20
drifters have been described (Schmidt et al. 2003) but their use is more experimental
than operational. Lagrangian methods are further described in the discussion of
sensor platforms (Chap. 3).
2.2.4 Ocean Tides
Astronomical ocean tides powered by gravitational forces between earth, moon, and
sun and allowed by the fluid nature of the ocean cause the periodic rise and fall of
seawater level observed throughout the world ocean except at amphidromic points
around which the tides are perceived to rotate. Tides are in effect, the longest waves
in the ocean (Sverdrup et al. 1942). Tidal periodicity varies according to the nature
of the astronomical forcing such that a number of partial tides are recognized ranging from the quasi-semidiurnal lunar and solar components through the fortnightly
30’
30’
45’
45’
18°N
68°W
15’
15’
cm/s
0
5
10
15
20
25
30
35
40
45
50
2015/09/08 00:00 GMT
30’
45’
Fig. 2.7 Surface currents in the Mona Passage to the west of Puerto Rico at 6  km resolution
obtained from the two HFR emplacements marked by black circles
2 Electronic Sensors and Instruments for Coastal Ocean Observing