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GEOG 4/521: Advanced Climatology Overview This document describes the climate anomaly analysis project. The aim of the project is twofold: 1) to gain experience in using on-line data sources to learn about recent climate anomalies, and 2) to understand the structure of those anomalies, i.e. how atmospheric circulation controls the nature of the climate anomaly at a particular location or region. The analysis of climate anomalies is quite straightforward, and consists of three steps:
The key years or cases are usually identified by examining a time series that describes the variation of a climate variable of interest. There are two basic kinds of variables used in such an analysis:
Other information can also be used to indentify the cases used to construct the composites, like a list of the years and months in which, say, large landslides occurred in western Oregon. Typically, a set of positive cases (e.g. the five warmest years at a specific location) are contrasted with a set of negative (or neutral) cases (e.g. the five coldest years). Sources of Information
Time Series
Maps
Text
Example What kind of upper-level circulation patterns favor dry Januaries in the Willamette Valley? To answer this question, we'll examine the precipitation January for the Willamette Valley Climate Division to identify the five driest years, and then produce maps of the 500mb height and vertical velocity anomalies that occur during those dry years. Before beginning, you might want to start up Word to provide a document that you can paste images into to save them. Step 1: Get and plot a time series of precipitation It may be helpful to look at these screen shots (or thumbnails) before doing the following: To get and plot a time series of January precipitation for the Willamette Valley climate division:
4. The plot should look something like:
5. Next, use the "Back" link on the plot page (not the "Back button!") to return to the "Create a monthly/seasonal mean time series from the US Climate Division Dataset" page, and repeat the above steps, only this time requesting "Ranked values" instead of "Anomaly" and "Raw data values" instead of "Plot data" (See the screen shots to verify that you've made the right selections.) 6. The five driest Januaries in the record are 1985 (driest), 1977, 1949, 1962, and 2001.
Step 2: Get some maps It may be helpful to look at these screen shots (or thumbnails) before doing the following:
After a few seconds, an image should appear. You can conveniently save the image by right-clicking on it, and selecting copy. Then in Microsoft Word, for example, you can paste the image in, and save the document. Use the "BACK" link to go back to the composites page, and repeat the steps above to obtain "Omega" (vertical velocity) at the 500mb analysis level, and "Vector Winds" (wind arrows and wind-speed shading) at the 500mb analysis level for the same selection of years. The resulting figures should look like the following:
To get seasonal anomalies (e.g. for winter in the Northern Hemisphere, i.e., December through February), enter beginning month = Dec and ending month = Feb in step 3. The years specified (step 4) are those of the ending month of the season (i.e. the 1976-77 winter is Dec 1976 through Feb 1977, and so the year to composite is specified as 1977). In the Northern Hemisphere, winter is DJF, spring is MAM, summer is JJA, and autumn is SON. In monsoonal regions, summer is sometimes taken to be JAS. Step 3: Interpretation The three maps of the composite anomalies (or differences from the long-term average) 500mb height, vertical velocity, and vector winds and wind speeds during the five driest Januarys for the Willamette Valley climate division provide a good example of how such anomalies can be interpreted. In particular, the dry conditions during the five years in the composite can be seen to be the product of a) generally weaker onshore flow of moisture, and b) the absence of precipitation-producing mechanisms, as indicated by large-scale sinking of air over the Pacific Northwest. The Assignment The project should consist of an illustrated discussion of either a) a particular climate anomaly, such as the unusually dry winter of 2000-2001 in western Oregon, or perhaps the floods of 1997, or b) of a systematic response of a region like the Pacific Northwest to variations in a large-scale control of climate as represented by a teleconnection index like the SOI. The report should include a page or two of introduction and description of the analysis, a presentation of maps and time series (2-4 pages), and a one- or two-page discussion of the results. The write up is due at the scheduled time of the final exam. By the time of the first exam, you should be able to indicate the specific climate anomaly or regional climate variations that you're going to explore. |
