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

Step 1 –
Investigate an El Niño Year

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?
Answer: A large plume of warmer than average surface water extending out from the coast of South America along the equator.
  1. 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.
    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.
    6. Select OK which will perform the calculation and return you to Visualize mode. The "Monthly Average, Dec 1997" will be displayed.
  2. 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."
    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"
  3. 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.
    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
    6. Does this difference image agree with your prediction? Is this pattern consistent with a strong El Niño year?

Step 2 –
Investigate a La Niña Year

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?

Answer: A large plume of cooler than average surface water extending out from the coast of South America along the equator.

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.

  1. Create a monthly average from the five weeks of 1998. Enter Monthly Average, Dec 1998 as the name for the result.
    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 1998.
    5. 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.
    6. Enter Monthly Average, Dec 1998 as the name for the result.
  2. Subtract the overall average from the 1998 average.
    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 1998 average and the Overall Average.
    2. Enter a Result name of "1998 Difference from Overall Average".
  3. 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.
    1. Double-click the "Weekly Means of SST layer." to open the edit appearance window. Choose the following settings:
      • Colorscheme: red-white-blue
      • Check reverse box. (Very important)
      • Min: -2 Max: 2
    2. Apply and close
    3. Does this difference image agree with your prediction? Is this pattern consistent with a strong La Niña year?

Step 3 –
Choose Another Year to Analyze

  1. Choose another year to do a similar analysis. Decide if that year was an El Niño , a La Niña year or a "normal" year.
    1. Create a monthly average for the weeks of the selected year
    2. Subtract the overall average from the selected year average
    3. Apply the standard color scheme to your difference image
    4. Interpret the difference image. Is it an El Niño , a La Niña or a "normal" year.
    Hint: 1982 was considered another strong El Niño year while the next year, 1983, was considered to be a La Niña year.
  2. Compare the following difference images for each year with your selected year.
  3. 1982-El Niño

    1983-La Niña

    1984-La Niña

    1985-Normal

    1986-El Niño

    1987-El Niño

    1988-La Niña

    1989-Normal

    1990-Normal

    1991-El Niño

    1992-Normal

    1993-Normal

    1994-El Niño

    1995-La Niña

    1996-Normal

  4. Compare your interpretation with a SST anomaly graph for the Pacific Ocean located along the equator. How does your year fit into the overall trend of SST? Try comparing this graph with another year.
  5. Hint: Try showing one year at a time from above while comparing it to the SST Anomaly graph below.
    SST anomalies for the years 1965-2003. Red indicates warmer than average temperatures and blue is colder than average.

    December
    1982-El Niño
    1983-La Niña
    1984-La Niña
    1985-Normal
    1986-El Niño
    1987-El Niño
    1988-La Niña
    1989-Normal
    1990-Normal
    1991-El Niño
    1992-Normal
    1993-Normal
    1994-El Niño
    1995-La Niña
    1996-Normal

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