OCEANOGRAPHIC CLIMATE OF HAWAIIAN ISLANDS REGION 



399 



basis of high salinit}' gradients shown in figures 10 

 and 9, respectively. 



The foregoing discussion can be further illus- 

 trated by computing intrinsic temperatures with 

 the help of the simplified heat budget, equation 6. 



Since only two processes, the heat exchange 

 across the sea surface and advection, are assumed 

 to contribute to the change of temperature, one 

 can compute what the change of temperature 

 should be if there were no heat exchange. The 

 heat budget equation 



At pCpZ 



then becomes 



At 



•\=-v.v.. 



the change of temperature due to advection only. 

 Considering the unit of time to be one month, 

 one can compute the intrinsic temperature, di, at 

 the end of a month for any location, if that month's 

 advection is known, from 



61 — 61=— v-ve or 

 6i = 6i-V-'^6 



where ^, is the temperature at the beginning of the 

 month. One can also obtain the intrinsic tempera- 

 ture bj' subtracting the net heat exchange during 

 a month from the temperature at the end of the 

 month: 



61=62 ' 



pCpZ 



A chart of the temperature distribution at the be- 

 ginning of the month and one for the intrinsic 

 temperature at the end of the month can now be 

 drawn and combined. This new chart then illus- 

 trates advection in terms of the displacement of 

 intrinsic isotherms or boundaries. 



For the Hawaiian region, four "advection 

 charts" were prepared (cliart VI, appendix B), 

 one each for the three advection periods, June- 

 July, October-November, and Decembei— January, 

 and one for the minimum or negative (warm) 

 advection period of March-April. 



In the first chart the solid and dashed isotherms 

 show the mean temperature distribution for the 

 beginning of March and April and the mean in- 

 trinsic temperature distribution for the end of 



March and April, respectively. The chart shows 

 that the displacement of the intrinsic isotherm is 

 small throughout most of the region and probably 

 not significant. The 24° and 25° C. isotherms are 

 displaced northward, however, bj- as much as 180 

 miles in the eastern portion of the region, indicat- 

 ing warm advection. 



The March-April advection chart therefore in- 

 dicates either no current flow, or currents parallel 

 to the isotherms, with some warm advection in 

 the eastern portion. This is compatible with the 

 gross winter geopotential topography (chart IV), 

 which indicates little flow in the northern half of 

 the region and flow parallel to the isotherms in the 

 southern half. 



The June-July advection diagram shows the 

 average isotherms for the beginning of these 

 months slope from northwest to southeast, whereas 

 the average intrinsic isotherms for the end of these 

 months, except for the southwest portion, show an 

 east-west direction. Thus, this advection chart 

 for the principal advection period shows very little 

 displacement of the 25° and 26° C. isotherms in 

 the eastern portion of the region, whereas in the 

 western portion displacement may be as high as 

 550 miles per month, indicating a southward com- 

 ponent of flow of 0.7 to 0.8 knot. Although there 

 are insufficient current data to compute the advec- 

 tion independently, a westerly setting current of 1 

 knot could accomplish this displacement. High 

 advection is also indicated in the northern portion 

 of the region, where the displacement increases 

 westward from 150 miles per month to more than 

 500 miles per month. 



This advection chart conveys a picture compati- 

 ble with salinity changes if one postulates water 

 entering the survey region from the east and push- 

 ing the intrinsic isotherms southward as if they 

 were movable boundaries. The water entering 

 the survej' region from the east also decreases the 

 salinity in the vicinity of the Hawaiian Islands 

 during the spring, reaching a minimum in July 

 (fig. 9), despite the fact that this is a period of 

 ma.vhnum evaporation minus precipitation (fig. 

 20). 



High heat advection in the vicinity of Midwaj- 

 Island during June and July is also compatible 

 with the sharp salinitv dechne during those months 

 (fig. 9). 



The characteristic advection diagrams of the 

 previous section (chart V) show that the advection 



