OCEANOGRAPHIC CLIMATE OF HAWAIIAN ISLANDS REGION 



389 



clieck in that they also bahmced tlic lioat budget 

 of the atmosphere. 



The last term to be considered in tlic iicl lieat 

 exchange across the sea surface is conduction of 

 sensible heat. It is a function of the vertical 

 temperature gradient and turbulence above the 

 sea surface. From Jacobs (1951), one again ob- 

 tains a smiple e.xpression when average climatic 

 data are considered: 



Q,= 1.45(e„-0j Wa cal. cm. -2 day-'. 



da and ^a are the temperatures in °F. of the sea 

 and air, respectively, and Wa is the wind speed 

 in knots. 



In the Hawaiian region, the exchange of sensible 

 heat across the sea surface is not important and 

 ranges from 5 to 40 cal. cm.~^ day"', with the 

 high values occurring during winter months in the 

 northern portion of the region. In the vicinity 

 of the main island group, it ranges from 4 cal. 

 cm.-^ day-' in August to 25 cal. cm.-^ day-' in 

 Februarv. 



To summarize, figure 15 shows the relative 

 magnitude of the terms discussed in this section, 

 as they affect the net heat exchange at 20° N., 

 100° W. It is apparent that the lieat absorbed 

 by the water, Q,, determines the seasonal pattern 

 of the net heat exchange, H, although this may 

 be modified, as it is during the months centered 

 about April, because of high evaporation rates. 



The total rate of heat loss from the sea surface 

 is at a maximum of 490 cal. cm.-^ day-' during 

 April and at a minimum of 370 cal. cm.-^ day-' 

 duiing July and August. During tlie former 

 period, the relative contribution to the total daily 

 heat loss from back radiation, evaporation and 

 conduction is 27 percent, 69 percent, and 4 percent, 

 respectively. During the latter period, these rela- 

 tive losses are 37 percent, 62 percent, and 1 per- 

 cent, respectively. This illustrates again the 

 unportance of evaporation in the net heat exchange 

 across the sea surface. 



The relative importance of the terms contribu- 

 ting to the net heat exchange across tlie sea surface 

 is also illustrated in figure 16 by meridional 

 profiles at 160° W. for June and December. Dur- 

 ing June (fig. 16A), the rapidly rising net heat 

 exchange north of 19° N. is seen to be due to 

 northward declining evaporation from 260 cal. 

 cm. -2 day-' at 19° N. to 130 cal. cm."' day-' 30° N. 



600 



-100 



1 I — r 



-1 — I — I — I — I — r 



H.QrQb-Qe-Qc 



_L 



J \ \ L 



J I I L 



JAN MAR MAY JUL SEP 



NOV 



JAN MAR 



Figure 15. — Relative magnitude of the components enter- 

 ing into the net heat exchange across the sea surface at 

 20° N., 160° W. (Q,— heat absorbed by the water, 

 Qb — back radiation, Qe — heat used for evaporation, 

 Qc — conduction of sensible heat, H — net heat e.xchange 

 across the sea surface.) 



In December (fig. 16B), the total daily heat loss 

 across the sea surface varies only by about 30 

 cal. cm.-' day-' between 10° and 30° N. 



It is now of interest to know how the net heat 

 exchange affects the surface temperature of the 

 water. According to the basic assumptions of 

 the previous section, the absorbed heat remaining 

 in the water after back radiation, evaporation, 

 and conduction, would be uniformly distributed 

 throughout the mixed layer. Therefore, accord- 

 ing to equation 6, one obtains the change of 

 surface temperature per month by dividing the 

 net heat exchange, H, by the depth of mixed 



