The oceanic circulation of the North Pacific closely resembles that of the North Atlantic. The drift from the Kuroshio current off Japan is propelled by the westerlies towards the west coast of North America and it acts as a warm current between 40° and 60°N. Sea-surface temperatures are several degrees lower than in comparable latitudes off western Europe, however, due to the smaller volume of warm water involved. In addition, in contrast to the Norwegian Sea, the shape of the Alaskan coastline prevents the extension of the drift to high latitudes (see Figure 7.29).
The Pacific coast ranges greatly restrict the inland extent of oceanic influences, and hence there is no extensive maritime temperate climate as in western Europe. The major climatic features duplicate those of the coastal mountains of Norway and those of New Zealand and southern Chile in the belt of southern westerlies. Topographic factors make the weather and climate of such areas very variable over short distances, both vertically and horizontally. A few salient characteristics are selected for consideration here.
There is a regular pattern of rainy windward and drier lee slopes across the successive northwest to southeast ranges, with a more general decrease towards the interior. The Coast Range in British Columbia has mean annual totals of precipitation exceeding 2500 mm, with
5000 mm in the wettest places, compared with 1250 mm or less on the summits of the Rockies. Yet even on the leeward side of Vancouver Island, the average figure at Victoria is only 700 mm. Analogous to the 'westerlies-oceanic' regime of northwest Europe, there is a winter precipitation maximum along the littoral (Estevan Point in Figure 10.16), which also extends beyond the Cascades (in Washington) and the Coast Range (in British Columbia), but summers are drier due to the strong North Pacific anticyclone. The regime in the interior of British Columbia is transitional between that of the coastal region and the distinct summer maximum of central North America (Calgary), although at Kamloops in the Thompson valley (annual average 250 mm) there is a slight summer maximum associated with thunderstorm-type rainfall. In general, the sheltered interior valleys receive less than 500 mm per year. In the driest years certain localities have recorded only 150 mm. Above 1000 m, much of the precipitation falls
as snow (see Figure 10.16) and some of the greatest snow depths in the world are reported from British Columbia, Washington and Oregon. A US national record seasonal total of 28.96m was observed at the Mt Baker ski area (1280 m) in 1998 to 1999. Generally, 10 to 15 m of snow falls annually on the Cascade Range at heights of about 1500 m, and even as far inland as the Selkirk Mountains snowfall totals are considerable. The mean snowfall is 9.9 m at Glacier, British Columbia (elevation 1250 m), and this accounts for almost 70 per cent of the annual precipitation (see Figure 10.16). Near sea-level on the outer coast, in contrast, very little precipitation falls as snow (for example, Estevan Point). It is estimated that the climatic snowline rises from about 1600 m on the west side of Vancouver Island to 2900 m in the eastern Coast Range. Inland, its elevation increases from 2300 m on the west slopes of the Columbia Mountains to 3100 m on the east side of the Rockies. This trend reflects the precipitation pattern referred to above.
Large diurnal variations affect the Cordilleran valleys. Strong diurnal rhythms of temperature (especially in summer) and wind direction are a feature of mountain climates and their effect is superimposed upon the general climatic characteristics of the area. Cold air drainage can produce remarkably low minima in the mountain valleys and basins. At Princeton, British Columbia (elevation 695 m), where the mean daily minimum in January is -14°C, there is on record an absolute low of -45°C, for example. This leads in some cases to reversal of the normal lapse rate. Golden in the Rocky Mountain Trench has a January mean of -12°C, whereas 460 m higher at Glacier (1250 m) it is -10°C.
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