Part III: Jet Stream Patterns during El Niño and La Niña Events
Part III of the Final Project completes the background investigative work needed for the Final Project. (A separate document describes the format of a report summarizing the results of your investigation.)
The Final Project is a research project broken into
four distinct parts:
- Part I: Analysis of
precipitation data (done for you, but you need to understand how it was done in order to interpret it properly).
- Part II: Analysis of Pacific equatorial
sea-surface temperature and statistical connections to precipitation
data (completed in lab on Wed., Dec. 12 or Fri., Dec. 14)
- Part III: Jet Stream Patterns during El Niño/La Niño Events (in lecture Mon., Dec. 17)
- A final, summative report (due on Friday, Dec. 21; we provide you with a template)
- Conduct a realistic research project to investigate possible
connections, both statistical and physical, between variations in sea
surface temperatures near the equator in the Pacific Ocean and
variations in winter precipitation on the west coast of the U.S.
- Access, analyze, interpret, and present data to test the assertion that there are such connections.
Objectives for Part III:
- Using observations analyzed for use with computer forecast models, calculate and plot averages ("composites") of wind speed in the upper troposphere (in particular, where the pressure is 300 mb) during the five-month rainy season that you identified for your four weather stations in Part I of the Final Project, for the following years since the 1950-1951 rainy season:
Based on any differences among these plots, decide whether jet stream position and strength might depend in some way on sea-surface temperature patterns in the equatorial Pacific, and hence possibly affect winter rainfall totals at various locations on the West Coast of the U.S.
- all years from 1950-1951 to the most recent rainy season for which you have data;
- years in which strong El
Niño events occurred; and
- years in which strong La Niña events occurred.
- A computer with internet access.
- Completed and verified analyses data from Part II of the Final Project.
In Parts I and II
of the Final Project, you probably discovered the following:
- The amount of rainfall that West Coast weather
stations receive during the five wettest months of the year can vary quite a bit from
year to year.
- For stations in some regions, at least, there might be statistically significant connections between rainy season precipitation totals and the occurrence of some types of El Niño and/or La Niña events (that is, ENSO, or El Niño/Southern Oscillation, events).
However, statistically significant connections don't, by themselves, demonstrate cause and effect—for that, we have to demonstrate that there is also a physical connection.
One possible physical connection between ENSO events and West Coast rainfall is through ENSO's influence on atmospheric temperature patterns in the lower troposphere. These influences most directly affect the tropical Pacific Ocean but can affect other areas indirectly, too. Here's how the connection might work:
- During El Niño events, the higher-than-normal sea surface temperatures (SSTs) in the central and eastern tropical Pacific should warm the lower atmosphere there through:
La Niña events, the colder than normal sea surface temperatures (SSTs) in the central and eastern tropical Pacific should produce cooler than normal temperatures in the lower atmosphere there through:
- increased conduction of heat into the atmosphere from the sea surface, and
- increased evaporation from the ocean surface (which converts heat in the ocean into latent heat in water vapor, cooling the ocean surface), followed by condensation of the increased water vapor to form clouds (which converts latent heat back into heat, warming the atmosphere where the clouds form); and
- increased emission of longwave infrared (LWIR) radiation from the surface, and hence increased absorption of LWIR radiation (especially in the lower troposphere, where most of the water vapor is).
- reduced or even reversed conduction of heat between the atmosphere and the sea surface; and
- reduced evaporation from the sea surface, and hence reduced cloud formation, and hence reduced latent heat release in the atmosphere; and
- reduced emission of LWIR radiation from the surface, and hence reduced absorption of LWIR radiation in the lower troposphere.
- Recall that the polar front is a narrow zone of relatively large temperature contrast (large temperature gradient) in the lower troposphere between the tropics and the poles, normally found at midlatitudes. Warming of the lower troposphere in eastern tropical Pacific during El Niño events should create a temperature gradient between the tropics in that region and the midlatitudes, a region where the temperature gradient is normally very weak or absent. This should shift the latitude of the polar front farther south, or perhaps create a second, more southern branch of of the polar front.
