Negrps and Mindanao), the Mindanao Sea is isolated from the Pacific 

 below a depth of 64 metres by the reef fringing the east side of the 

 islands. Communication with the Pacific through San Bernardino 

 Strait farther north is equally restricted. 



The next deepest connection with outside waters is Balabac Strait. 

 The sill depth here is less than 100 metres. .To the south there are two 

 deeper troughs which connect the Sulu Sea with the Celebes Sea. One 

 of these, east of the Island of Jolo, has a sill depth of 225 metres. A 

 deeper one is Sibutu Passage between Tawitawi and Borneo, where 

 the sill depth is about 275 metres. 



The deepest connection between the Sulu Sea and outside waters is 

 between the South China Sea to the north by way of Cuyo East Pass 

 west of Panay and Mindoro Strait west of Mindoro. The sill depth here 

 is about 450 metres. 



The depth of these sills is of the greatest importance to the under- 

 standing of the hydrography of the area. 



Wind Direction 



The winds in the Sulu Sea are definitely seasonal, although the two 

 monsoons are not so well developed here as in more northern parts of 

 the Phihppine waters. This is particularly true of the southern part 

 of the sea (Selga, 1931). 



At Cuyo, in the northernmost part of the Sulu Sea, the north-east 

 monsoon extends from October to April, reaching its height from 

 January to March. The south-west monsoon extends from June to 

 September, with greatest development in August. Neither the intensity 

 nor the duration of the south-west winds is as great as the north-east. 



At Jolo, in the Sulu Archipelago, the seasons are much less marked. 

 North-easterly winds predominate from December to April, while 

 southerly winds are more common from -May to November, but there is 

 a high percentage of cahns throughout the year. 



Thus the " Baird's " operations in the northern half of the Sulu 

 Sea in October were conducted at the beginning of the north-east 

 monsoon when it might be expected that the surface currents stiU main- 

 tained their pattern of the south-west monsoon period, while the survey 

 in the southern half of the area in December was made after the north- 

 easterly winds had probably had time to establish a pattern more 

 characteristic of the north-east monsoon. 



The second survey of the Sulu Sea, in June and July, was conducted 

 in the first half of the south-west monsoon, when one can reasonably 

 expect to find at least a partial development of the current pattern 

 characteristic of that season. 



Surface Currents 



The islands of the Phihppines lying to the -east of the Sulu Sea are 

 in the direct path of the North Equatorial Current. Since this is a strong 

 current which persists the year around it can be expected to have a 

 strong influence on the waters to the west throughout the year. As 

 this current strikes the Philippines it is deflected part to the north and 



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