The El Niño Southern Oscillation enso background
Activity 2: Make graphic representations of El Nino and La Nina Conditions, sea surface pressure
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- Navigate this page:
- Step Action
- Activity 3: Make graphic representations of El Nino and La Nina Conditions, sea surface Temperatures
- Exercise 4: More on SST’s
- Exercise 5: Look at Sea Surface Height
- Exercise 6: El Niño, La Niña and Seasonal Precipitation
- Exercise 7: Current Conditions (June 2011)
| Activity 2: Make graphic representations of El Nino and La Nina Conditions, sea surface pressure
In this first example, the sea-saw mechanism in sea surface level atmospheric pressure associated with ENSO and referred to as the Southern Oscillation is revisited. As previously indicated, the sequence of ENSO warm and cold events during the late 1980s represented a clear signal of the phenomenon and is used here as an example. In the following section, you will be lead through stepped through the procedure of graphically and interactively displaying the sea surface level pressure. It is recommended that the you are connected to the internet and follow the instructions outlined in Table 1. Changes in pressure, temperature etc. (see below) are always represented as anomalies, that is the difference between the actual, present-day temperature or pressure and a climatological value. The latter is calculated as a mean value from many years of observations; the period is also referred to as the reference period and is representative of the climatological mean state of the Earth’s climate system.
Table 1: Instructions to graphically display sea surface level pressure during the ENSO warm and cold event in 1987 – 1988. For the warm event choose data averaged for January to May 1987, for the cold event choose data averaged for August to December 1988. The distribution in sea surface level pressure anomaly, i.e. the difference in the pressure observed during the period January to May 1987 and the climatological mean value is characterized by higher than usual values (positive anomaly) in the western and lower than usual values (negative anomaly) in the eastern Pacific Ocean. During this period, unusually warm surface water appeared in the eastern Pacific Ocean (see exercise below). This is the so called El Niño, or ENSO warm event. Print your map to turn in. 
Figure 3: The graphic shows the anomaly in sea surface level pressure (in millibars) during the period January to May 1987. Australia on the left, South America on the right. Positive contour labels east of about 180o indicate higher than normal sea surface level pressure. During this period a very strong El Niño event, i.e. an unusual warming of the eastern Equatorial Pacific Ocean was observed. About 12-14 months after the warm event, ENSO approaches another extreme state referred to as an ENSO cold event or La Niña. While the eastern Pacific surface ocean is unusually cold during this period, the distribution of sea level surface pressure is the opposite to that shown in Figure 3. The regions in the western Pacific Ocean characterized by higher than usual pressure are now characterized by lower than usual pressure (Figure 4), while the eastern Pacific Ocean exhibits higher than usual pressure. The figure is obtained by replacing the averaging period of step 4 (Table 1) with the period August to December 1988. The process, which is identified here from plots of the atmospheric observations is the so-called sea-saw mechanisms in sea surface level pressure. The Southern Oscillation fluctuates between these two distinct phases with a period of about 3-4 years, and the SOI is a measure for the current state of the oscillation. 
Figure 4: The graphic shows the anomaly in sea surface level pressure during the period August to December 1988. Positive contour labels east of about 180o indicate higher than normal sea surface level pressure. During this period a very strong La Niña event, i.e. an unusual cooling of the eastern Equatorial Pacific Ocean was observed. Activity 3: Make graphic representations of El Nino and La Nina Conditions, sea surface Temperatures Changes in sea surface temperature are dramatic during ENSO warm and cold events and oscillate in a manner similar to that of atmospheric pressure. This exercise involves displaying graphically the sea-saw mechanism in sea surface temperature associated with ENSO, data are used for the period 1987 – 1988, and the instructions are identical to those listed in Table 1 with the exemption that the variable to be selected is that of sea surface temperature. So, when following the instructions of step 2 in Table 1, choose Hadlsst, select time begin and end with either April 1987 (Figure 4) or August 1988 (Figure 5). The resulting graphic displays the anomaly in sea surface temperature during the ENSO warm event (Figure 5) and cold event (Figure 6). The sea-saw mechanism already established in pressure data (Figure 3 and 4), is clearly evident from the record of sea surface temperature. Within a period of about 18 months the temperature distribution reverses from cold to warm. This represents a temperature change of more than 4o C over the width of the Pacific Ocean. 
