the basic concepts of physical oceanography

7/29/2019 The Basic Concepts of Physical Oceanography

1/16

The BasicConcepts of

PhysicalOceanograph

y

1. What drives the ocean currents?

1.1. Rotation of the Earth1.2. Wind stress

2. The heat flux through the ocean surface

3. Vertical distribution of water properties

4. Water circulation

4.1. Horizontal circulation

Geostrophic flow

Rossby wavesEkman drift

4.2. Vertical circulation

Upwelling

1. What drives the ocean currents?
https://www.eeb.ucla.edu/test/faculty/nezlin/PhysicalOceanography.htm#Section1https://www.eeb.ucla.edu/test/faculty/nezlin/PhysicalOceanography.htm#Section1https://www.eeb.ucla.edu/test/faculty/nezlin/PhysicalOceanography.htm#Section2https://www.eeb.ucla.edu/test/faculty/nezlin/PhysicalOceanography.htm#Section2https://www.eeb.ucla.edu/test/faculty/nezlin/PhysicalOceanography.htm#Section3https://www.eeb.ucla.edu/test/faculty/nezlin/PhysicalOceanography.htm#Section4https://www.eeb.ucla.edu/test/faculty/nezlin/PhysicalOceanography.htm#Section4https://www.eeb.ucla.edu/test/faculty/nezlin/PhysicalOceanography.htm#Section4https://www.eeb.ucla.edu/test/faculty/nezlin/PhysicalOceanography.htm#Section3https://www.eeb.ucla.edu/test/faculty/nezlin/PhysicalOceanography.htm#Section2https://www.eeb.ucla.edu/test/faculty/nezlin/PhysicalOceanography.htm#Section1


2/16

If we exclude tidal forces, the oceanic circulation is driven by four external influences:

- rotation of the Earth;

- wind stress;

- heating and cooling;

- evaporation and precipitation.

The last three are ultimately driven by radiation from the sun.

The solar heating is unevenand at different latitudes: more sunlight falls in equatorial regions

than strikes the poles (This and many other illustration in this lecture were taken from the

book written by Tom Garrison, "Oceanography: An Invitation to Marine Science",

Wadsworth Publishing Company, Belmont, 1993, 540 pp. Figure 8.2).

Warm air rises and cool air sinks; a convection current forms in a room resulting from uneven

heating and cooling (Garrison, 1993, Figure 8.3).


3/16

(Garrison, 1993, Figure 8.4).

The amount of heat radiation is of maximum at the

equator. The cold air at the poles is denser than the

warm air at the equator; hence, air pressure at sea level

is higher at the poles than at the equator. In other

words, the pressure gradient at sea level is directed

from the poles toward the equator, and the pressuregradient in the upper part of the atmosphere has the

opposite sign.

In fluid and gases pressure gradients produce flow

from regions of high pressure to regions of low

pressure. If the earth were not rotating, the response to

these pressure gradients would be direct and simple.

(Garrison, 1993; Figure 8.5).

At the higher latitude each location

travels a shorter path on the rotating Earth

than at the equator.

A cannonball shot north from the cannon

located at the equator is also moving east

at the speed of the Earth rotation at the

equator and veers to the right from its

northward path.

A cannonball shot south travels overportions of the Earth that are moving

increasingly faster in an eastward

direction and also veers to the right.

This effect is called Coriolis effect.


4/16



The rotation of the Earth modifies the pattern of atmospheric circulation in two ways. Firstly,

as air moves toward the equator, the rotation of the earth shifts ocean and land eastward

under it. The result is "easterly" winds (Polar Easterlies and Trade winds).

Secondly, the zonal flow of high speed becomes unstable, creating eddies which reshape air

pressure distribution resulting in air pressure maximum in mid-latitudes. It creates a band of

"westerly" wind in the Roaring Forties.

At this animated image you can see the seasonal variations of wind over the World Ocean

averaged during several decades of observations.


5/16


In coastal zones the atmospheric

circulation pattern is modulated by the

difference between the heat balance over

land and sea zones.

During summer land accepts more heat

and onshore wind dominates.

During winter land is cooler than sea and

offshore wind dominates.

