling. Hint: it is best to do this on images that are not stacked, applying a stretch to a stack will apply it to all the images.

Select your stacked images and use the cursor to examine a selection of high and low chlorophyll points in different parts of the image. Use the tab key to flick between the SST and chlorophyll images, noting the data values displayed on the Bilko status (bottom right corner of the Bilko window). Take some time to do this, it should reinforce your visual analysis of the image.

Finally we will use the Bilko transect tool to investigate the image. Select your image stack and take three transects:

Along the main upwelling areas running through any features that interest you. From the warmer offshore water into the cooler upwelling. From the warmer offshore water in the South East into the upwelling area.

Once you have drawn on your transect, with the line tool, right click on the image and select new transect document, make sure apply stretches is unticked. You should be able to see that the chlorophyll concentrations are very small compared to the temperatures, making them hard to compare. Bilko does not support multiple scales on an axis, so to compare the two variables on the same graph we would need to extract the data and analyse it in another application.

If you wish to do so, this is easily done, simply click the copy button on the Bilko tool bar, open a spread-sheet in another software package and paste the columns corresponding to the two transect lines. In Bilko you can do this by creating two separate transect documents:

1. Draw your transect using the line tool then press[CTRL-N] or right click and select new » transect document to create a new transect. Do this twice so that you have two identical transects.
2. Right click on each transect and select redisplay, hide one line in each transect by selecting it from the drop down box and ticking hide.
3. You now need to change the scale. Right click and select scale then choose suitable scales to expose the details.
4. Minimise all documents other than the two transects and select tile horizontally to evaluate the detail.
5. Repeat this for each of the three transects.