Why Does West Coast Precipitation
Vary from Year to Year?
The last two lab meetings of the semester, on Wednesdays, Dec. 5 and 12 (Lab Section #1) or Fridays, Dec. 7 and 14 (Lab Section #2), will be devoted to supporting the final project, which will be worth 15% of your final course grade. (Some of each of the last three lecture class meetings on the Mondays of Dec. 3, 10, and 17 will also support this assignment.)
The final project is a research project broken into four distinct parts:
Objectives for Part I:
Water is arguably the most vital natural resource for human and nonhuman life. This is particularly true in places where water is relatively scarce, such as much of the western U.S. The water that we depend on for drinking, irrigating crops, industry, and recreation, and that sustains the natural ecosystems on which we also depend, comes ultimately via precipitation.
The ways and places in which we live and work depend deeply not only on the average precipitation of the region but on its variability from month to month and year to year. To deal with variability and try to provide a reliable supply of water its residents, the state of California and the federal government have built a complex system of dams, pumps, and canals to capture, store, and divert water to places where people grow irrigated crops or prefer to work and live and where there wouldn't otherwise be enough water for those activities. However, California only stores in reservoirs or pumps out of the ground about half of all of the water that it currently uses, and it will be very hard to change that in the future. As a result, we depend heavily on the snow pack that accumulates in the Sierra Nevada Mountains (in eastern California) each winter to store water for us. As temperatures warm in late spring and summer, the winter snow pack largely melts, releasing the water gradually over a period of months, and we are able to capture and use some of it.
(Note that one of the likely impacts of global warming of greatest concern to California and other western states in particular, is on the winter snow pack. As the planet warms, more precipitation in the Sierra Nevada Mts. will fall as rain rather than as snow and will run off immediately instead of being stored as snow. We won't be able to capture as much of it as we need, and there will also be more winter flooding because the precipitation will run off in short periods instead of over several months.)
Most of the precipitation that falls in California, Oregon, and Washington is associated with midlatitude cyclones in the fall, winter, and spring. These storms form and travel along the polar front beneath the jet stream, so the location of the jet stream has a big impact on where midlatitude cyclones go and how strong they are, and hence where and how much precipitation falls. Although it's not possible to forecast individual storms with any confidence beyond a few days to (sometimes) a week in advance, there are some influences on weather patterns that are longer lasting and that can be predicted with some confidence months in advance. Sea surface temperature in the Pacific Ocean near the equator is one of those influences. Do sea surface temperature patterns affect the position and strength of the jet stream, and hence the path and strength of midlatitude cyclones, and hence precipitation patterns? And if so, how?
In this project, we are interested in the year to year variability of precipitation in California in particular but also other parts of the West Coast, and will try to determine what might account for some of the variability. There is reason to believe that the phenomenon of El Niño/La Niña, a quasi-periodic variation in equatorial Pacific sea surface temperatures that we can predict with some confidence months in advance, might West Coast rainfall. We will investigate the extent to which this might be true, and if it seems true, see if it might be connected to the position of the jet stream.
To investigate these connections, we will start in Part I by walking through an already performed, partial analysis of precipitation data recorded at several weather stations distributed along the West Coast.
You will use precipitation data from four weather stations, including one from each of the following four regions of the West Coast (see map):
We selected these stations because (1) they provide a representative distribution of stations up and down the west coast of the U.S.; and (2) each has a relatively continuous record of precipitation (missing no more than three days from any one month) from the current year going back to at least 1950. We got the data from the Western Regional Climate Center (http://www.wrcc.dri.edu/coopmap).
Attached to this assignment is a list of particular station assignments for Lab Section #1 (Wednesdays) or Lab Section #2 (Fridays).
We've partially analyzed the data in Microsoft Excel (a spreadsheet calculating program) and saved them as PDF files. The files (one file per station) are accessible at http://funnel.sfsu.edu/courses/metr104/F12/labs/FinalProject/PrecipData/.