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Figure 10.— Mean monthly atmospheric sea level pressure (mbar) over the eastern North 

 Pacific and the west coast of North America during February, May, August, and November. 

 The contour interval is 2.5 mbar (after Reid et al. 1958). 



North America during 4 mo of the year (adapted from Reid et al. 

 1958). During spring and summer (May and August) the subtropical 

 high reaches its maximum northward position and maximum pressure, 

 and at the same time a thermal low pressure system develops over Cali- 

 fornia and Arizona. The large high pressure area shunts migratory 

 cyclones, which pass across the coasts of Washington and Oregon in 

 the fall and winter, well to the north. For winds in geostrophic balance, 

 the surface wind blows parallel to isobars as indicated by the arrow 

 barbs on the isobars. Frictional effects in the planetary boundary layer 

 deflect the surface winds toward low pressure. The composite monthly 

 mean windspeed distributions for June and December, respectively 

 (Figs. 11. 12) illustrate the character of seasonal changes in the surface 

 wind field. During summer (Fig. 11). the region north of lat. 40°N 

 shows relatively low windspeeds of 6 to 7 m/s over almost the entire 

 area. Maximum windspeeds in excess of 9 m/s are found off Cape 

 Mendocino. This offshore maximum is aligned parallel to the coast and 

 maxima can be traced to the south as far as Punta Eugenia, at a distance 

 of about 200 to 300 km from the coast. In contrast, during winter strong 

 onshore winds north of lat. 40°N (Nelson 1977) blow at speeds of 9 to 

 1 1 m/s, while the area to the south experiences speeds of 5 to 8 m/s, 

 with a somewhat disorganized pattern (Fig. 12). 



During the summer, relatively strong alongshore equatorward 

 winds induce an offshore transport of surface water due to the rota- 

 tion of the earth acting upon the oceanic response to equatorward 

 wind stress. Conservation of mass requires replacement by conver- 

 gence in the equatorward surface flow or compensation from 

 below, giving rise to an upwelling of water from intermediate 

 depths along the coast. This upwelling forms a large area of rela- 

 tively cold water along the coasts of Oregon and California. Fig- 

 ures 13 and 14 show the long-term composite sea surface 

 temperature fields for the months of June and December, respec- 

 tively, which have been computed from historical surface marine 

 data. The California Current brings relatively cold, freshwater 

 along the west coast of the United States as a branch of the North 



Pacific Current, which is deflected to the south by the North 

 American continent. In December (Fig. 14) the coldest water is 

 found at the northernmost latitudes, but as the current is deflected 

 to the south the isotherms bend to the south and colder water is 

 found along the coast relative to the water farther offshore. As the 

 water continues south it is warmed by mixing with warmer water 

 offshore and an increase in solar radiation. In June (Fig. 13) cold 

 water is evident at the northerly latitudes, but a large area of cold 

 water (11°-13°C) exists along the coasts of Oregon and northern 

 California. Another region of relatively cool water is found near the 

 coast of Baja California, north of Punta Eugenia. Such conditions 

 persist off Oregon and northern California through September. 

 During the summer, the only possible source for this cold water is 

 from below, because warmer sea surface temperatures occur to the 

 north. 



Nelson (1977) described the seasonal variation of the alongshore 

 component of wind stress and discussed the northward progression of 

 the maximum alongshore wind stress from April and May oft" the coast 

 of Baja California to July and August off Cape Mendocino and Cape 

 Blanco. The movement of this alongshore wind stress maximum, 

 which is an indicator of conditions favorable for upwelling. is the result 

 of the northward shift and strengthening of the subtropical high pres- 

 sure center and the coincident development of the continental thermal 

 low. An important characteristic of this dome of high pressure is that 

 the atmospheric circulation around the eastern edge of the system is 

 divergent and large-scale subsidence occurs. The combination of the 

 large-scale subsidence of warm air from aloft and the upwelling of cold 

 water along the coast results in the formation of a strong low-level 

 inversion in the marine atmospheric boundary layer. This inversion 

 suppresses deep cloud formation and greatly inhibits precipitation. The 

 effect of the large-scale subsidence is noted in the true desert climate of 

 Baja California, particularly south of Punta Eugenia, and in the almost 

 complete lack of rainfall along the coasts of California and Oregon dur- 

 ing summer. Within 10 to 20 km of the coast, upwelling influences the 



17 



