JP 3-50 National Search and Rescue Manual Vol I - US Navy

B. Sea Current (SC) is the residual current when currents caused
by tides and local winds are subtracted from local current. It is the
main large-scale flow of ocean waters. Near shore or in shallow waters,
sea current is usually less important than the tidal current or the
local wind-driven current. The strongest sea currents exist near the
edge of the continental shelf and are usually referred to as boundary
currents. Sea currents are driven by the energy of large-scale wind
systems and the interaction of ocean water masses of different
densities.
1. The preferred source of sea current information is the Naval
Oceanographic Office Special Publication series 1400. Other
sources include Section 1 of the Oceanographic Atlas, the
Atlas of Surface Currents, Pilot Charts, and the Defense
Mapping Agency. This data is useful for estimating drift of
many objects for a long period, but less useful for the drift
of one object for a short period.
2. Sea currents are not always steady. Their variability in both
speed and direction can be great. Averages should be used
with caution.
3. Charts and atlases from which sea current data is obtained are
derived from averaging past shipboard observations of drift.
In regions dominated by trade winds (persistent winds from the
same quadrant 75 percent of the time), observations are biased
by a persistent wind current as part of the observed sea
current. Unless sea current for these regions is obtained
from a better source, such as special current charts based on
the average hydrography of the area, the resultant drift
vector, illustrated in Figure 5-3A, may not be accurate. For
regions of persistent winds:
a. If winds blow persistently within normal range of
direction and speed, find the drift vector by omitting wind
current effect from the vector diagram, and using a sea
current vector and leeway vector as shown in Figure 5-3B.
b. If winds blow outside normal range of direction or
speed for a period greater than 24 hours during the preceding
48 hours, use a correction vector to subtract the effect of
persistent wind from sea current. The correction vector is
found by computing the wind-driven current that the persistent
wind would cause and reversing its direction. Considering the
normal range of persistent wind, the best estimate of median
direction and speed should be used. See Figure 5-3C.
C. Wind Current (WC), or wind-driven current, is generated by the
wind acting on the water surface over a period of time. As a wind blows
over water, it causes horizontal water movement that grows with wind
speed and duration.
1. Two methods can be used to calculate the wind current vector.