Part 3—Discover SST Associated with El Niño and La Niña

1997 was considered a strong El Niño year. Let's see if the data supports that claim. What pattern of SST would you expect to observe in the Pacific if it were a strong El Niño year?

Show me the answer

We have already calculated the average SST for the period of 1982 to 1998 (Overall Average). Now calculate the average SST for Dec. 1997. Enter Monthly Average, Dec 1997 as the name for the result.
Show me

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1. Click the Analyze tab.
2. Under Add field(s) to Layer, choose By Math Operation.
3. In the "Add Field to the Table of" dialog box, use the drop-down menus to Add a Field to Weekly Means of SST by computing a Mean (average).
4. Click the radio button to compute the value from more than two fields, and check the boxes next to the grids that represent weekly SSTs for Dec 1997.
  Hint: Use the Deselect All before making your selection of the 1997 fields. This way you won't include any other years in the analysis.
5. Enter Monthly Average, Dec 1997 as the name for the result.
  Show me how the dialog box should look
6. Select OK which will perform the calculation and return you to Visualize mode. The "Monthly Average, Dec 1997" will be displayed.
To view the differences between two grids is to generate a "difference" image. A difference image will show the "departure from normal" of each location as red if it is warmer than normal and blue if it is cooler than normal. This is known as an anomaly, or difference, map. Subtract the overall average from the 1998 average and name the result "1997 Difference from Overall Average."
Show me how

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1. Click the Analyze tab and Add a new field to the table of the Weekly Means of SST layer by Computing a difference between the 1997 average and the Overall Average.
2. Enter a Result name of "1997 Difference from Overall Average"
  Show me how the dialog box should look
Apply a standard color scheme to your difference image that you'll be able to copy to other ones. Choose the red-white-blue color scheme and change the minimum temperature to -2 and the maximum temperature to 2.
Show me how

Hide
1. Double-click the "Weekly Means of SST layer." to open the edit appearance window.
2. Colorscheme: red-white-blue
3. Check reverse box. (Very important)
4. Min: -2 Max: 2
5. Apply and close
  Show me what the window looks like
6. Does this difference image agree with your prediction? Is this pattern consistent with a strong El Niño year?
Show me what the 1997 Anomaly looks like

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The following year after this strong El Niño event, a La Niña set up in the Pacific Ocean. As you did for the El Niño year, predict what pattern in SST would you expect to observe in the Pacific if it were a strong La Niña year?

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You have already calculated the average SST for the period of 1982 to 1998. Next you will need to calculate the average SST for Dec 1998 and then subtract the Overall Average from it. This calculation will show you the difference between "normal" and the 1998 year.

Create a monthly average from the five weeks of 1998. Enter Monthly Average, Dec 1998 as the name for the result.

Show me

Click the Analyze tab.
Under Add field(s) to Layer, choose By Math Operation.
In the "Add Field to the Table of" dialog box, use the drop-down menus to Add a Field to Weekly Means of SST by computing a Mean (average).
Click the radio button to compute the value from more than two fields, and check the boxes next to the grids that represent weekly SSTs for Dec 1998.

Hint: Use the Deselect All before making your selection of the 1998 fields. This way you won't include any other years in the analysis.

Enter Monthly Average, Dec 1998 as the name for the result.

Show me how the dialog box should look

Subtract the overall average from the 1998 average.

Show me

Apply a standard color scheme to your difference image that you'll be able to copy to other ones. Choose the red-white-blue color scheme and change the minimum temperature to -2 and the maximum temperature to 2.