Robert W. ChristophersonCharlie Thomsen

Chapter 6Atmospheric and

Oceanic Circulations

Wind is a vector variableTemperature is a scalar variable.

Atmospheric and Oceanic Circulations

Wind Essentials  Driving Forces within the Atmosphere  Atmospheric Patterns of Motion  Oceanic Currents  

Wind EssentialsAir Pressure and Its Measurement  

Mercury barometerAneroid barometer

Wind: Description and Measurement  Wind Anemometer Wind vane

Global Winds  

Barometers

Figure 6.2

Air Pressure Readings

Figure 6.3

1013.25mb=101.325kpa=14.66lb/in2

Atmospheric Pressure and Elevation  

p1

p2

Which point (p1 vs. p2) has higher air pressure? Why?

How are pressure change with elevation?

(1) Uniformly decrease with elevation (2) decreases faster with higher close to

see level than at high elevations.

Wind Vane and Anemometer

Figure 6.4

Wind: horizontal movement of air across Earth surface.

Vector: Speed measured by Anemometer

Direction measured by wind vane wind direction is defined as the direction from which it originates. Standard measurement of wind is 10 m above ground.

Old weather forecast refer wind speed in scales, a commonly used one is Beaufort Wind Scale.

Driving Forces within the Atmosphere  

Pressure Gradient Force  Coriolis Force  Friction Force  Gravity

Pressure Gradient Force

Figure 6.7

Without pressure gradient force, the air will not move, then there will be no Coriolis force, no friction force.

Pressure difference is primarily caused by uneven heating of Earth surface

Coriolis Force

Figure 6.9

It deflects anything that flies or flows across Earth surface: wind, airplane, ocean currents etc.

Coriolis Force only changes the direction of movement, not the speed. It is always perpendicular to the direction of movement, to the right hand side on Northern Hemisphere.

F=2vΩsin(φ), where v=wind speed, Ω=angular velocity of earth rotation 7.29x105 radians per second, φ=latitude

The stronger the wind, the stronger Coriolis force.

Pressure + Coriolis + Friction

Figure 6.8

Pressure Gradient Force only

Pressure Gradient +Coriolis Forces

Pressure Gradient+Coriolis+Friction Forces

Friction force: Always in the opposite direction of wind.

Strength: depending on wind speed, surface condition (topography, vegetation, …)

Scales of Atmospheric Movement  The movement of atmosphere around the globe is a

composite of multiple scale motion, like a meandering river contains larege eddies composed o f smaller eddies containing still smaller eddies.

Macroscale: Large Planetary wide movement of atmosphere, e.g. trade winds, monsoon, hurricanes, which can blow for weeks or longer.Mesoscale: lasts for several minutes or hours, usually less than 100 km across, e.g. thunderstorms, tornadoes  Microscale: smallest scale of air motion, lasts for seconds at most for minutes, e.g. wind gust, dust devils  

The Westerlies

Earthsun

tropopause

Equator N. Pole

Pressure gradiant

1000mb

700mb800mb

600mb

900mb

500mb

Single Cell Model (Hadley 1735)

Figure 6.12

sun

Three-Cell Model (1920s)

Figure 6.12Hadley CellFerrel CellPolar Cell

General Atmospheric Circulation and pressure zones

Figure 6.12

General Atmospheric Circulation

Figure 6.12

Primary High-Pressure and Low-Pressure Areas

Equatorial low-pressure trough: thermalPolar high-pressure cells: thermalSubtropical high-pressure cells: dynamicSubpolar low-pressure cells: dynamic

Equatorial Low-Pressure TroughIntertropical convergence zone (ITCZ)

Clouds and rainTrade winds: The trade winds were named during the era of sail ships that carried trade across the seas.

Global Barometric Pressure

Figure 6.10

Aleutian LowIcelandic Low

Azores HighHawaiian High

Siberian High

The subtropical high pressure zone broke into three high pressure centers: Hawaiian, Azores, Siberian HighsThe subpolar low pressure zone broke into two low pressure centers: Aleutian and Icelandic Lows

High Pressure Center

East side drier and more stable, feature cooler ocean currents than west side. Earth major deserts extend to the west coast of each continent.

Global Barometric Pressure

Figure 6.10

Pacific high

Bermuda high

The high pressure centers are pushed northern. As a result, the subpolar low pressure centers are weakened significantly.

Global Patterns of Pressure

Wind Portrait of the Pacific Ocean

Figure 6.6

Wind pattern derived from a radar scatterometer aboard Seasat on a day in September.

Note: compare wind pattern and the visible earth below:

Westerlies

Trade wind

Pacific High

June–July ITCZ

Figure 6.11

Monsoonal Winds

Figure 6.20

Regional wind systems seasonally changes direction and intensity associated with changes temperature and precipitation.

Winter: cold dry wind blow off the continents Summer: warm moist-laden wind blow from sea toward land

Larger than average northward migration of ITCZ.

Upper Atmospheric CirculationJet stream: a fast flowing narrow air currents in the upper atmosphereRossby waves

Jet Streams

Figure 6.17

An concentrated band of wind occurring in the westerly flow aloft. Flat in vertical direction Speed up to 190 mph Stronger in winter

It is caused by the large pressure gradient caused by the large horizontal temperature difference over short distance.

Influence surface weather systems.

30-70oN

20-50oN

Rossby Waves

Figure 6.16

Note: in the upper atmosphere, artic area has low pressure, thus the air circles counterclockwise parallel to the pressure gradient (Why?). But this circle is not perfect. Instead, it follows a wavy path.

Discovered by Carl G. Rossby in 1938. It refers to the waving undulations of geostrophic winds of the arctic front.

Rossby Waves

Figure 6.16

Smooth westward flow of upper air westerlies

Develop at the polar front, and form convoluted waves eventually pinch off

Primary mechanism for poleward heat transfere

Pools of cool air create areas of low pressure

Local WindsLand-sea breezesMountain-valley breezesKatabatic winds

Land-Sea Breezes

Figure 6.18

Mountain-Valley Breezes

Figure 6.19

Katabatic Wind: A regional scale gravity driven wind, usually needs a high plateau to cool the air, and become dense and flow downslope.

Oceanic CurrentsFunction: Mixing sea water Surface warm water with deep cold water CO2 absorption Climate Biogeochemical processes: phytoplankton growth

Driving force: the frictional drag of winds Thus we have an Atmosphere-Sea are coupled system. Once the

current starts to move, the Coriolis force will kick in. Then there is “friction” between upper and lower water, the shear stress.

  

Major Ocean Surface Currents

Figure 6.21Surface ocean currents are driven by air circulation around subtropical high pressure cells.

Equatorial Currents/Western IntesificaitnCorresponding to trade winds on both sides of the Equator, these winds drives the surface current westward along the equator, called equatorial currents.The equatorial currents push water piles up against the eastern shores of the continent. This is called western intensification. The piled up water will go either up north or down south . The Gulf Stream is one caused by western intensification.

Upwelling/Downwelling Currents

Figure 6.22

Upwelling Currents: When surface water is swept away from a coast, an upwelling current occurs. This cool water generally is nutrient rich, e.g. Pacific Coast of North and South America

Downwelling Currents: Accumulation of surface water (e.g. western end of equatorial current) can gravitates downward to generate a downwelling current.