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currents or by a rapid current setting over an irregular carbonate has been converted to oxide, the bromine and
bottom. Termed tide rip when a tidal current is involved. iodine replaced by chlorine, and all organic matter
See overfalls. completely oxidized. The following is approximate.
rip current A narrow intense current setting sea- S(0 ) = 1.806,55 x Cl (0 )
ward through the surf zone. It removes the excess water Where Cl(0 ) is chlorinity in parts per thousand. See
brought to the zone by the small net mass transport of chlorinity.
waves. It is fed by longshore currents. Rip currents usually Sargasso Sea The west central region of the sub-
occur at points, groins, jetties, etc., of irregular beaches, tropical gyre of the North Atlantic Ocean. It is bounded by
and at regular intervals along straight, uninterrupted the North Atlantic, Canary, North Equatorial, and Antilles
beaches. Currents, and the Gulf Stream. It is characterized by the
river current The gravity-induced seaward flow of absence of any well-marked currents and by large
fresh water originating from the drainage basin of a river. quantities of drifting Sargassum, or gulfweed.
In the fresh water portion of the river below head of tide, Saros A period of 223 synodic months correspond-
the river current is alternately increased and decreased by ing approximately to 19 eclipse years or 18.03 Julian
the effect of the tidal current. After entering a tidal years, and is a cycle in which solar and lunar eclipses
estuary, river current is the depth-averaged mean flow repeat themselves under approximately the same con-
through any cross-section. See head of tide and estuary. ditions.
river estuary See estuary. sea level datum (SLD) An obsolete term. See Na-
rotary current A tidal current that flows contin- tional Geodetic Vertical Datum of 1929 and mean sea
ually with the direction of flow changing through all points level.
of the compass during the tidal period. Rotary currents are second reduction Same as phase reduction.
usually found offshore where the direction of flow is not secondary control tide station A tide station at
restricted by any barriers. The tendency for the rotation in which continuous observations have been made over a
direction has its origin in the Coriolis force and, unless minimum period of 1 year but less than 19 years. The
modified by local conditions, the change is clockwise in series is reduced by comparison with simultaneous ob-
the Northern Hemisphere and counterclockwise in the servations from a primary control tide station. This station
Southern. The speed of the current usually varies provides for a 365-day harmonic analysis including the
throughout the tidal cycle, passing through the two maxima seasonal fluctuation of sea level. See tide station, primary
in approximately opposite directions and the two minima control tide station, tertiary tide station, and subordinate
with the direction of the current at approximately 90° from tide station (1).
the directions of the maxima. secular trend See apparent secular trend as pre-
22
ferred term. sidereal day The time of the rotation of the Earth
seiche A stationary wave usually caused by strong with respect to the vernal equinox. It equals approximately
winds and/or changes in barometric pressure. It is found in 0.997,27 of a mean solar day. Because of the precession
lakes, semi-enclosed bodies of water, and in areas of the of the equinoxes, the sidereal day thus defined is slightly
open ocean. The period of a seiche in an enclosed less than the period of rotation with respect to the fixed
rectangular body of water is usually represented by the stars, but the difference is less than a hundredth part of a
formula: second.
Period (T) = 2L / gd sidereal month Average period of the revolution of
in which L is the length, d the average depth of the body of the Moon around the Earth with respect to a fixed star,
water, and g the acceleration of gravity. See standing wave. equal to 27.321,661 mean solar days.
seismic sea wave Same as tsunami. sidereal time This is usually defined by astrono-
semidiurnal Having a period or cycle of approxi- mers as the hour angle of the vernal equinox. The sidereal
mately one-half of a tidal day. The predominant type of tide day is the interval between two successive upper transits of
throughout the world is semidiurnal, with two high waters the vernal equinox. It is to be noted that when applied to the
and two low waters each tidal day. The tidal current is said month and year the word sidereal has reference to motion
to be semidiurnal when there are two flood and two ebb with respect to the fixed stars, while the word tropical is
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