Cooling the lower troposphere in the eastern tropical Pacific during La Niña events should weaken the (already weak) temperature gradient between the tropics and midlatitudes, which might leave the polar front farther north than usual.
- The pattern of pressure aloft between the tropics and midlatitudes should shift along with the polar front, because temperatures in the lower troposphere largely determine the pressure aloft. In particular, the narrow zone of large pressure gradient aloft that occurs directly above the polar front, might shift southward or form a southern branch during El Niño events and shift northward during La Niña events, following the polar front.
- As the pattern of pressure aloft shifts, the pattern of winds aloft (in particular, the location of the jet stream, which forms in the zone of large pressure gradient directly above the polar front), should shift as well. During El Niño events, we might see the jet stream shift southward or form a southern branch in the eastern Pacific.
- Since midlatitude cyclonic storms track along the jet stream, and midlatitude cyclones bring most of the rainfall received on the West Coast, any alteration in the jet stream position might affect rainfall patterns on the West Coast.
One relatively simple test of this possible physical connection is to analyze upper tropospheric wind speed data to see if the average jet stream position during the rainy season during El Niño and during La Nina events differs from the jet stream's overall average position during the rainy season. If it does differ, and in particular differs in ways consistent with changes in observed patterns of rainfall during El Niño and/or La Nina events, then we will have confirmed (but of course not proven) the hypothesis that ENSO events affect the latitude of the jet stream, and hence midlatitude cyclone tracks, and hence rainy season precipitation totals. That's as far as this project will go, but confirming the possible explanation would help justify searching for more evidence, which is how it works in science!
Instructions for Part III
The National Atmospheric and Oceanic Administration's Earth Systems Research Laboratory (ESRL), in Boulder, Colorado, provides Web access to many years of atmospheric observations analyzed originally for use with computer forecasting models. Among other things, the Web site allows you to construct "composites" (by which ESRL means averages over time of spatial patterns) of a variety of atmospheric quantities, including wind speed at various levels in the atmosphere.
We will take advantage of ESRL's Web site to test the hypothesis that ENSO events influence the jet stream along the West Coast in winter in ways consistent with statistical connections between rainfall and ENSO events at some West Coast weather stations.
To do this:
- Access ESRL's Monthly/Seasonal Climate Composites Web site at http://www.esrl.noaa.gov/psd/cgi-bin/data/composites/printpage.pl.
(Alternatively, to get to this page step by step:
- start with ESRL's Physical Science Division at http://www.esrl.noaa.gov/psd/;
- from the menu of links across the top of the page, pull down the "Products" menu and select "Plotting and Analysis", which gives you access to a wide range of different sorts of data and ways of analyzing them;
- click on the link to "Monthly/Seasonal Mean Composites".)
- Specify the quantity that you want to analyze and plot:
- Pull down the "Which variable?" menu and select "Scalar Wind Speed".
- Specify the level in the atmosphere where you want to analyze the wind speed:
- Pull down the "Level?" menu and select "300 mb". [The altitude where the pressure is 300 mb is around 9 or 10 kilometers, or around 30,000 feet, which is in the upper troposphere near where the jet stream has its maximum wind speeds.]
- Specify the period of particular months of the year (the "season") during which you want to analyze the wind speed at 300 mb:
Specify the range of years for which you want to compute a composite average of 300 mb wind speed during your five-month rainy season:
- Pull down the "Beginning month of the season" menu and select the first month of the five-month rainy season that you identified in Part I of the Final Project (probably November ["Nov"]).
- Pull down the "Ending month" menu and select the last month in your five-month rainy season (probably March ["Mar"]).
You are going to create a "color-filled" contour plot, which is a contour plot (of lines of constant wind speed, or isotachs) in which the area between each pair of adjacent contour lines is filled in with a different color.
Specify a plot color:
- In the "Enter range of years" text box, enter "1951" to the last year for which you had data for a full "rainfall season" in Part I of the final project.