1.2. Wind stress

Wind stress t (kg m-1 s-2 or Newton per m2) is an important quantity in the process of

wind driving ocean currents.

where


6/16

Cd is the dimensionless "drag coefficient" (about 0.0013),

is air density (about 1.2 kg m-2),

U is wind speed at 10 m above sea level (m s-1).

Wind stress is a square function of wind speed

because the wind forcing depends on wind speed

and sea roughness, which in turn depends on

wind speed.

2. The heat flux through the ocean surface

The heat flux is determined by the balance between four components:

- incoming solar radiation;

- outgoing back radiation;

- heat loss from evaporation;

- mechanical heat transfer between the ocean and the atmosphere.

200 W m

-2

warm a layer of water 50 m deep by about 2.5C per month if unopposed by heatlosses from other effects.


7/16

Annual mean solar radiation (W m-2) received at sea level (Illustration from the book written

by Matthias Tomczak and J. Stuart Godfrey "Regional Oceanography: An Introduction",

Pergamon Press, Oxford, 1994, 414 pp.; Figure 1.5).

Annual mean heat flux into the ocean depends on solar radiation and sea surface temperatur

(Tomczak and Godfrey, 1994; Figure 1.6).


8/16

Sea surface temperature in degrees Celsius during Northern Hemisphere winter (Garrison,

1993; Figure 7.20).

3. Vertical distribution of water properties



9/16

Pressure p (kiloPascal, 10 kPa = 1 dbar = 1 m);Temperature T (degrees C);Salinity S (Practical Salinity Units - psu) correspond to promille (g salt/kg sea water);

Density (kg m-3) represented by


The equation of state

indicates that water

density is a function of

temperature, salinity, and

pressure.

The pressure fieldThe pressure in the water column increases with depth and depends on the vertical

distribution of water density. We can calculate differences between pressures at different

depths or depth differences between two surfaces of constant pressure. For the latter purpose

a quantity called steric height is introduced.

Its meaning is the height by which the water column between depths z1 and z2 with standard

temperature T = 0C and salinity S = 35.0 expands if its temperature and salinity arechanged to the observed values.

Typically h is a few tens of centimeters. Oceanographers map the shape of the sea surface byshowing contours of equal steric height relative to a depth of no motion, where pressure is

assumed to be constant (usually 1500 or 2000 m).


10/16

Dynamic height D (m2 s-2) is equal to g * h, i. e., the product ofgravity and steric height.

From steric height ordynamic height we can estimate the horizontal pressure gradient

resulting in geostrophic water circulation.

4. Water circulation

4.1. Horizontal circulation

Mass transport and volume transport

Mass transport is the transport of water through an area of unit width (1 m 2), units (kg m-2 s-

1).

Volume transport is a mass transport integrated over the width and depth of a current, divided

by density. Units are m3 s-1 or Sverdrup (Sv), 1 Sv = 106 m3 s-1.

Geostrophic flow

Distribution of isobars and isopycnals at any depth levelabove z = z0(Tomczak and Godfrey, 1994; Figure 2.7).

Water at station A is

denser than water at

station B. As the weight of

the water above z = z0 isthe same, the water

column must be longer at

B than at A.

In geostrophic flow, water

moves along isobars, with

the higher pressure on its

right in the Northern

Hemisphere (away from

the equator).

The magnitude (mass transport per unit depth) of geostrophic flow:

where is an average water density,

g is acceleration of gravity (g = 9.8 m s-2),Td is the length of a day (86,400 s),


11/16

is the latitude,

is the difference in steric height between two adjacent isobars.

is known as Coriolis parameter.

Mean dynamic height (m2 s-2), or steric height multiplied by gravity, for the World Ocean at 0

m relative to 2000 m. Arrows indicate the direction of the implied geostrophic movement of

water (Tomczak and Godfrey, 1994; Figure 2.8).

Illustration of the

relationship between a

map of steric height

(dynamic topography),

geostrophic flow, and

the evaluation of the

geostrophic mass

transport per unit depth

M' between two

streamlines (contours ofconstant steric height) in

the Southern

Hemisphere (Tomczak

and Godfrey, 1994;

Figure 3.2).

