# Presentation: 084063

## Understanding El Nino

- Science Olympiad Oceanography Resource
- Anand Gnanadesikan
- NOAA/Geophysical Fluid Dynamics Lab

## In this lesson you will

- Learn about the largest pattern of climate variation on the planet...
- Learn how scientists track this variation
- Learn to predict it yourself!

## A key tool we use in this presentation...

- _[http://www.pmel.noaa.gov/tao/jsdisplay/](http://www.pmel.noaa.gov/tao/jsdisplay/)_
- Site showing data from the Tropical Atmospheres and Oceans (TAO) array of moorings deployed by NOAA’s Pacific Marine Environmental Lab In the equatorial Pacific. Click on the above site

- _[http://www.pmel.noaa.gov/tao/proj_over/proj_over.html](http://www.pmel.noaa.gov/tao/proj_over/proj_over.html)_
- Click here to learn more about the project.

## ... you get a picture that looks like this

- Top: Five-day mean for temperatures (in colors) and winds (shown with the arrows). Notice that the winds are blow from the east to the west.
- Bottom: **Anomalies** in temperatures and winds-i.e. how different this time period is from a “normal” time period (the average for this time of year).
- Click on the tab that says “Section plots” and then on the tab below that says “Depth”, and then on the red Make plot! Button.

## You get a picture that looks like this..

- This shows a cross-section of temperatures along the equator. Notice that the warm water is piled along the west coast, just where one would expect if it were pushed by the winds (in contrast to the rest of the ocean where the Coriolis force plays an important role in determining the shape.
- Where is the thermocline on the upper plot? At what temperature is it found?

## Returning to the latitude-longitude plots...

- Call up March 28, 1997.

- Notice that there is a strong anomaly of eastward winds- a westerly wind burst

## Now change the plot to look at the depth of the 20C isotherm...

- The isotherm is deeper than it usually is across a broad region of the Pacific. This means that water is being piled up locally.

## Watch what happens to the anomaly across the next 40 days

- April 7

- April 17

- April 27

- May 7

## The propagation of the warm anomaly is known as a Kelvin wave

- Northern Hemisphere

- Southern Hemisphere

- To understand how a Kelvin wave works, you need to understand the Coriolis force, and that warm water is less dense.
- A bulge of warm water will thus try to flow out over the surrounding cold waters. In the northern Hemisphere this will cause it to turn to the right, while in the Southern Hemisphere it will turn to the left. This moves water from the rear of the bulge to the front, causing it to propagate to the east.

## Now switch to looking at Sea Surface temperature

- Where the thermocline (20C isotherm) has deepened, the surface has now warmed!

- There is also another westerly wind burst starting

## This illustrates a process known as the Bjerknes feedback- based on two principles

- 1, Along the equator winds blow from cold waters to warm waters.
- 2. The winds pile warm water up against coasts.

- A. Weaker winds

- C. Leading to less upwelling of cold water

- D. Which leads to less of a surface temperature  difference between the east and the west... causing the winds to weaken further

- Warm

- Cold

- B. Cause the warm water to surge to the east.

## Look at Sea surface tempeatures on Nov. 28, 1997

- The anomaly has now grown to over 4 C! What causes it to shut off?

## Go back to looking at the 20C isotherm depth on August 10

- While the 20C isotherm is deeper on-equator- it is shallower off equator
- Less  warm water has been pushed there by the weaker winds.

## Look at what happens to this anomaly over time

- Sept. 9, 1997

- Oct.. 9 1997

- Nov. 8, 1997

- Dec. 8, 1997

- A shallow thermocline builds

## This is a phenomenon known as a Rossby wave

- Equator

- A low pressure bulge off-equator generates a weak flow far from the equator, where the Coriolis force is stronger, and a strong flow closer to the equator where the Coriolis force is weak. This tends to move water from the front of the bulge to the rear, causing the bulge to move to the west.
- See whether you can make this logic work in the Southern Hemisphere. Can you understand why long Rossby waves always propagate to the west?

## Eventually this shallowing of the thermocline moves east as a Kelvin wave 

- Jan. 8, 1998

- Feb. 6, 1998

- Mar. 8, 1998

- April 7, 1998

## ...and as it does, the surface eventually begins to cool

- 20C depth, April 7, 1998

- 20C depth, June 1, 1998

- SST, April 7, 1998

- SST, June 1, 1998

## Over time, this causes the ocean to flip to the opposite state.

- Notice now that a positive anomaly is building up off equator!

- 20C depth, July 16,1998

- SST, July 16, 1998

- La Nina

## So can we use this to predict El Nino?

- Well, sort of. If we see a bulge of warm water along the equator, we can predict that it will propagate to the east and cause warming...

## So in terms of this picture

- We can predict this feature!

- But this westerly wind burst may be affected by weather... it may not be as predictable

## Summary of how El Nino works...

- Changes in winds along the equator cause ....
- Changes in the depth of the thermocline which propagate to the east where the thermocline is very shallow.
- Causing warming/cooling in the East Pacific that reinforces the winds....
- But causing opposite-sign changes in the thermocline off-equator...
- Which propagate to the west as Rossby waves....
- And eventually hit the boundary and reflect back to the west.
- This is the basic picture of the “delayed oscillator” but...

## Issues I’ve left out

- Role of heat fluxes in damping out El Nino (cold anomalies get warmed, warm anomalies get cooled).
- Role of currents as well as waves in moving anomalies around.
- How changes in 20C depth off-equator can propagate to the equator and change the system. (This is another reason El Nino is harder to predict).
- Role of biology in changing El Nino (you can read a paper I wrote on the subject here _[http://www.gfdl.noaa.gov/reference/bibliography/2007/wga0701.pdf](http://www.gfdl.noaa.gov/reference/bibliography/2007/wga0701.pdf)__)_
- PMEL site has a lot of discussion of the impacts of El Nino.