GLOSSARY - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 379

fresh water, where the water column
becomes thermally stratified in spring.
spring-neap cycle Variation in the range of
the tide over a ∼ 14.5 day period as the
Moon's position changes relative to the
Sun during the lunar orbital cycle.
spring tides Largest tides in the fortnightly
cycle usually occurring soon after new
Moon and full Moon.
standing wave A wave where the nodes
(points of zero amplitude) and anti-nodes
(points oscillating with the full wave
amplitude) are fixed in position. Standing
waves do not involve net energy transfer.
Also sometimes called a 'stationary wave'.
(See also progressive wave.)
Stokes drift

tidal constituent A periodic component of
the tide in the form of a sine wave at a
frequency which is set by the motions of
the Moon and the Sun relative to the
Earth. The real tide, surface elevation and
tidal currents, can be represented as a sum
of a number of such constituents.
tidal current ellipse The shape traced out by
the tip of the Eulerian tidal current vector over
the tidal cycle. For a single tidal constituent,
theshapeisatrueellipseandcanbescaled
to show the corresponding displacement
of a water particle in the tidal flow.
tidal mixing front Boundary zone with

distinct dynamical and biogeochemical
properties between seasonally stratified
and mixed regimes.
tidal species

Net motion in the direction of
wave travel due to non-linear effects in
surface waves of finite amplitude.
subsurface chlorophyll maximum (SCM)

In a spectrum of tidal energy,
the tidal constituents are concentrated into
three narrow bands of frequency referred
to as 'long period', 'diurnal' and
'semi-diurnal', tidal species.
tidal straining The periodic tilting of
isopycnals by shear in an oscillatory tidal
current which, in the presence of a
horizontal density gradient, induces a
periodic component of stratification.
tide generating force The body force
producing tidal movements in the ocean.
Lunar and solar tide generating forces arise
from imbalances between gravitational
attraction and centrifugal forces in the
orbital motions of the Earth-Moon and the
Sun-Earth systems.
top-down control The control of the biomass
of an organism, or group of organisms, at
one trophic level by the grazing/predation
pressure exerted by organisms at a higher
trophic level. The opposite is 'bottom-up'
control, where biomass is limited by one or
more of the resources required to generate
growth.
total or material derivative The time
derivative of a particle property following
the particle as opposed to the 'local
derivative' which is that applying at a fixed
point.

In

stratified shelf seas, where surface layer
nutrients have been removed by an earlier
spring bloom, phytoplankton are often
found to have their highest biomass in a
layer within the thermocline.
Taylor hypothesis An assumption that the
advection of a field of turbulence past a fixed
point can be taken to be entirely due to the
mean flow, thus allowing the determination
of the spatial spectrum of turbulence from
measurements by a fixed sensor.
thermal wind A term adopted from
meteorology for the flow driven by
horizontal gradients of density. The
thermal wind equation, which relates the
vertical shear in the current to the
horizontal density gradient, can be
integrated to determine the current profile
from density observations if the current is
known at one level.
Thorpe scale A displacement lengthscale

found by re-ordering the particles in the
density profile to remove instabilities and
then calculating the r.m.s. displacement.
The Thorpe scale is closely related to the
Ozmidov length, which limits the size of
overturning eddies.