Lecture 13 Notes  October 22, 2004

Most of the figures below where prepared by Dr. Nate Mantua

Pacific Northwest Climate (PNW) - The Mean

Factors that influence the mean are mountains, prevailing winds and proximity to the ocean. The following figure shows the winter (Dec-Feb) and summer (Jun-Aug) sea level pressure maps. As we learned in chapter 4, ocean surfaces generally have low pressure centers in winter and high pressure centers in summer due to the ocean-land temperature contrast (warm ocean - cold land in winter and vice versa in summer).

Mountains map the winds from the large-scale pressure patterns onto local PNW landscapes.
Depending on the wind direction, the puget sound can experience cold air outbreaks, a convergence zone, or even a rain shadow. Cold air outbreaks or "Arctic Blasts" occur when northeasterly winds come down the Fraser River Valley (the green colored vein reaching the coastal lowlands just south of Vancouver, BC) and down the Columbia river from east to west. These winds from the Fraser River Valley yield the best possibility for snow in Seattle. Snowfall is frequent snowfall in the mountains because the elevation is high enough to maintain subfreezing temperatures, but in Seattle we are often on the temperature margin for snow or rain. 

The Puget Sound Convergence Zone results when winds hit the Olympic mountains just right so they spit into two low level branches that then reunite in the Puget Sound.

If atmospheric scientists are sure about anything, we know how to explain a rain shadow (paraphrased from Dr. Gerard Roe, seminar Nov. 10, 2004). Upslope winds lift air upwards, which causes air to cool and water  to condense. The prevailing westerlies in wintertime hit the Olympics, coastal range, and Cascades and cause high precipitation there. Much less rain falls on the Rockies because the air is much drier after condensing most of the available water near the coast. The maximum color range shown at is about 100in of rain. The actual maximum is probably more like 200 inches on Mt Olympus.


Climate variability in the PNW is due primary to three things (known to date): El Nino, the Pacific Decadal Oscillation (PDO), and random perturbations. These three thngs are inter-related to some extent too.  We have not yet talked about the PDO, and it is not covered in the text at all, yet it has substantial influence in the PNW! University of Washington scientists coined the term PDO. Local expert Nate Mantua wrote a brief description for non-specialists

The University of Washington Climate Impacts Group (of which Nate Mantua is a member) has an excellent web page about PNW climate and climate variability. Check out their page comparing climate impacts of ENSO and PDO in the PNW.

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Contact the instructor at: atms211@atmos.washington.edu

Last Updated: 10/22/2004