The Deep Water 



The deep water of the Sulu Sea is very homogeneous in its character- 

 istics. As pointed out above, at the 800-metre level the temperatures 

 vary less than 0-1° c. and the salinities less than 0-05 %o from station to 

 station. The uniformity with depth is equally striking. Between 800 

 metres and 4,000 metres the temperature does not vary as much as 

 0-5° c. nor the salinity as much as 0-5 %o (Fig. 1). 



To date the " Baird " has occupied only one deep station in the Sulu 

 Sea. Unfortunately, the salinity samples for this station above 400 

 metres were lost. In the presentation of conditions at this deep station, 

 Station 37, salinity values for a nearby station, Station 36, have been 

 substituted (Figs. 1 and 2). 



The temperature of the deep water reached a minimum of 10-05° c. 

 at a depth of about 1,400 metres, from which depth it increased to 10-48° c. 

 at about 4,000 metres. This is an adiabatic increase in temperature and 

 does not represent an instability of the water column. As shown in 

 Fig. 1, the potential temperature decreases with increase in depth. This 

 agrees with the observations of the " Snellius " in the Sulu Sea, as well as 

 in the Mindanao Trench and other deeps of the western Pacific (Van Riel, 

 1934). It must be emphasized at this point that the " Baird " temperature 

 ■data are accurate only in a relative sense. Exact absolute temperatures 

 must await the calibration of the two reference thermometers. That 

 the absolute values are not greatly in error is shown by the fact that Van 

 Riel (1934) reports a minimum temperature in the Sulu Basin of 10-08° C. 

 while the minimum temperature obtained by the " Baird " was 10-05° c. 



According to Van Riel (1934), the deep water of the Sulu Basin is 

 renewed from the north by inflow of water from the South China Sea. 

 As pointed out above, the deepest sill separating the Sulu Sea from 

 adjacent waters lies to the north in Mindoro Strait. The sill depth in 

 that area is at about 450 metres. At this level in the South China Sea 

 the water has a temperature of about 10° c. and saUnity about 34-5 °/oo 

 (Sverdrup, Fleming, and Johnson, 1942). These are the values obtained 

 in the deep water of the Sulu Sea. Thus the temperature and salinity 

 of the deep water of the Sulu Sea seems to be determined b^^ the nature 

 of the water entering over the northern sill. 



The oxygen content of the deep water indicates that renewal of the 

 basin water is fairly strong. The " Dana " reported uniform oxygen 

 content below a depth of 400 metres. At " Baird " station 37 the oxygen 

 content decreased to 2,000 metres, and then remained constant at 0-11 mg 

 at/L to a depth of 4,000 metres. Although there was no increase toward 

 the bottom characteristic of the open Pacific and South China Sea, 

 yet there was no depletion of oxygen so characteristic of some isolated 

 laasins. A further study of the renewal of the deep water of the Sulu 

 Sea must await more deep stations in the area. 



The characteristics of the deep and intermediate water of the Sulu 

 Sea are shown by the temperature-salinity curves. Figure 2 shows the 

 temperature-salinity curves for "Dana" station 3685, "Snellius" 

 station 65, and "Baird" stations 36 and 37 (data from Station 36 are 

 used above 400 metres). The shapes of these curves are very simUar 

 except for values in the upper 100 metres. These differences can be 



255 