For both station pairs, A and B and A' and B', in Equation is given by h2 - h1.

The geostrophic velocity is inversely proportional to the distance between streamlines, or

equal to M' divided by density and by the distance between points A and B, because the

section AB is perpendicular to the streamlines.If station pairA' and B' is used for the calculation, similar Equation still produces the correct


12/16

geostrophic mass transport M' between streamlines h1 and h2, but the velocity derived from

M' and distance A'B' is only the velocity component Vn perpendicular to the section A'B'.

1-layer model

Side view of a 1-layer ocean (Tomczak and Godfrey,

1994; Figure 3.3).

1-layer model is is an

approximation to the ocean's density

structure. The ocean is divided into a

deep layer of constant density r2

and much shallower layer above it,

again of constant density

The lower layer

is considered motionless. Thethickness of the upper layerz =

H(x, y, t) is allowed to vary.

The factor is of the order0.01 or less. Hence, in a 11/2 layer

ocean the sea surface is a scaled

mirror of the depth of the pycnocline

(100-300 times larger).

Rossby wave in the Southern Hemisphere (Tomczak and

Total poleward flow in greater

in magnitude between A and

B than between C and D

because the Coriolos force fissmaller in magnitude at A andB than at C and D; the

thermocline deepens in

ABCD. By the same

argument, the thermocline

shallows in A'B'C'D'; the

eddy moves west.


13/16

Godfrey, 1994; Figure 3.4).

The direction of circulation in the eddy structures is important.

In the Northern Hemisphere, the eddies with cold core are rotating counter-clockwise.

In the Southern Hemisphere, these eddies are rotating clockwise.In both cases, this type of rotation is called cyclonic.

The eddies with warm core are rotating clockwise in the Northern Hemisphere and counter-

clockwise in the Southern Hemisphere.

These eddies are called anticyclonic.

Ekman drift

The Ekman spiral and the mechanism by which it operates. (a) The Ekman spiral model. (b)

A body of water can be thought as a set of layers. The top layer is driven forward by the

wind, and each layer below is moved by friction. Each succeeding layer moves at a slower

speed, and at an angle to the layer immediately above it (to the right in the Northern

Hemisphere, to the left in the Southern Hemisphere) until friction becomes negligible. (c)

Though the direction of movement is different for each layer in the stack, the theoretical

average direction of flow of water in the Northern Hemisphere is 90 to the right of theprevailing surface wind (Garrison, 1993; Figure 9.5).


14/16

The movement of water away frompoint B is influenced by the Coriolis

effect and gravity (Garrison, 1993;

Figure 9.6).

The current moves at an angle to the wind (to right in the Northern Hemisphere), turning

further away from the wind direction and becoming weaker with depth. Therefore, the wind-driven component of water transport is directed perpendicular to the mean wind stress to the

right in the Northern Hemisphere. The magnitude (kg m-1 s-1) is

where is wind stress and fis Coriolis force.

The combination of geostrophic flow and wind forcing results in the general pattern of ocean

currents (Garrison, 1993; Figure 9.8).


15/16

The general circulation in all oceans is

anticyclonic, i.e., clockwise in the NorthernHemisphere and counterclockwise in the

Southern Hemisphere (Garrison, 1993; Figure

9.2).

4.2. Vertical circulation

Upwelling

In the eastern parts of theoceans permanent

equatorward winds generate

offshore Ekman drift and

coastal upwelling of

rich in nutrients waters

resulting in high primary

production.

A prolonged poleward wind

along a west coast can result

in downwelling (Garrison,1993; Figure 9.14).

Some useful links:

Oceanography Science & Technology
http://www.onr.navy.mil/focus/ocean/regions/default.htmhttp://www.onr.navy.mil/focus/ocean/regions/default.htm


16/16

Ocean Surface Currents

Wind-Driven Circulation in the Open Ocean
http://oceancurrents.rsmas.miami.edu/http://www.rsmas.miami.edu/personal/edk/Class/Protected/Wind/chapter.phtmlhttp://www.rsmas.miami.edu/personal/edk/Class/Protected/Wind/chapter.phtmlhttp://oceancurrents.rsmas.miami.edu/

the basic concepts of physical oceanography

Documents