386 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



in determining the climates of the world. Much 

 work has been done to investigate the man\i'old 

 problems associated with the energy exchange at 

 the air-sea boundary. A complete review of this 

 research will not be attempted, but such general 

 results as are of interest to this investigation may 

 be found in Sverdrup et al. (1942) and Jacobs 

 (1951). 



Briefly, the processes of concern here are the 

 radiant energy from sun and sky absorbed by 

 the water (Qs), the radiant energy lost due to back 

 radiation (Qt,), heat energy lost by the processes of 

 evaporation (Qe), and conduction of sensible heat 

 to or from the water (Qc). The net heat ex- 

 change across a unit area of sea surface per unit 

 of time, H, or the heat available to change the 

 temperature of the water can be expressed by 

 the formula : 



Q-Q,~Qe-Qc=H 



These terms will now be discussed separately. 



After entering the atmosphere, solar radiation 

 is modified by scattering and absorption so that 

 Qo represents the energy reaching the earth's sur- 

 face with a clear sky. Not all of this energy is 

 absorbed by the water because of reflection, and 

 further reduction may have occurred because of 

 cloud cover. To compute the energy absorbed 

 by the water (Q,) the following expression has 

 been given by Jacobs (1951) : 



(?,= (l-r)(l-0.071 C)Qo 



Here r is tlie percentage reflection and < ' is the 

 cloudiness on a scale from to 10. Qs is generally 

 expressed in cal. cm.~^ day~^ 



Observations for the direct and diffuse solar 

 radiation reaching the earth's surface, Qo, are not 

 available in the Hawaiian region, so that these 

 values were obtained from the Smithsonian Mete- 

 orological Tables, table 135 (List, 1951), using a 

 transmission coefRcient a=0.9, and have been 

 plotted in figure 11. At 10° N. the seasonal range 

 of Qo is 140 cal. cm."^ day"'; from 640 cal. cm.~^ 

 day"' in December to 780 cal. cm.~^ day"' in 

 summer. At 30° N. the seasonal range is 460 cal. 

 cm.~^ day"'; from 400 cal. cm."^ day"' in Decem- 

 ber to 860 cal. cm."^ day"' in June. It should 

 also be noted that from April to September the 

 difference between Qo at 10° N. and at 30° N. is 



1000 



JAN MAR MAY 



SEP NOV 



FiauRE 11. — Seasonal variation of the direct and diffuse 

 solar radiation reaching the earth's surface at 10°, 20°, 

 and 30° N. 



less than 100 cal. cm.~^ day"', whereas in Decem- 

 ber this difference is 240 cal. cm."^ day"'. 



On the basis of these data, even though the 

 direct and diffuse radiation will be modified by 

 factors, such as back radiation and evaporation, 

 one would anticipate tiie following effects on the 

 surface water temperature: (1) the seasonal tem- 

 perature range increases northward, and (2) the 

 water temperature difference between 10° N. and 

 30° N. would be greatest during the minimum 

 temperature period of the year. 



In order to use Jacolis' fornuila, the cloud cover, 

 which ranges from four- to six-tenths of sky 

 covered in the Hawaii region, was obtained from 

 the U.S. Weather Bureau Atlas of climatic cliarts 

 of the ocean (McDonald, 1938). The amount of 

 radiation reflected back to the sky was quoted by 

 Jacobs (1951) to range from 3.3 percent at the 

 equator to 8 percent at the poles. With these 

 data and Qo from the Smithsonian tables, the 

 amount of radiation absorbed in the water, Q,, 

 was computed. 



