BATHYTHEWOGRAPH OBS'^HVATIONS 
Bathytheriaoffraph (BT) casts were taken at 4-.hour intervals 
* 
throuprhout the cruise, except when/ in port or anchored off islands, 
BT instrument number KK09297 was lost during the night of November 
5-6j number KK09299 was used for the remainder of the cruise. The 
calibrated grid supplied with the instrument was used to determine 
temperature values at various depths* From these, a runnin/r plot 
was kept of the thermal structure of the sea along the cruise track, 
■S 
The BT casts regularly reached 200 meters depth, and frequently reached 
250 meters* 
The surface layer of the ocean normally is well-mixed and 
isothermal, or nearly so. This mixed layer usually has a sharply 
defined base below which is the therraocline where the temperature 
cools abruptly with depth, 
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V?hen leaving Johnston Island (near 17®N) November 11, the base 
of the mixed layer was fotmd at 75 meters depth. As the ship proceeded 
a 
sothwestward, the mixed layer became shallower. Its base rose 
gradually but steadily to a depth of only 30 meters between 10° and 9® 
north latitude. As the cold water below the mixed layer rose closer to 
the surface, the temperature at I 50 meters depth cooled steadily from 
22,5°C near Johnston to 13*5° at 10°N (see Figure 1), This region 
e 
where the base of the surface mixed layer tilts upward toward the south, 
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and the underlying water cools toward the south, is the region of the 
westwardly flowing North Bquatoriail Current (NEC), 
At about 10°N, the ship passed out of the irEC into the enstwardiy 
flowing Equatorial Countercurrent (ECC), The surface mixed layer 
