398 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



October-November and another, slightly lower, 

 during August-September. Minimum advection 

 of —0.1° C. mos."' occurs during April. The 

 primary distinction between area B and the other 

 areas of the region is the absence of both the 

 June-July and the December-January advection 

 peaks. The August-September advection period 

 occurs in only two of the nine advection diagrams 

 of this climatic area, leaving October-November 

 as the important advection period of area B. 



In area B,, the southwestern portion of the 

 region, the advection pattern appears more 

 complex. Maximum advection of 1.1° C. mos.-' 

 occurs in December-January, with a noticeable 

 peak of 0.9° C. mos."' in October-November 

 and another distinct peak of 0.8° C. mos."' in 

 June-July. Minimum advection of —0.2° C. 

 mos."' occurs during February. 



Area B2, north of areas B and B,, has two 

 pronounced advection peaks; one of 0.9° C. 

 mos."' in June-July and another of 1.1° C. mos."' 

 in December-January. Minimum advection of 

 — 0.3° C. mos."' occurs in March. 



Finally, in area B3, east of Hawaii, the advection 

 pattern again appears more complex. Maximum 

 advection of >0.5° C. mos."' occurs in October- 

 November and, in addition, there are peaks of 

 0.5° C. mos."' in July-August and December- 

 January. Minimum advection of —0.4° C. mos."' 

 occurs in March-April. In addition, there is an 

 advection peak of 0.2° C. mos."^ in May-June, 

 which is also apparent one month earlier in areas 

 Bi, B2, and A4. 



To summarize, the Hawaiian region was divided 

 into areas on the basis of time of occurrence of 

 positive advection peaks. In the northwest 

 portion, a primary peak occurs during June-July 

 and a secondary peak during December-January. 

 In the southeast portion, the dominant peak occurs 

 during October-November. The areas between 

 exhibit varying magnitudes of these advection 

 periods. Differences from the basic patterns in 

 areas A3, A4, and B3 may be due to varying effects 

 of the island barrier in a changing current field. 

 Negative advection, generally of small magnitude, 

 peaked during March and April. 



Since heat (temperature) advection is the scalar 

 product of the current velocity and horizontal 

 temperature gradient, it is difficult to visualize it 

 as a water displacement perpendicular to an iso- 



therm. In order to do so and to explore the 

 physical significance of advection further, the 

 intrinsic temperature and advection charts will be 

 developed in the next section. 



B. Intrinsic Temperature and the Heat Advection Chart 



A measure of advection as obtained in the pre- 

 vious section can provide, if the horizontal 

 temperature gradient is known, information only 

 about the component of the current in the direc- 

 tion perpendicular to an isotherm. For example, 

 if the advection is 1° C. mos."' and the horizontal 

 temperature gradient 1° C. per 60 miles, then this 

 advection is equivalent to water shifting 60 miles 

 perpendicular to the isotherm. If the gradient is 

 0.5° C. per 60 miles then the shift would be 120 

 miles. 



Now assume that the surface temperature is 

 conservative or an "intrinsic" property of a parcel 

 of water. Then, as before, there would be no 

 advection for currents parallel to an isotherm. 

 However, a component of flow perpendicular to an 

 isotherm, the temperatm-e now being an intrinsic 

 property of the water, would result in a shift of the 

 isotherm. In other words, advection can be 

 interpreted as a shift of "intrinsic" isotherms with 

 the displacement distance depending upon the 

 temperature gradient. An "intrinsic" isotherm, 

 therefore, exhibits properties of a movable stream- 

 line or boundary in that there is no flow across the 

 isotherm. Thus, no information can be gained 

 about the flow parallel to an isotherm, but a com- 

 ponent of flow perpendicular to the isotherm, 

 using "intrinsic" temperatures, must result in a 

 shift of the boundary or the "intrinsic" isotherm. 



Although any intrinsic isotherm or salinity 

 isopleth in the ocean is a movable boundary, the 

 resulting advection due to their displacement is 

 magnified in transition zones marked by higher 

 temperature and (or) salinity gradients. Thus, 

 the areas of high advection in the northwest and 

 southeast portions of the Hawaiian region (fig. 23) 

 can be interpreted as transition zones, with moving 

 boundaries, between climatic regions. These cli- 

 matic regions were recognized by the character- 

 istic advection diagrams of areas A and B in the 

 previous section. Ai-eas of high salt advection 

 due to boundary movement are also to be expected 

 during summer at 21° to 23° N., 155° to 160° W., 

 and during fall at 13° N., 155° to 160° W., on the 



