OFFSHORE 

 J FMAM] J A S NO 



COASTAL 

 J FMAM J J AS ON D 



m=68 40 N 



-2-J- 

 2 



m=.64 37 N 



m=.63 33 N 



m=.68 3QN 



m-,66 27 N 



m=26 



Figure 16.— Monthly anomalies of low cloud amount (tenths) for lat. 40°N, 

 37°N, 33 °N, 30 C N, and 27°N in the 1 ° squares near the coast and 10° offshore. 

 The mean annual low cloud amount from which the anomalies were computed 

 is indicated in each plot by the value (m). 



coastal cloud cover near Punta Eugenia at lat. 27°N shows very little 

 seasonal change, reflecting the coastal desert climate of southern Baja 

 California, with cloud cover <0.3 throughout the year. 



Although the annual cycles of low cloud cover show local maxima at 

 locations corresponding to the sites of upwelling along the central and 

 southern California coasts, the long-term composite monthly total 

 cloud cover distributions reveal spatial minima in cloud cover at the 

 coast, coincident with areas of cold upwelled water. Figure 17 displays 

 the composite monthly mean total cloud cover for July. Cloud cover 

 values of about 0.5 of sky obscured are found in the Cape Bianco-Cape 

 Mendocino area and south of Point Conception. Beyond 200 to 300 km 

 from the coast, cloud cover varies from 0.7 to 0.8 of sky covered. The 

 low cloud cover south of Punta Eugenia. < 0.4, is a permanent feature 

 associated with the persistent offshore flow of dry continental air. The 

 pattern of cloud minima at the coast during summer is substantiated in a 

 number of recent climatological atlases, e.g., U.S. Naval Weather 

 Service Detachment (1977) and U.S. Department of Commerce and 

 U.S. Air Force (1971). Figure 18 displays the composite mean cloud 

 cover over the North Pacific on a 2.5° latitude-longitude grid for July 

 derived from a compilation of satellite images during the period 1965 to 

 1972. Isopleths are labeled in units equivalent to oktas of sky covered. 

 Values of 3 to 3.5 oktas along the coasts of Oregon and California and 

 off the coast of Baja California are equal to cloud cover of 0.4 to 0.45 



^Sadler. J. C. L. Oda.andB. J. Kilonsky. 1976. Pacific ocean cloudiness from 

 satellite observations. UHMET 76-01, Dep. Meteorol., Univ. Hawaii, 137 p. 



of sky covered. These estimates are independent of the surface marine 

 data. 



Simon (1977) analyzed mean cloudiness in an area off California 

 from May to September 1975 using 6-hourly photographs from the 

 geostationary satellite SMS-2. He found a minimum cloudiness zone 

 off the coasts of northern and central California, which was closely 

 associated with a region of maximum divergence in the surface winds, 

 particularly during July. The seasonal variation of the minimum cloudi- 

 ness line appeared to follow the variations of the summer "monsoon" 

 circulation along the California coast. The minimum mean cloudiness 

 values near 0.5 may reflect the tendency for cloudiness in the 1° 

 squares to be either clear or completely overcast, as noted by Simon in 

 his analysis. 



A relative minimum in cloud cover and cool sea surface tempera- 

 tures at the coast during summer increase the short-wave radiation and 

 decrease the net long-wave radiation reaching the sea surface, cause 

 sensible heat flux to the ocean, and reduce the loss of heat from evapo- 

 ration. In July, cloud cover increases from 0.5 of sky covered near the 

 coast, between lat. 39°N and 44°N, to 0.8 of sky covered offshore 

 (Fig. 17). This change in cloud cover results in a 25% decrease in 

 short-wave radiation, from approximately 275 W/m 2 near the coast to 

 205 W/m 2 offshore (Appendix I, Chart 7). In this same region, vapor 

 pressure and sea surface temperature increase in the offshore direction 

 as well. The observed zonal gradients in cloud cover vapor pressure, 

 and sea surface temperature lead to a 55 % reduction in computed back 

 radiation, from 40 W/m 2 between Cape Mendocino and Cape Blanco 

 to 18 W/m 2 , 10° of longitude offshore (Chart 19). The net effect of the 

 onshore-offshore gradients in cloud cover is to produce a relatively 

 large net radiative flux to the ocean near the coast in comparison with 

 the areas farther offshore. 



SEASONAL VARIATION OF THE OCEAN- 

 ATMOSPHERE HEAT EXCHANGE PROCESSES 



The mean annual cycles of the various components of surface 

 heat flux presented in Appendix I are shown in Figure 19 for the 

 eight 1 ° squares indicated in Figure 1 . Four of the locations along 

 the coast were chosen to illustrate the effects of coastal upwelling 

 on the heat fluxes and to show the range of conditions from near 

 Vancouver Island (lat. 50°N, long. 127°W) to Punta Eugenia (lat. 

 27°N, long. 1 14°W). The other four locations are 10° of longitude 

 offshore from the coastal squares, and were selected to show the 

 contrast of open ocean conditions. 



Radiation from sun and sky is a function of the seasonal variation 

 in the declination and altitude of the sun and the cloud cover at a 

 particular location. In the range from lat. 20°Nto50°N, the incom- 

 ing radiation from a cloudless sky reaches maximum values at all 

 latitudes in mid-June and minimum values in December. The 

 effects of the larger mean total cloud cover in the offshore squares 

 during nearly all months is evident in the smaller Q s values (curve 

 1) at the offshore locations, except at lat. 50°N where the coastal 

 square experiences greater cloud cover during winter. During sum- 

 mer (Appendix I, Charts 6-8) the gradient in cloud cover from 

 about 0.8 of sky obscured in the offshore region to about 0.5 at the 

 coast amounts to a 15 to 20% increase in incident solar radiation at 

 the coast. The onshore-offshore difference is even greater at lat. 

 27°N where the coastal desert climate results in a persistent low- 

 mean cloud cover of <0.4. The effects of an alongshore gradient in 

 cloud cover near Punta Eugenia are particularly evident in June and 

 July (Fig. 17; Charts 6, 7). A 30% increase in incident solar radia- 

 tion is realized, and £) 5 varies from 220 W/m 2 north of lat. 29°N to 

 290 W/m 2 just south of Punta Eugenia. A relatively constant cloud 

 cover throughout the year is evident in the smooth sinusoidal varia- 



23 



