Lab5_Potter_M (1)
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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.
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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 -
Figure 1:
In January 1997, the National Oceanic and Atmospheric Administration (NOAA) recorded anomalies in the Equatorial Pacific.
The anomalies included latitude/longitude degrees, temperature, and wind vectors.
Figure 2:
In January 1998, NOAA recorded anomalies indicating a powerful El Niño year in the Equatorial Pacific. The wind vectors
were observed to be changing direction, and temperatures had significantly increased towards the west.
Figure 3:
Based on limited data from January 1992 recorded by the NOAA, temperatures in the equatorial pacific are slightly warmer
than average on the west side of the graph
.
Figure 4:
NOAA recorded anomalies indicating a powerful La Niña year in the Equatorial Pacific starting from January 1999. The
temperatures remained at the average except for a decrease right at the equator.
Figure 5:
According to NOAA's data from January 2002, temperatures have remained steady and trade winds have moved from West to
East, indicating normal conditions
.
Figure 6:
The National Oceanic and Atmospheric Administration (NOAA) reported on the temperatures across the equatorial pacific in
January 1997, showing consistent temperatures with only minor variation of around 4 degrees Celsius
.
Figure 7: According to a report by NOAA, temperatures in the west of the equatorial Pacific have increased from around 110 - 100
degrees West, while a large upper portion of the East half has cooled off. The report was taken in January 1998.
Figure 8:
The National Oceanic and Atmospheric Administration (NOAA) reported consistent temperatures across the equatorial
Pacific from January 1999, with a slight cooling in the upper western half. The thermocline began to tilt up at around 150 degrees
West, and warmer waters were observed in the East.
.
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Figure 9: According to a report by the NOAA, a cross-section of temperature data from the equatorial Pacific in January 1992 shows
limited data. However, it appears that temperatures in the Western surface areas have increased by approximately four degrees
Celsius..
Figure 10: The National Oceanic and Atmospheric Administration (NOAA) reported a cross section of equatorial Pacific temperatures
from January 2002. Temperatures have increased by about 4% near the middle surface areas, and a thermocline can be noted from
about 140 degrees West. Warmer temperatures move from West to East.
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)
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.
a.
Conditions that are usually typical have the trade winds going from the West to the East. The
waters are typically warmer with a temperature of about 28 degrees Celsius to 30 degrees
Celsius pushed to the East. The cooler temps tend to stay around 23-24 degrees Celsius. The
equatorial Pacific tends to have warmer water on the surface, whereas the deeper water is
colder. Thermocline begins to go upward at around 180ish meters deep and 150ish meters West
and goes down in the deeper oceanic parts.
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.]
-
In January 1998, during El Niño, the trade winds moved in the opposite direction
compared to normal conditions, going from east to west instead of west to east. The
sea surface temperatures (SSTs) were more evenly matched in El Niño, whereas in
normal conditions they varied from warm to cool. In comparison to El Niño, the SSTs in
La Niña were generally cooler. The trade winds in La Niña were like normal conditions.
The thermocline in El Niño was flatter compared to normal conditions and La Niña,
which had steeper thermoclines. In 1997, the thermocline started rising at around 180
meters deep at 140 degrees West and stopped at around 100 meters at 80 degrees
West. In 1999, the thermocline started at 200 meters at 160 degrees West and went
up to 50 meters deep at 80 degrees West.
3.
Based on your descriptions above, how would you characterize conditions during 1992 and 2002 – in
other words are these periods most like El Niño, La Niña, or neutral conditions. Describe your reasoning.
a.
In 2002, it was a La Nina year. The thermocline depth was around the same ange of the La Nina.
In 1992, I would say the year was average or neutral because it is similar to January of 1997. The
sea surface temps were in the same range of each other. Both years did have some of the same
wind patterns though.
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 your 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).
-
Upwelling near the equator occurs from the strong winds due to high pressure
between the Pacific Oceans. Causing La Nina in the Peruvian Coast. The warmer waters
would be present in the Eastern Pacific and the water that is colder would be placed
into the Western Pacific.
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.
-
El Nino years are expected to bring smaller trade winds moving across the equator.
There may be decreased upwelling of nutrient rich water. Therefore, almost flattening
the thermocline. Warm water would be greater in the eastern parts due to these
changes. The pressure gradient of the atmosphere changes the trade winds direction.
These sea surface temperatures are heavily reliant and affected by the differeing
pressures.
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