Geog. 321 -- Climatology
Exercise 6 -- Atmospheric circulation

This exercise examines atmospheric circulation, in particular the current circulation, and recent "anomalies" or differences from long-term averages..  Links to the various images and text can be found on the Current Weather & Climate web page, in the "Upper Air and Surface Winds" and "Climate Anomaly Maps and Diagrams" sections.

Note:  The exercise once again makes use of "live" images, which might not be available at the time you work on the exercise.  Before starting the exercise, scroll down the page to make sure all images are being displayed.  There should be two images (each with a left-hand panel and a right-hand panel, OR (depending on the resolution of your monitor) a top panel and a bottom panel) .  If some of the images are missing or the pages don't open up, you could try doing the exercise later, or without losing any points, just type "image or page not available," and the time, into the answer form.

Note that some of these images use color schemes that may not be interpretable to "color deficient" viewers.  Let us know if you have trouble interpreting the images.

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The image from the Climate Diagnostics Center below shows current 500mb heights on the left (or top) and "height anomalies" on the right (or bottom).  Winds at the 500mb level blow parallel to the height-contour lines (which are located where the color changes on the first (left-hand or top) image.  The second image (on the right or below the first one) shows 500mb height anomalies (or differences from normal heights) by colors.  Positive height anomalies, or locations where the 500mb surface was higher than normal are indicated by red and yellow hues, while negative height anomalies are indicated by blue and purple hues.  Recall that the flow of air at the 500mb level is generally parallel to the (black) contour lines, in a west to east direction around the globe. The anomalous component of flow at the 500mb level can be inferred from the height anomalies:  it is clockwise around positive height anomalies, and counter-clockwise around negative height anomalies. 

Here's a summary:

  • negative anomalies (blue) indicate deeper troughs or weaker ridges, and the "anomalous component" of flow around the anomaly is in a counter-clockwise direction (i.e. easterly (from the east)  on the poleward side of an anomaly, westerly (from the west) on the equatorward side);
  • positive anomalies (red) indicate shallower troughs or stronger ridges, and the "anomalous component" of flow around the anomaly is in a clockwise direction (i.e. westerly (from the west) on the poleward side of an anomaly, easterly (from the east) on the equatorward side);
  • if the anomalous component is westerly in a region that ordinarily experiences westerly winds, then that indicates that the westerlies are stronger than normal;
  • if the anomalous component is easterly in a region that ordinarily experiences westerly winds, then than indicates that the westerlies are weaker than normal.

Current (probably yesterday, but current enough) 500mb heights and height anomalies

Question 6-1:  On the top or left-hand map, describe the upper-level wind patterns over the eastern Pacific and western North America.  (Where are ridges located on the left-hand map, where are troughs located?)

Question 6-2:  Describe the 500mb height anomalies on the bottom or right-hand map.  (Where do positive anomalies occur and where do negative anomalies occur?)

Question 6-3:  Focusing on the Pacific Northwest (i.e. Washington and Oregon), describe the observed upper-level wind direction (inferred from the shape of the height contours), and attempt to infer the anomalous component of flow using the ideas in the bullets above.  How do the current upper-level winds differ from the expected or "normal" winds for this time of year?  (Hint:  imagine looking at the anomaly height contours (right-hand map) as if they were sea-level pressure maps and imagine the flow around high (positive height contours) and low (negative height contour) systems.

 

Sea-level pressure maps

Question 6-4:  Describe the current sea-level pressure over the eastern Pacific and western North America.  (Where is the pressure high on the top or left-hand map, where is it low?)  Is there any correspondence between the sea-level pressure anomalies (bottom or right-hand map) and 500mb height anomalies?

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