The wind speed data available from ERSL's Web site is in meters per second. One meter per second is almost 2 knots (or 2.24 miles per hour). By convention, the jet stream is defined to be a relatively narrow "tube" of air aloft moving with a speed of at least 60 knots, which is about 30 meters/second. However, the jet stream position can vary somewhat from one day, week, month, and year to the next, so averaging the wind speeds for many months will tend to smear out the position of the jet stream and the winds will be weaker at any particular spot (because sometimes the jet stream will be there and sometimes not). To account for this "smearing out" of the averaged jet stream position and better highlight the average location of the jet stream, you'll want to construct a plot of wind speed that doesn't show winds slower than about 25 meters/second (rather than the conventional cut-off of 30 m/s). To this end, and to help optimize the jet stream plot more generally, change the default wind speed contour interval and the range of values to plot:
- Pull down the "Color" menu and select "Black and White".
Rather than viewing a plot for the entire world, create one for North America (which focuses more closely on the area of interest to us, the West Coast of the U.S.):
- Under "Override default contour interval?", in the "Interval" text box, enter "2.5" (which means 2.5 meters per second).
- In the "Range: low" text box, enter "25" (that is, 25 meters/second).
- In the "Range: high" text box, enter "50" (that is, 50 meters/second).
(Note that wind speeds in some places in the jet stream at any particular moment routinely exceed 50 meters/sec, which is almost 100 knots, but because the jet stream position wobbles back and forth as troughs and ridges migrate eastward, and the jet stream shifts north and south and back again to some degree over a period of weeks and months, the maximum wind speeds at any particular place and time are averaged with lower wind speeds at that place at other times, so the maximum averaged wind speeds are never as fast as the maximum wind speeds at any particular moment. As a result, plotting wind speeds only between 25 and 50 meters/second will still capture all or nearly all of the interesting behavior while optimizing the contrast in colors between lower and highest average wind speeds, which makes the plot easier to read.)
Click on the "Create plot" button. This should create the specified plot and display it in your Web browser.
- Pull down the "Map projection" menu and select "North America".
Capture the plot for use in your summary report for the Final Project:
Now you're ready to create another plot, this time a composite of average rainy-season wind speeds during years in which strong El Niño events occurred.
- Open a new document in Microsoft Word (or other word processing software) and drag the plot from your Web browser window and drop it into your Word document. (If this doesn't work, right-click on the plot in your Web browser window, pull down the "File" menu and select "Save image as", assign a name to the file you're about to save, and save it somewhere where you can find it on your computer. Then try importing it into a page in your word processing software, by dragging and dropping or by other means.)
- Enter a caption for the plot, either above beneath it (something like "Mean Jet Stream, NDJFM 1951-2012") should be enough.
- Failing this, print a hard copy of your plot and title it by hand.
Click on your browser's "Back" button to get back to the "Monthly/Seasonal Climate Composites" page. Repeat Steps 1 through 10 except for Step 5. Erase the existing entry (if any) for the "Enter range of years" item. This time, instead of a requesting a full range of years from 1951 to the more recent "rainfall season", refer to the option immediately above that, called "Enter years for composites (from 1 to 16)". In the text boxes beneath it, enter the years in which strong El Niño events occurred, which you determined in Part II of the Final Project. In Step 10, be sure to give your new plot an appropriate title.
Repeat for years in which strong La Niña events occurred. By the time you're done, you should have three plots altogether.
If you're not working on the computer on which you plan to write your summary of results for the Final Project, then email yourself a copy of the file containing your three captioned plots, or save a copy on a thumb drive, or ask the instructors for advice about how to save the document in a place and form where you can access it later.
You're now in a position to see if the hypothesis is confirmed, that ENSO events affect the position of the jet stream in a way that can help account for any statistical connections that you saw in Part II of the Final Project.
At this point, you should be ready to write your summary report for the Final Project (see "The Summary Report" for guidance).