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Surface temperatures typically reveal parallel isothermal structures
between November and April, with a latitudinal temperature gradient of
about 0.6-0.8 o C per degree of latitude 47 . These parallel isotherms break
down in the summer. Minimum and maximum temperatures generally
occur in March and September, respectively. The annual range in surface
temperature at latitude 12 o N was only 1.7 o C, but at 26 o N it increased to
5.3 o C. Below the mixed layer, the average temperature gradient is about
15 o , 6 o and 5 o C per 100 m at latitudes 10 o , 20 o and 30 o N, respectively.
Seasonal and diurnal thermoclines may be superimposed on the
permanent thermocline.
5.5.2.2. Ocean circulation
As speculated for many years and confirmed in the DOMES
programme 48 and later in the TOPEX/Poseidon remote-sensing programme,
the North Equatorial Current dominates surface-water movement in the
study area. The North Equatorial Current is a broad flow east to west
extending between 9 o and 20 o N, and has an average speed of about 10
cm/sec. The surface-water mass in this area, called the North Pacific
Subtropical Water, is between 50 and 200 m thick. Beneath the North
Pacific Subtropical Water lies a 1000-m layer of colder water called the
North Pacific Intermediate Water. This layer is characterised by a salinity
minimum. The water originates at high latitudes in the Pacific Ocean and
sinks beneath the North Pacific Subtropical Water in a broad area north of
approximately 45 o N.
The nearly homogeneous North Pacific Deep Water is found in a
zone approximately 3600 m thick beneath the North Pacific Intermediate
Water. North Pacific Deep Water originates in the North Pacific Basin and
flows very slowly southward. Antarctic Bottom Water is found close to the
seafloor in a zone several hundred metres thick. AABW originates in the
Antarctic Ocean and moves slowly northward.
The wind-generated open ocean wave climate is typified by four
general wave types: Northeast Trade Wind waves, South Pacific and North
Pacific swells, and hurricane-generated waves. Simultaneous arrival of
waves from more than one source is common. Trade-wind waves may occur
throughout the year but they dominate from April to November, when they
are present 90-95% of the time as compared to 55-60% of the time from
December through March. Waves generated by these winds typically have
periods of 5-8 sec with heights of 1-4 m. They usually approach from the
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