OC201_Lab5_ElNino
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Oregon State University, Corvallis *
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201
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Date
Dec 6, 2023
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OC 201, Oceanography
Lab 5: Understanding El Niño – Southern Oscillation
In today’s lab you will work with real archived data that has been collected by the Pacific Marine Environmental
Laboratory (PMEL) located in Seattle.
You will use data from the Tropical Atmosphere Ocean (TAO) project,
which is a real-time observation system that collects data from a network of moored buoys in the equatorial
Pacific Ocean. The TAO observations facilitate the detection and study of ENSO and provide an ideal opportunity
to investigate interactions between the atmosphere and ocean.
Open a web browser – Safari is preferred on Macs, Chrome on PC – and enter the URL of the TAO homepage:
http://www.pmel.noaa.gov/tao/index.shtml
[Clicking on
About > Mission > TAO/TRITON
from the TAO home page will provide additional information about
the TAO project and details about how the data is collected. You may also click on the
About > El Niño
tab for
more information on these phenomena.]
The map below, which was taken from the TAO website, shows the location of moorings (blue and yellow) in the
tropical Pacific Ocean from which data are being collected and continuously sent back to shore. Each mooring
location contains an array of sensors that measure surface weather conditions and temperature and salinity at a
variety of depths below the surface. In addition, the red station locations include acoustic Doppler current
profilers (ADCPs), which measure the velocity of water movement (currents) at different depths in the ocean.
Take note of the fact that the Pacific Ocean spans the 180° longitude line (dashed line below) where degrees
longitude transition from degrees west of the prime meridian to degrees east of the prime meridian (which runs
through Greenwich England). Consequently, the Western Pacific is in the vicinity of 140° E to 180° E whereas the
Eastern Pacific is the region between 90° W and 140° W.
Eastern
Pacific
Western
Pacific
Overview of the Activity
For this exercise you will access archived data collected from TAO moorings and create plots that allow you to
identify and interpret some of the characteristic features of El Niño and La Niña events. The objectives of this
activity are:
(1) To develop observational and descriptive skills relevant to the interpretation and communication of complex
(real) oceanographic data sets.
(2) To identify and interpret evidence of the connection between the atmosphere and the ocean
(3) To provide an introduction to some of the fundamental oceanographic features associated with ENSO events.
Getting Started
To begin this exercise, you should click on the
Data > Data Display and Delivery > Assorted Plots
tab from the
TAO homepage. This page displays a figure like the one shown below.
The plots you will be making are similar to
the one shown below.
They show the most recent observations of sea surface temperature (color coded) and
surface winds (as arrows) in the region. Note that the top figure shows observed
mean
temperatures and winds
averaged over a 5-day period, whereas the bottom panel shows
anomalies
.
Anomalies are the difference
between the actual observed conditions and the long-term (many years) average or “normal” conditions in the
region. If the temperature anomalies are between light yellow, green and light blue in color then they are very
close to long-term mean conditions, if the anomalies are red, conditions are warmer than normal, and if the
anomalies are dark blue, conditions are cooler than normal. Likewise, if the wind vectors (arrows) are short then
winds are near average conditions, if they are long then wind speed (and direction) is different relative to
normal.
Once you feel comfortable with what is being displayed in these figures it is now time to make plots from the
archived data. Click on the
Data > Data Display and Delivery
tab.
You should see the following page:
Step 1
– Create surface maps of normal, El Niño, and La Niña years:
To generate sea surface temperature and surface wind maps like the one shown on the previous page, follow
these steps:
1. On the Data display and delivery page press the “
Clear
” button to reset the program.
2. Press the “
Lat Lon Map
” button
3. Select the radio button that says “
Anomaly
” under the right-most menu that says “
Temperature
”
4. Select “
Monthly
” from the menu that says “
5-day
”
5. Select a year and month using the pull down menus (you will do this 5 times for each of the month/year
combinations listed below)
6. Press the
display
button – this should open up a new window with the appropriate figures in it.
7. Save the image or copy and paste it into a word processing document to include in your written response to
this assignment.
You should create and save surface maps for the following periods:
(10 pts)
January 1997 – typical of “normal conditions”
January 1998 – a very strong El Niño year
January 1999 – a very strong La Niña year
January 1992 (note the data are more limited b/c there were fewer moorings in 1992)
January 2002
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Step 2
– Create cross sections of temperature conditions across the equatorial pacific. These should look like the
example below from January of 2009:
Again, the top panel shows actual temperatures and the bottom
panel shows anomalies. The thermocline (region of rapid
temperature change) is visible as the region where the
temperature contours are most closely spaced.
Follow the following procedure to generate temperature cross
sections:
1. Press the “
Clear
” button to reset the interface
2. Press the “
Depth Section
” button.
