Chapter 4Atmospheric and Oceanic Circulation

Atmospheric & Oceanic Circulation

Major things you need to know:

• What causes wind to happen  • Global pressure areas and wind

patterns, and explanations for them • What causes ocean currents• Global ocean current geography 

Air Pressure: the weight of a column of air at a location

• Air pressure decreases with altitude, as there is less “weight” of air above

Forces Creating and Affecting Air Movement  

• Pressure Gradient Force  • Coriolis Force  • Friction Force  • Gravity

Primary Cause of Wind: difference in pressure between two places (pressure gradient). Closer spacing of isobars indicates stronger presssure gradient.

one example of how pressure gradients are created:

At the surface, cold air, located next to warm air, is denser and has higher pressure.

Pressure Gradient

Fig. 4.7

The Coriolis Effect

• Explanation: earth rotation “deflects” movement to the right in the N. Hemisphere, to the left in S. Hem.

• Effect is greater at higher latitudes• Friction reduces wind velocity, thus reduces

Coriolis Effect• In upper atmosphere with negligible friction,

wind flows nearly parallel to isobars (geostrophic wind)

• Merry-go-round analogy

Coriolis Force

Figure 4.9

Pressure + Coriolis + Friction

Figure 4.8

Global High Pressure and Low Pressure Areas - Why they Develop:

1. Thermal (heating or cooling)– Great amounts of solar radiation promote heating,

low pressure, and convergence (example: circum-equatorial area of low pressure known as “Inter -Tropical Convergence Zone” or ITCZ)

– Minimal or no solar radiation promotes cooling, high pressure, and divergence (Polar Highs)

2. Dynamic (mechanical forcing) results from movements of air masses (result in Subtropical Highs, Sub Polar Lows)

The Basic Oversimplified Model of Global Wind, Ignoring the Coriolis

Effect:

• The hot Equator and the cold Poles would create Equatorial low pressure (thermal low), Polar Highs (thermal high)

• Surface winds would flow from poles to equator

• Rising air at the Equator would flow to the north & south in the atmosphere and descend at the poles…BUT

But the Coriolis Effect will not allow this model to work

• Winds from polar areas are deflected, becoming Easterlies rather than remaining northerlies – thus they never make it to the Equator to replace the warm, rising air there.

Therefore, a more complex model is needed…

This is your model to learn - worth at least 15 pts on exam:

Barometers

Figure 4.2