4.3 Hovmöller diagrams.

Creating a Hovmöller diagram Interpreting the plot

LESSON 4

Overview

References:
List of journal references

Downloads:

Useful information:

East Greenland Current

North Atlantic Drift

Canary Current

South Equatorial Current

Creating a Hovmoller diagram.

In the previous activity you created a transect plot, interesting exercise but the plot is a little messy and difficult to interpret properly. A better way to look a time series of data along a transect line, known as a Hovmöller diagram. In such a plot all the data extracted along the line in space (x-axis) are plotted with time along (y-axis) to give a space-time diagram or image, where they numerical pixel value represents the data (in this case sea surface temperature). The plot is typically used to study time varying phenomena at a given latitude or longitude, but there is no reason why the line could not also be diagonal.

Hovmöller plots are used widely in oceanography and meteorology, for example to study how features such as eddies, cyclones or planetary waves move with time. They are easier to interpret than the transect plot you have just been using. The best way to think of this plot is as a vertical slice through the ASST data cube from top to bottom.

Activity

Return to the ASST image stack, and redraw the transect line you made earlier (starting at [017° 15`W,83°45`N] ending at [017°15`W, 76°15`S]).
Right-click to bring up the pop-up menu, and select New » Image Document. Bilko presents a new widow containing the SST values extracted from each image in the stacked set plotted from left to right (in this case representing north to south along the transect) and from the first image to the last image moving from the top to the bottom of the window.
To view this data more clearly, right click in the new image and select the Zoom » Don`t care option. You can now re-size the resulting image window and the data set will always fill the window space.
Add a colour table to the image(File » open), for example rainbow.pal as used earlier.
Finally, data displayed in this way are by convention displayed so that the earlier data in the time series are at the bottom of the page. Use the View » reflect » in Y function from the Bilko menu to flip the time series in the y-axis.

Select the cursor point tool from the Bilko toolbar and examine the image data set that you have produced.

Question 1

a)	Use the Hovmöller diagram to determine the SST value at [017°15`W, 19°15`N] in October 1999.(Hint: there are 59 months of data, each month is a row. You will need to turn on Coords as shown earlier.)

Answers:
(Resizable pop-ups)

Answer 1

Answer 2

Back up to:
Q1 Q2

Interpreting the plot.

You can look at the variability of the SST at this location using the Hovmöller diagram by extracting a vertical transect. Of course, this is exactly the same plot you would have obtained if you had simply placed the cursor at a location in the stack and data `cored` this location. However, there is more information than this in the Hovmöller plot. For example, the impact of the seasonal movement of the earth relative to the sun can be seen in the equatorial regions, the permanent upwelling throughout the year at ~22°N, the seasonal upwelling stretching to latitudes of ~11°N.

To highlight particular features you can modify the contrast stretch applied to the data. For example, if we are interested in the dynamics of the tropical waters at higher temperatures, we can apply a stretch of DN 180=0, DN255=225. Experiment with the stretch values using the Image » Stretch » Manual option from the Bilko drop-down menu.

Activity/Question 2:

a)	In which season did the most intense upwelling event along the African coast occur? Use dates and latitudinal extent in your answer. Hint: refer to the image description earlier in the lesson and relate the rows of the Hovmöller diagram to the dates.

Next: Western boundary current variability.