OCEANOGRAPHIC CLIMATE OF HAWAIIAN ISLANDS REGION 



385 



This equation is similar to equation V,5 in Svcr- 

 drup et al. (1942), except that the diffusion 

 terms have been replaced by the heat exchange as 

 a result of the simplih-ing assumptions. 



Equation 6 can be divided into two portions: 

 the term representing the temperature observa- 

 tions, and the terms representing the physical 

 processes which determine the temperatures. 



iid 



the local time change of temperature, 



Tl 



dt 



represents the observed effect of those processes 

 which nppear on the right side of the equation. 



The fii-st one of these is associated with the net 

 lieat exchange across the sea surface, H. This is 

 modified by the depth through which it is dis- 

 tributed, the density, and specific heat of the 

 water, to appear as it affects the temperature of 

 the water, in terms of temperature per unit mass 



I H 



per unit time, 



The second term on the right side of equation 6 

 which affects the water temperature is the process 

 of heat advection. It is an abstract term con- 

 taining both the horizontal temperature distribu- 

 tion and the velocity and should not be mistaken 

 for the heat transport. In expanded form the heat 

 advection can be written |F||Ve|cos 0, i.e., the 

 magnitude of the velocity (the speed) times the 

 magnitude of the temperature gradient times the 

 cosine of the angle between the direction ot the cur- 

 rent and the direction of the gradient. Thus, if 

 <t> is 90°, then the current flows parallel to the 

 isotherms and the heat advection is zero, although 

 the heat transport is not. Other cases where heat 

 advection is zero occur when there are no currents 

 or wlien there is no temperature gradient. The 

 heat advection, therefore, is simply a process 

 affecting the temperature of the water, here 

 expressed in temperature per unit mass per unit 

 time, resulting from a component of tlie current 

 perpendicular to the isotherm. Even though the 

 velocity enters into the expression, heat advection, 

 if determined from budget considerations, can 

 never give any information about the component 

 of velocity parallel to the isotherm. 



To summarize, the heat budget equation repre- 

 sents the manner in which the association between 

 observed temperatures and physical processes has 



been formalized. It provides a qualitative appre- 

 ciation of the processes involved and determines 

 what parametei-s must be measured and the man- 

 ner in which they should be measured. For 

 example, it is necessary to ol)tain time series of 

 observations together with horizontal distributions 

 in order to be able to interpret temperature obser- 

 vations in terms of processes. Furthermore, if an 

 understanding of the seasonal variation of proc- 

 esses is desired, then a plot of the local time 

 change rather than the absolute temperature 

 should be made. 



The preceding discussion applies equally well to 

 other parameters. For example, the salt budget 

 eqnation is 



(7) 



^=1 (E-P)-F.Vs 



where s is the salinity, and E—P the evaporation 

 minus precipitation. Here the processes are the 

 water exchange across the sea surface and salt 

 advection. 



In the following sections the heat budget con- 

 siderations will be applied to the Hawaiian region. 

 Fii-st, the processes which transpire at the sea sur- 

 face and affect the surface temperature will be 

 described. These wiU then be combined with the 

 data of part I to gain some understanding of the 

 advection processes. 



2. EVENTS AT THE SEA SURFACE 



Events at the sea surface which affect the 

 physical and dynamic properties of the surface 

 water include the net heat exchange across the 

 sea surface, evaporation minus precipitation, and 

 the wind stress. In this part of the atlas, atten- 

 tion will be focused on those processes which 

 affect the distribution of the sm'face temperature. 

 Therefore, the net heat exchange across the sea 

 surface will be of prunary concern. The evapo- 

 ration minus precipitation will be briefly discussed, 

 omitting wind information which is commonly 

 available in cHmatic compilations, such as the 

 U.S. Navy Marine CHmatic Atlas of the World, 

 vol. II (1956), and others. 



A. The Net Heat Exchange Across the Sea Surface 



It has long been recognized that the ocean is a 

 vast heat reservoir which has an important role 



