Of the incoming radiation corrected for the 

 screening effects of cloud cover, some is 

 reflected at the sea surface. The amount 

 reflected depends on the latitude and time of 

 year. The computational expression is as 

 follows: 



4.70 (e 



- e ) W 

 a' — 



Q 



R 



Q x x 



where r = percentage of radiation reflected 

 presented in a table by Budyko (1956); this 

 table is listed in Appendix B- 2. The percentage 

 varies from about 6 percent in low latitudes 

 to more than 20 percent in high latitudes in 

 winter. 



Back radiation, Qg (in cal./cm. /day), is 

 the difference between long-wave radiation 

 from the sea surface and long-wave radiation 

 from the atmosphere. The following semi- 

 empirical equation proposed by Berliand and 

 Berliand (1952), and used by Roden (1959) for 

 studies of the California Current system, has 

 been incorporated in this study: 



= so8 



(0.39 - 0.050V~i) (1 - k C 2 ) 



+ 4so6 



it 



J 



where _s = 0.97 - the ratio of the radiation of 



the sea surface to a black body 

 8 8 = absolute temperature in ° C; 



ai £ = 1.175 x 10-7 . the Stefan-Boltz- 

 mann constant; 

 e = vapor pressure (mb.); 

 k = constant *; 

 C = cloudiness in tenths. 



Off the California coast, Roden (1959) has 

 found that Q R varies from about 80 to 140 



2 — 



cal./cm. /day and Seckel (1962) has found that, 



in the Hawaiian Islands region, values range 

 from 115 to 150 cal./cm 2 /day. At Triple 

 Island, British Columbia, and at ocean station 

 "Papa," Tabata (1958, 1961) has recorded 

 average values of about 100 cal./cmr/day 

 with little seasonal variation. 



The incoming radiation corrected for cloud 

 cover minus that which is reflected and lost 

 by back radiation may be called the "effective" 

 radiation. Additional energy enters or leaves 

 the sea surface as evaporation (Qg) and sensi- 

 ble heat (Qu), Jacobs (1951) discussed evapora- 

 tion and conduction of sensible heat at length 

 and presented seasonal charts of these values 

 for the north Pacific Ocean. 



Evaporation depends upon (1) the velocity 

 of the wind and of the vapor pressure differ- 

 ence between the sea surface and air above it, 

 and (2) a constant coefficient of proportionality. 

 There is little agreement as to the value of 

 the constant that should be used; the one 

 selected for our computations is given by 

 Tabata (1958): 



where e„ = vapor pressure at temperature of 

 sea surface (mb.); 

 e a = vapor pressure of air (mb.); 

 and W = wind speed (m./sec. ). 



Appendix Table B-4 lists values of satura- 

 tion vapor pressure over water used in compu- 

 tation of evaporation. 



In general, regions of greatest evaporation 

 are those wherein northerly transport of 

 surface water is greatest and which are 

 subjected during winter to frequent invasions 

 by cold, dry air masses from the interiors 

 of continents. In the Pacific, this region is 

 situated off Japan where cold dry air of 

 continental Asiatic origin frequently traverses 

 the northward-flowing, warm Kuroshio Cur- 

 rent. Another region of high evaporation is 

 in the trade wind zone as a result of relatively 

 strong winds and dry, descending air associated 

 with the semipermanent fields of high pressure 

 (Jacobs, 1951). A region of low evaporation, 

 in contrast, is in the eastern Pacific over the 

 southward-flowing, cool California Current. 



Bowen (1926) established the relation be- 

 tween evaporation and the heat exchange at a 

 water surface. The equation used here is 

 derived from the relation found by Bowen: 



'H 



3(T 



T ) W 

 a — 



* Values for k are given in Appendix table B-3. 



where T s = sea temperature (° C); 

 T a = air temperature (° C); 

 and W = wind speed (m./sec.). 



Values of sensible heat exchange are gen- 

 erally low in summer and relatively high in 

 winter but nowhere approach the magnitude of 

 heat flux through evaporation. 



Caution must be exercised in interpreting 

 the energy exchange values in regions having 

 limited observational coverage. Small errors 

 in observation and transmission can cause 

 large errors in some of the computations. In 

 quadrangles having few observations, consid- 

 erable bias can be introduced by the relative 

 positions of the reporting ships and their timing 

 with respect to the calendar month. All com- 

 putations presented below assume the data 

 centroid to be at the center of each respective 

 quadrangle and for the middle of the calendar 

 period involved. Energy exchange calculations 

 are not performed for 5-degree quadrangles 

 having fewer than five observations per month, 

 though summarized meteorological data are 

 listed. 



Representative charts of the seasonal varia- 

 tion of total energy exchange and its components 

 are presented for August 1963 and February 

 1964 in figures 26-37. 



Until further experiments on the nature of 

 the heat flux at the air-sea interface are com- 

 pleted, equation models are refined, and the 



31 



