Part A | 2.3
18 Part A Fundamentals
2.3 Oceanic Processes
Like the atmospheric winds, ocean currents at a single place/time is a superposition of contributions from
a complex set of processes many with different spacetime scales. The following profiles many of the most
important ocean processes in terms of their basic physical elements including forcing and restoring forces and
relevance in the overall ocean system. The corresponding ranges of time and space scales of these processes
are presented in Table 2.1 and in the dispersion diagram
(Fig. 2.5) – a diagram that is particularly relevant to
waves:
Capillary waves. Physics: surface tension, inertia,
wind-driving – First effect of winds in wave generation
Surface gravity waves (short). Physics: gravity,
inertia, wind-driving – Mid-ocean wind waves/
surface chop, shipping, off-shore construction.
Surface gravity waves (long). Physics: gravity, inertia, bottom topography, wind-driving or earthquake – Surf and breakers, storm surges, tsunamis,
dangerous flooding.
Internal gravity waves. Physics: gravity or buoyancy, inertia, topography, indirect wind- driving –
Energy dissipation, mooring stresses, dead water.
Turbulence, mixing. Physics: inertia, buoyancy,
nonlinear interaction, friction – Dissipation of energy, mixing of momentum and dissolved chemicals
including O 2 .
Upwelling (and downwelling). Physics: buoyancy,
local winds and currents – Brings up nutrients/
formation of water masses.
Mid-Ocean mean flows. Physics: buoyancy, Coriolis force, wind and heating driving, friction – Maintaining climate, distributing chemicals, and heat for
biology.
Continental shelf currents. Physics: buoyancy,
Coriolis force, nonlinear inertia – Transfer of heat
Table 2.1 Characteristic time and length scales associated with different ocean processes and currents
Type
Time Scales T
Length Scales L
Amplitude H
Capillary waves
< 0:1 s
< 1 cm
< 1 cm
Short surface gravity waves
0:160 s
cmkm
m
Long surface gravity waves
s=h
m10’s km
m
Internal gravity waves
minh
m10’s km
m
Turbulence (Mixing)
s
cmm
mmcm=s
Upwelling/downwelling
Days–seasonal
100 km < 1
mm=day
Mid-ocean mean flow
1000km
1000 km
cm=s
Western boundary currents
Seasonal–decadal
100 km
200 cm=s
Continental shelf currents
Daysmonths
100 km
cm=s
Tides
Day
1000 km
m
Internal tides
Day
10100 km
10 m
Estuarine currents
Dayseasonal
100 km
cm=sec
and vorticity, climate and local weather, biology,
chemistry.
Tides. Physics: gravity, inertia, sun and moon gravitational pull, Coriolis force – Piloting and shipping, coastal construction.
Internal tides. Physics: gravity or buoyancy, inertia,
topography, indirect wind-driving – Internal waves
at tidal periods with large vertical excursions.
Estuary flows. Physics: gravity-buoyancy, Coriolis,
tidal, and wind forcing – Pollutant dispersal, biological nutrient renewal, inflow of fresh water.
The energy distribution of these different oceanic
processes tends to sort by time scale and can be conveniently presented in terms of frequency spectra in different forms. The energy density (i. e., energy per unit
–2
0
2
4
6
8
10
14
12
Log 10 (wavelength) L (cm)
1 year
Gulf stream
variability
Seasonal
variability
Long gravity
Internal
gravity
Tides
Western
boundary
currents
Short gravity
Capillary
Inertial
1 month
f 1 day
1 h
1 min
1 s
Log 10 (period) T (s)
12
10
8
6
4
2
0
–2
–4
Thermal
circulation
wind driven
flow
Fig. 2.5 Dispersion diagram for the principal oceanic processes
18 Part A Fundamentals
2.3 Oceanic Processes
Like the atmospheric winds, ocean currents at a single place/time is a superposition of contributions from
a complex set of processes many with different spacetime scales. The following profiles many of the most
important ocean processes in terms of their basic physical elements including forcing and restoring forces and
relevance in the overall ocean system. The corresponding ranges of time and space scales of these processes
are presented in Table 2.1 and in the dispersion diagram
(Fig. 2.5) – a diagram that is particularly relevant to
waves:
Capillary waves. Physics: surface tension, inertia,
wind-driving – First effect of winds in wave generation
Surface gravity waves (short). Physics: gravity,
inertia, wind-driving – Mid-ocean wind waves/
surface chop, shipping, off-shore construction.
Surface gravity waves (long). Physics: gravity, inertia, bottom topography, wind-driving or earthquake – Surf and breakers, storm surges, tsunamis,
dangerous flooding.
Internal gravity waves. Physics: gravity or buoyancy, inertia, topography, indirect wind- driving –
Energy dissipation, mooring stresses, dead water.
Turbulence, mixing. Physics: inertia, buoyancy,
nonlinear interaction, friction – Dissipation of energy, mixing of momentum and dissolved chemicals
including O 2 .
Upwelling (and downwelling). Physics: buoyancy,
local winds and currents – Brings up nutrients/
formation of water masses.
Mid-Ocean mean flows. Physics: buoyancy, Coriolis force, wind and heating driving, friction – Maintaining climate, distributing chemicals, and heat for
biology.
Continental shelf currents. Physics: buoyancy,
Coriolis force, nonlinear inertia – Transfer of heat
Table 2.1 Characteristic time and length scales associated with different ocean processes and currents
Type
Time Scales T
Length Scales L
Amplitude H
Capillary waves
< 0:1 s
< 1 cm
< 1 cm
Short surface gravity waves
0:160 s
cmkm
m
Long surface gravity waves
s=h
m10’s km
m
Internal gravity waves
minh
m10’s km
m
Turbulence (Mixing)
s
cmm
mmcm=s
Upwelling/downwelling
Days–seasonal
100 km < 1
mm=day
Mid-ocean mean flow
1000km
1000 km
cm=s
Western boundary currents
Seasonal–decadal
100 km
200 cm=s
Continental shelf currents
Daysmonths
100 km
cm=s
Tides
Day
1000 km
m
Internal tides
Day
10100 km
10 m
Estuarine currents
Dayseasonal
100 km
cm=sec
and vorticity, climate and local weather, biology,
chemistry.
Tides. Physics: gravity, inertia, sun and moon gravitational pull, Coriolis force – Piloting and shipping, coastal construction.
Internal tides. Physics: gravity or buoyancy, inertia,
topography, indirect wind-driving – Internal waves
at tidal periods with large vertical excursions.
Estuary flows. Physics: gravity-buoyancy, Coriolis,
tidal, and wind forcing – Pollutant dispersal, biological nutrient renewal, inflow of fresh water.
The energy distribution of these different oceanic
processes tends to sort by time scale and can be conveniently presented in terms of frequency spectra in different forms. The energy density (i. e., energy per unit
–2
0
2
4
6
8
10
14
12
Log 10 (wavelength) L (cm)
1 year
Gulf stream
variability
Seasonal
variability
Long gravity
Internal
gravity
Tides
Western
boundary
currents
Short gravity
Capillary
Inertial
1 month
f 1 day
1 h
1 min
1 s
Log 10 (period) T (s)
12
10
8
6
4
2
0
–2
–4
Thermal
circulation
wind driven
flow
Fig. 2.5 Dispersion diagram for the principal oceanic processes