Lecture 5:The General Circulation

of the Atmosphere

EESC V2100The Climate System

spring 2004

Yochanan Kushnir Lamont Doherty Earth Observatory

of Columbia UniversityPalisades, NY 10964, USA

kushnir@ldeo.columbia.edu

Latitudinal Radiation Imbalance

The annual mean, averaged around latitude circles, of the balance between the solar radiation absorbed at the ground (in blue) and the outgoing infrared radiation from Earth into space (in red). The two curves must balance completely over the entire globe , but not at every single latitude. In the tropics , there is an access of radiation (solar radiation absorbed acceeds outgoing terrastrial radiation) in middle and high latitudes all the way to the poles, there is a deficit (Earth is radiating into space more than it receives from the sun). The atmosphere and ocean systems are forced to move about by this imbalance, and bring heat by convection and advection from equator to the poles. Here we examine how the atmosphere responds to this imbalance.

The Response of the TropicsHeated by the sun and fed by moisture from evaporation over the surrounding oceans, the tropics are a hotbed for intense moist convection and rainfall, as can be seen in the figure.

The main dynamical balance in the tropics is dictated by the thermodynamics and entials the cancelation between and the heating due to condensation and latent heat release on one hand and the adiabatic cooling in the rising moist air on the other hand.

Mathematically, we can express this balance by going back to the thermodynamic energy equation (see lecture III on convection) and expressing it as a change per unit time in the heating (dividing by dt):

dQ/dt = dT/dt + Γsdz/dt

where Q is the condensational heating. Setting dT/dt=0 we find that the steady state balance is:

dQ/dt = Γsw

where w is the vertical motion (=dz/dt). The rising motion in the tropics is the starting point for understanding the general circulation of the atmosphere.

Global, annual-mean rainfall distribution. Red areas in the tropics are regions of strong vertical motion - as explained here

The Hadley Cells

The rising motion in the tropics is capped from above by the stratosphere (where the air warms with height, thus suppressing upward motion - see lecture on convection). The law of mass continuity requires the air to move away from the tropics, northward and southward as in the diagram above. This motion amounts to an upper level mass divergence, forced by the rising motion. Again, for reasons of mass continuity, the diverging upper-level tropical air must return to the surface poleward of the equator. At the same time, mass continuity at the surface requires low level convergence and the movement of air towards the equator.

The two cells formed north and south of the equator by the tropical uplift are called Hadley Cells, after the astronomer Hadley who first proposed their existence.

CELLS

Jet Streams

The upper atmospheric air diverging from the tropics and moving poleward is subjected to the Coriolis force which exerts an eastward directed acceleration on both the nortward moving air in the Northern Hemisphere and the southward moving air in the Southern Hemisphere (see lecture IV on atmospheric dynamics). The acceleration, which depends on the latitude and the poleward (meridional) wind speed, leads to the formation of a westerly (eastward-directed) upper-level air flow both north and south of the equator. This flow reaches maxima at about 30°N and S of the equator, referred to as the jet streams. In the figure above, the jet streams are the areas of largest windspeed (red color)

equator

30°N

30°S

Coriolis force

Coriolis force

Hadley Cell Divergence

Jet Stream

Jet Stream

Upper Level Winds

The January and July averaged 300 mb (~9km) wind. Arrows depict the monthly averaged wind

vector (in m/s) see arrow scale below picture). The colors depict the vector

magnitude (in m/s) according to the colorscale below. Note the seasonal differences due

mainly to the shift in the location of the maximum in

tropical heating.

January

July

The climatological distribution of zonally average temperature °C (left panel) and zonal wind (right panel) during the month of January, as a function of latitude and height.

The Zonal wind and temperature fields are in thermal wind balance.

Zonally Averaged Circulation

Trade winds

At the surface, the tropical winds are converging towards the equator to close the Hadley Cell. The Coriolis force acts here to accelerate the winds westwards. Friction acts to slow the winds and the final balance is achieved with the winds moving both westward (easterly winds) and towards the equator forming the so-called trade winds.

The Surface Winds

The January and July averaged surface winds.

Arrows depict the monthly averaged wind vector (in m/

s) see arrow scale below picture). The colors depict

the vector magnitude (in m/s) according to the colorscale

below. Note the seasonal differences due both to the shift in the location of the

maximum in tropical heating and the heating and cooling

of the continents.

January

July

The Ferrel Cell and the Meridional Mass Circulation

The Hadley Cell ends at about 30° north and south of the equator because it becomes dynamically unstable, creating eddies that are the reason for the weather disturbances of the midlatitude belts (see Lecture 4). These eddies force a downward motion just south of the jet axis and an upward motion between 40 and 60° north and south of the equator, forming the Ferrel Cell. The eddies are also responsible for spreading the westerlies down to the surface.

The Zonally Averaged Mass Circulation

The annually-averaged atmospheric mass circulation in the latitude pressure plane (the meridional plan). The arrows depict the direction of air movement in the meridional plane. The contour interval is 2x10 10 Kg/sec - this is the amount of mass that is circulating between every two contours. The total

amount of mass circulating around each "cell" is given by the largest value in that cell. Data based on the NCEP-NCAR reanalysis project 1958-1998.

The January and July sea level pressure (SLP) field

shown in colors and contours (in mb) Note the seasonal

differences due both to the shift in the location of the

maximum in tropical heating and the heating and cooling

of the continents.

The reversals of SLP over the tropical continents are the MONSOONS (of India, Africa, America’s) they are

dew to the differential heating between land and

ocean and are referred

January

July

The Sea Level Pressure Field

The January and July surface temperature field shown in colors and contours (in °C).

The seasonal differences due both to the shift in the

location of the maximum in tropical heating and the

heating and cooling of the continents.

A link between warm/cold surface temperatures and

low/high SLP can be seen by comparing with the previous

slide

The Surface Temperature

Field January

July

The difference between January and July surface temperature is shown in

colors and contours (in °C).

Large differences occur over the continents and smaller ones over the oceans - an

indication for the heat capacity of the surface (the

larger the capacity the smaller the difference).

Seasonal Temperature Differences

Climate Zones

The information we gathered through our study points at the diversity of climate over Earth determined by the incoming solar radiation, atmosphere (and ocean) circulation, the land-ocean distribution, and the topography. There are different ways to classify the climate zone. The map above is one of them, which is relatively simple. In the next slide is a more scientific one proposed by Koeppen in the 1920’s.

Climate Zones according to Koeppen

For more information about this map see:

http://www.blueplanetbiomes.org/climate.htm