|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