3. Select the radio button that says “
Anomaly
” under the right-
most menu that says “
Sea Surface Temperature
”
4. Select “
Monthly
” from the menu that says “
5-day
”
5. Select a year and month from the pull-down menus.
6. Press the display button. (Again, these images can be enlarged
and copied/saved by clicking on them).
You should create and save cross sections for the same five periods for which you created surface maps.
(10pts)
Figure 1 TAO/Triton data during January of 1997, closest to normal weather conditions, top panel shows winds were strong and
shifting west, the bottom panel shows the winds were weaker
Figure 2 TAO/Triton data during January 1998 a strong El Nino year, High winds throughout the west shifting East
Figure 3 TAO/Triton data during January 1999, a strong La Nina year, winds were less strong and less consistent in direction
Figure 4 TAO/Triton data during January 1992, winds were strong and blowing both east and west
Figure 5 TAO/Triton data during January 2002, winds were strong and shifting north
Figure 6 Sea Surface temperature January 1997, temperature in top panel shows that surface decreases in temp with depth, bottom
graph shows very little temperature anomolies
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Figure 7 Sea Surface Temperatures January 1998, shows stronger anomalies in temperature, and warmer waters
Figure 8 Seas Surface Temperature January 1999, back to very few anomalies in temperature and colder waters
Figure 9 Seas Surface Temperature in January 1992, increase in temperature anomalies and water temperature
Figure 10 Sea Surface temperature in January 2002, increase in water temperature and decrease in temperature anomolies
Step 3
–
Provide a written (typed) response to the following questions that includes the 10 figures you made in
steps 1 & 2.
Each of these figures should have a brief caption.
(30pts)
**
Note that questions 1 and 2 ask for purely descriptive answers based on your own observations of the
available data – you shouldn’t be making interpretations until you get to questions three, four, and five.
1. Describe the characteristics of the equatorial Pacific during typical conditions (using the January 1997
observations as a guide). Your description should address the general pattern of sea surface temperature (SST)
and winds and the vertical pattern of water temperatures across the equatorial Pacific, including a description of
how the depth to the thermocline varies from west to east across the Pacific.
The equatorial Pacific during January of 1997 shows that winds were blowing east. The Sea surface
temperature showed that consistently the deeper the water the colder the temperature, with a few anomalies
occurring around 100-200m deep specifically within the eastern waters.
2. Describe how conditions during El Niño (January 1998) and La Niña (January 1999) differ from the typical
conditions described above. Focus on the same features you described above. [Note that one way to answer
both questions 1 and 2 would be to create a table listing the various characteristics (e.g., SST, thermocline depth)
for the 2 cases.]
During the El Nino event in January 1998 winds were significantly stronger and there was a spike in
anomalies within the west. The water temperatures also altered with warmer surface temperatures being
recorded in the west and colder surface temperatures being recorded in the East. This is the opposite of the La
Nina event that occurred January 1999 in which the winds were colder and blowing in multiple directions. The
water temperatures were also colder within the west and east compared to the El Nino event.
3. Based on your descriptions above, how would you characterize conditions during 1992 and 2002 – in other
words are these periods most similar to El Niño, La Niña, or neutral conditions. Describe your reasoning.
The conditions during 1992 and 2002 are most similar to an El Nino event. This can be seen as there is a
spike in surface temperature anomalies. In the west the temperature of the water became warmer and in the
east they became cooler, similar to what occurred in 1998.
4. Discuss when (if) upwelling and/or downwelling are likely occurring in the
eastern
equatorial pacific (near the
Peruvian Coast). Describe the evidence for this or you line of reasoning. (Note: upwelling refers to the vertical
movement of water when deep water is brought to the surface. Downwelling refers to the downward vertical
movement of water).
In the Eastern equatorial pacific upwelling is likely to occur due to the high-pressure gradient between
the Easter and Western Pacific the winds would plow from west to east causing an upwelling, as seen in Figure 1
and 6. A La Nina event would take place on the Peruvian coast because of this upwelling causing waters I the
west to get colder and waters in the west to get warmer. These same effects can be seen in the La Nina event in
figure 3 and figure 8.
5. Based on your observations of surface winds during El Niño and La Niña years and your understanding of
atmospheric-ocean interactions, discuss how atmospheric processes may be linked to the changes that occur in
the ocean during ENSO events.
The pressure gradients within the atmosphere affect the strength of the winds. The strength of the winds
directly causes El Nino and La Nina events. If there was a larger pressure gradient the winds would carry a large
amount of strength leading to an El Nino event. This would cause downwelling in the west causing warmer
waters and upwelling in the east leading to colder waters.
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