Figure 5: Anomaly in sea surface temperature during the 1987 El Niño. The eastern equatorial ocean is unusually warm with temperature more than 2 degree higher than normally, i.e. contour labels are positive numbers in the east and negative in the west. 
Figure 6: Anomaly in sea surface temperature during the August 1988 La Niña event. The eastern equatorial ocean is unusually cold with temperature more than 2 degrees lower than normally, i.e. contour labels are negative numbers in the east and positive in the west. (Modified from http://eprints.usq.edu.au/1074/1/Ribbe_2002_Climate.pdf) The data and images were provided by the NOAA-CIRES Climate Diagnostics Centre, Boulder, Colorado, USA, from their Web site at http://www.cdc.noaa.gov/ and by the Pacific Marine Environmental Laboratory, Seattle, USA, from their Web site at http://www.pmel.noaa.gov/. Exercise 4: More on SST’s Go to the following website: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensocycle/meansst.shtml It describes the average or "normal" sea surface temperatures in the Southern Pacific over a year's time. 1. Describe the sea surface temperatures along the equator going from west to east. 2. What process contributes to the low temperatures you observe in the eastern equatorial Pacific? 3. Look at the four diagrams carefully. Do you observe any differences between the various seasons? If yes, are they significant? The animations on the following web-page will show you the sea surface temperature anomalies in the Southern Pacific before, during and after four recent El Niño events. The data originally come from the Along Track Scanning Radiometers (ATSRs): http://www.cdc.noaa.gov/map/clim/sst_olr/old_sst/sst_anim_4panel.shtml 4. Look carefully at every event individually. What is the feature on these graphs that tells you that there is an El Niño event taking place in every case? How is that feature different from normal conditions? 5. Do the four El Niños look the same? Which seems to be the strongest and which lasts the longest? And a word about La Niña… Look at the images on the following web-site: http://www.pmel.noaa.gov/tao/elnino/la-nina-anomaly.html 6. Describe in a few sentences how La Niña conditions are different from normal conditions. Exercise 5: Look at Sea Surface Height Go to the following web page: http://sealevel.jpl.nasa.gov/science/elninopdo/elnino/ This site shows data from the TOPEX/Poseidon satellite altimeter indicating sea surface height changes in the period of time between March 1997 and December 2002.
1. What happened in March of 1997 to trigger the onset of the El Niño event? 2. What is the name of the wave that propagates from the Western to the Eastern Pacific? 3. How long did it take for it to reach South America? 4. What does the wave help transport and how does it affect sea level in the South-eastern Pacific? 5. How did this El Niño event affect sea level and temps? 6. Estimate the duration of the 1997-98 El Niño event in months (note beginning and end dates). Did the conditions return to normal after this event? Exercise 6: El Niño, La Niña and Seasonal Precipitation 
1. What correlations do you see about winter precipitation in San Francisco and El Niño /La Niña years?
years?
Niño /La Niña years?
Look at the following recent image of sea level height anomalies: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_update/sealev.gif The lines connect points of equal height. The brighter the colors, the more positive the sea level anomaly. 1. Record the date the image was released: ______________________
· the western equatorial Pacific? · the eastern equatorial Pacific?.
· Click the blue-green button called “Data Display” - Click the blue-green button called “Section plots” · Click the second orange button from the left called “Depth” right under the word “Section” · Click the red button called “Make plot!”
Look at the last two months' sea surface temperature data by going to the following page: http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml Select the "SST Animation - Tropical Pacific." You will see sea surface temperatures and anomalies for the last twelve weeks.
conditions: normal or unusual? Look at the El Niño Advisory by the Climate Prediction Center of NOAA: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ 10. According to this agency, is there an El Niño event under way? 11. How extensive in area is the sea surface warming? Download 0.84 Mb. Share with your friends:
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