Robert W. ChristophersonCharlie Thomsen

Chapter 6Atmospheric and

Oceanic Circulations

Wind EssentialsAir Pressure and Its Measurement  

Mercury barometer

Aneroid barometer

Wind: Description and Measurement  Wind

Anemometer

Wind vane

Global Winds  

Barometers

Figure 6.2

Air Pressure Readings

Figure 6.3

Wind Vane and Anemometermeasures wind direction and speed

Figure 6.4

N

S

W E

NE

SW

NEN

ENE

SE

NW

ESE

SESSWS

WSW

WNW

NWN

Driving Forces within the Atmosphere  

Pressure Gradient Force  

Coriolis Force  

Friction Force  

Pressure Gradient : changes in air pressure over a horizontal distancePressure gradient force (PGF): points from higher to lower pressure, perpendicular to isobars.Isobars: lines of equal air pressure.

Figure 6.7

Coriolis Force: an apparent force caused by the rotation of the earth;1) deflects to the right (of the movement) in northern hemisphere.2) Maximum in the poles and zero in the equater3) Proportional to the wind speed.

Figure 6.9

Geostrophic wind (Vg)Results from the balance between the PGF and Coriolis force

Flows parallel to straight isobars at a constant speed

Speed in determined by the PGF

1016mb

1012mb

PGF

COF

Vg

Frictional forceOpposite to the wind direction

Slows down the wind speed

Reduces Coriolis force

Creates surface wind (Vs):

Flows from high to low pressure across isobars at an angle 1016mb

1012mb

PGF

Vs

COF

Cyclone (low pressure) and Anticyclone (high pressure)

In northern hemisphere:Cyclone: wind flows counter-clockwise towards the center across isobars at an angleAnticyclone: wind flows clockwise away from the center across isobars at an angle

Figure 6.8

Cyclone: air converges on the surface and moves upwards over the center; clouds forms and is likely to be associated with precipitation

Anticyclone: air diverges away and subsides over the center; clear sky and sunny

Atmospheric Patterns of Motion  Primary High-Pressure and Low-Pressure Areas  

Upper Atmospheric Circulation  

Local Winds  

Monsoonal Winds  

equator

N. Pole

warm L

cold HGeneral circulation:If the earth were not rotating: simple one cell model

L equator

30N

60NH

H

L

H

L

H

General circulation pattern with rotation

General Atmospheric Circulation

Figure 6.12

Hadley Cell: rises from equator subsides over Subtropical High

Primary High-Pressure and Low-Pressure Areas

Inter-tropical convergence zone-ITCZ (equator)

Polar High Pressure (poles)

Subtropical high pressure (30N/S)

Subpolar low-pressure cells (60N/S)

June–July ITCZ

Figure 6.11

Global wind pattern

Northeast trade wind (between equator and 30N/S)

Westerlies (between 30N/S-60N/S)

Polar northeasterly (between 60N/S-poles)

Global Barometric Pressure -winter

Figure 6.10

Global Barometric Pressure-summer

Figure 6.10

Semi-permanent system associated with Subtropical High

Bermuda high

(Azores high; Atlantic high)

Pacific high

(Hawaii high)

Figure 6.13

Semi-permanent system associated with Subpolar Low Pressure

Aleutian low

Icelandic low

General Atmospheric Circulation

Figure 6.12

Local WindsLand-sea breezes

Mountain-valley breezes

Katabatic winds

Land-Sea Breezes

Figure 6.18

Sea breeze: wind flows from ocean to land; occurs during the day.

Land breeze: wind flows from land to ocean; occurs during the night

Mountain-Valley Breezes

Figure 6.19

Wind flows from valley to hill during the daytime

Wind flows from high to valley during the night

Monsoonal Winds: reversal of wind directions between seasons

Figure 6.20

Oceanic CurrentsSurface Currents  

Deep Currents  

Major Ocean CurrentsGyre: a circular flow pattern occupies the entire

ocean basin

Figure 6.21

Deep-Ocean Thermohaline Circulation

Figure 6.22