General Circulation Nili Harnik DEES, Lamont-Doherty Earth Observatory nili@ldeo.columbia.edu
Latitudinal Radiation Imbalance The annual mean, averaged around latitude circles, of the balance between the solar radiation absorbed at the ground and the outgoing infrared radiation from Earth into space.
Poleward Heat Transport Total heat transport Atmospheric heat transport PW=10 15 W The radiation imbalance of the top of the atmosphere implies that the climate system must transport heat poleward by the combined action of ocean and atmosphere. The transport calculated from space measurements of the imbalance is about 6x10 15 W at 30 latitude North and South. We can calculate the Atmospheric transport from relatively abundant atmospheric data. The difference is the ocean transport.
The Response of the Tropics In the tropics, a belt of warmest surface temperatures, surrounds the Earth. Here there is abundant moisture so that small vertical movements of air can lead to spontaneous generation of deep convection. This region is the ITCZ. It is characterized by massive thunderstorms and rains.
The Response of the Tropics Global, annual-mean rainfall distribution. Red areas in the tropics are regions of strong vertical motion - as explained here
The Hadley Cells 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.
Mid latitudes- the Jet Streams Hadley Cell Divergence Jet Stream 30 S Jet Stream Coriolis force 30 N Coriolis force equator 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)
The Jet Stream and Trade Winds
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 Jet Streams The January 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.
The Trade Winds The January 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.
Zonally Averaged Circulation 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.
Thermal Wind Balance The geostrophic relationship, together with the hydrostatic balance and the equation of state imply an important relationship between the horizontal gradient of temperature and the rate of change of wind with height or vertical wind shear. The vertical shear of the meridional wind, v is proportional to the zonal (west-east) gradient of temperature, and the shear of the zonal wind u, proportional to the meridional (south-north) gradient of temperature. Thus, where temperature decreases northward the geostrophic zonal wind must increase with height. The thermal wind equation arises from the slower drop of pressure in warm air compared to that in cold air.
Midlatitude weather systems NOAA NCEP-NCAR CDAS-1 DAILY 300 mb height (m) and winds (m/s) 1 Apr 1997
Midlatitude weather systems Midlatitude weather systems ("storms ), develop spontaneously on the midlatitude jet stream.
Midlatitude weather systems These storms transport heat poleward and upward and are thus contribute to the large scale equator to pole heat transport.
Midlatitude weather systems Global, annual-mean rainfall distribution. Red areas in the tropics are regions of strong vertical motion - as explained here
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 IV). 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 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 Surface Temperature Field January 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. July
Seasonal Temperature Differences 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).
Monsoons Winter thermal contrast pressure gradient mass continuity Coriolis force Summer
The Sea Level Pressure Field 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 (India, Africa, America s) are referred to as MONSOONs January July
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.
Effects of Mountains on Local Climate Moist convection can explain the local climate effect of mountains, namely the tendency for large mountain ranges to have excess precipitation on the upwind side and a desert or rain shadow on the downwind side. The former is due to the lifting of the incoming air by the mountain. The latter is due to the warming of the rising air due to latent heat release.
Climate Zones according to Koeppen For more information about this map see: http://www.blueplanetbiomes.org/climate.htm