global atmospheric circulation - reasons

In low latitudes the absorption of solar radiation is much greater than the emission (energy surplus)

In polar regions the emission is greater than the absorption (energy deficit)

The nonradiative transport of energy is necessary. The direction of this transport is from low towards high latitudes.

single-cell model of circulation by Hadley

Assumptions:- non rotating Earth- uniform surface- equator in the surface of ecliptic

This model was described by British meteorologist George Hadley (1685-1768) in 1735.

Three-cell model of atmopheric circulation

described by American meteorologist William Ferrel in 1856

http://www.physicalgeography.net/fundamentals/7p.html

Assumptions:- rotating Earth- uniform surface- equator in the surface of ecliptic

Trade winds – north-eastern winds on the northern hemisphere and south-eastern wind on the southern hemisphere

Intertropical convergence zone – the line of convergence of trade winds

subtropical highs – zone of quasi-constant highs located near tropics

Westerlies – zone with winds with strong western component

Polar front – front between tropical and polar air masses

Eastern winds zone – zone of prevailing eastern wind near the poles

Three cell model

Coriolis Effect

Global Atmospheric Circulation

Jet Streams

The Jet Stream and Rossby Waves

Seasonal Variations in Latitude - Monsoons

Seasonal variations are less likely to affect the equatorial region and the poles

Seasonal Pressure and Precipitation Patterns

Air masses and fronts

Air mass

An air mass is an extremely large body of air whose properties of temperature and moisture are fairly similar in any horizontal direction and in vertical direction

Regions where air masses originate are known as source regions. The longer the air remains stagnant over its source region, the more likely it will acquire properties of the surface below.

Ideal source region:

high pressure

flat uniform surface

huge area

Air masses

Air mass classification:

Arctic and antarctic air APolar air PTropical air TEquatorial air E

continental and maritime air

mA, cAmP, cPmT, cTE – always humid air

Air mass transformation

After leaving the source region air masses transform. The process of transformation depends on the temperature of surface it encounters.

Warm air over the colder surface cools in the lower layer. Warm air above cooler air produces a stable lapse rate with little vertical mixing. In moist air stratiform clouds form accompanied by drizzle or/and fog.

A stable lapse rate slows the mixing of the air and air mass transformation

When the air mass is colder than the underlying surface it is warmed from below,which results in steeper lapse rate and instability in low level. In this case convection can appear.

The instability makes the transformation faster, because of strong mixing.

Air mass transformation

Fronts

A front is a transition zone between two air masses of different properties (temperature, humidity, density)

If they have different densities (temperatures) one of them is called warm the other cold

If they have different humidities one of them is called wet the other dryThe air masses have both horizontal and vertical extent; the upward extension is referred to as a frontal surface or frontal zone.

Warm front

The warmer, less-dense air rides up and over the colder, more-dense surface air. This rising of warm air over cold, called overrunning, produces clouds and precipitation well in advance of the front's surface boundary. warm air overriding the cold air creates a stable atmosphere. A temperature inversion - called a frontal inversion – exists in the region of the upper-level front at the boundary where the warm air overrides the cold air. Under the Nimbostratus rain (or snow) falls. It can also accompany As cloud.

Cold front

A mass of cold, dense air wedges under the warm air, forcing it upward. As the moist, unstable air rises, it condenses into a series of cumuliform clouds. At the front itself, a relatively narrow band of thunderstorms produces heavy showers with gusty winds

occluded fronts

cold-type occluded front

warm-type occluded

front

The movement of water around the oceans has two parts which are strongly linked:

a density driven circulation which is driven by the differences in the density of seawater at different locations.  The density of seawater depends on its temperature and how salty it is.  As a result, this movement is known as the thermohaline circulation (Greek: thermo = heat, háls = salt).  a wind driven circulation which results in huge surface currents like the Gulf Stream.

Oceanic circulation

Wind driven circulation

The Gulf StreamThe Gulf Stream is one of the most important wind driven currents.  It transports very warm tropical water from the Caribbean Sea and the Gulf of Mexico across the North Atlantic to northern Europe.  The warmth of the water heats the air above and the movement of this warm air is a very important way by which heat is transported northwards.  As a result of this heat transport, northern Europe is very much warmer than corresponding latitudes in North America and countries around the Pacific Ocean.

For example, the yearly average temperature at Iqaluit (64oN, 068oW) in the Northwest Territories of Canada is -9.1 oC.  This compares with an average for Trondheim (63oN, 010oE) in Norway of +4.8 oC.  Long term records suggest that, as a result of the Gulf Stream, average temperatures in Northern Europe are 9 oC higher than the average temperatures for the same latitude elsewhere.The Gulf Stream is an example of a western boundary current, a current which flows along the western side of a major ocean basin.  The corresponding current in the Pacific Ocean is the Kuroshio Current, and in the Indian Ocean, the Aghulas Current.  They result from an interaction between the shape of the ocean basin, the general direction of the wind and the rotation of the earth.  They all have a high velocity (the Gulf Stream has an average velocity of 1 m s-1, thats 3.6 km h-1) they are all quite narrow (between 100 and 200 km wide) and all have a very important influence on the climate of the region.  Eastern boundary currents also occur; these transport cold surface waters from the poles to the equator.  They tend to be weaker than their western counterparts.

Pidwirny, M. (2006). "Surface and Subsurface Ocean Currents: Ocean Current Map". Fundamentals of Physical Geography, 2nd Edition. Date Viewed. http://www.physicalgeography.net/fundamentals/8q_1.html

Normal conditions:

El Niño conditions:

http://www.enn.com/specialreports/elnino/what.asp

Weakened Walker circulation

Australia/Indonesia

Australia/Indonesia

South America

South America

Typical walker circulation

Sir Gilbert Walker

StrongEl Niňo

Jacob Bjerkness

El Niňo

La Niňa

La Niňa

El Niňo

La Niňa

El Niňo

http://www.oc.nps.edu/webmodules/ENSO/effects.html