ARTICLE INDEX:
Figure 3 and 4 describes the actual surface pressures for the Earth as determined from 39 years of record. The circulation patterns produced by these pressures seem different somewhat from the three cell model in Figure 2. These differences are caused primarily by two factors. First, the Earth's surface is not composed of uniform materials. The two surface materials that dominate are water and land. These two materials behave differently in terms of heating and cooling, causing latitudinal pressure zones to be less uniform. The second factor influencing actual circulation patterns is elevation. Elevation tends to cause pressure centers to become intensified when altitude is increased. This is especially true of high pressure systems.
In figures 3 and 4 monthly average sea-level pressure and prevailing winds for the Earth's surface are for the years 1959-1997. Atmosphere pressure values are adjusted for elevation and are described relative to sea-level. Pressure values are indicated by color shading (see the legend in the graphic). Blue shades indicate pressure lower than the global average, while yellow to orange shades are higher than average measurements. Wind direction is shown with arrows. Wind speed is indicated by the length of these arrows (see the legend on the graphic).(1)
On these modified graphics, we can better visualize the intertropical convergence zone ( ITCZ ), subtropical high pressure zone , and the subpolar lows. The intertropical convergence zone is identified on the figures by a red line. The formation of this band of low pressure is the result of solar heating and the convergence of the trade winds. In January, the intertropical convergence zone is found south of the equator ( Figure 3 ). During this time period, the Southern Hemisphere is tilted towards the sun and receives higher inputs of shortwave radiation. Note that the line representing the intertropical convergence zone is not straight and parallel to the lines of latitude. Bends in the line occur because of the different heating characteristics of land and water. Over the continents of Africa, South America, and Australia, these bends are toward the South Pole. This phenomenon occurs because land heats up faster then ocean.
Figure 3 - Mean January prevailing surface winds and centers of atmospheric pressure, 1959-1997. The red line on this image represents the intertropical convergence zone (ITCZ). Centers of high and low pressure have also been labeled.
During July, the intertropical convergence zone (ITCZ) is generally found north of the equator ( Figure 4 ). This shift in position occurs because the altitude of the sun is now higher in the Northern Hemisphere. The greatest spatial shift in the ITCZ, from January to July, occurs in the eastern half of the image. This shift is about 40° of latitude in some places. The more intense July sun causes land areas of Northern Africa and Asia rapidly warm creating the Asiatic Low which becomes part of the ITCZ. In the winter months, the intertropical convergence zone is pushed south by the development of an intense high pressure system over central Asia (compare Figures 3 and 4 ). The extreme movement of the ITCZ in this part of the world also helps to intensify the development of a regional winds system called the Asian monsoon.
Figure 4 - Mean July prevailing surface winds and centers of atmospheric pressure, 1959-1997. The red line on this image represents the intertropical convergence zone (ITCZ). Centers of high and low pressure have also been labeled. (1)
The subtropical high pressure zone does not form a uniform area of high pressure stretching around the world in reality. Instead, the system consists of several localized anticyclonic cells of high pressure. These systems are located roughly at about 20 to 30° of latitude and are labeled with the letter H on Figures 3 and 4 . The subtropical high pressure systems develop because of the presence of descending air currents from the Hadley cell . These systems intensify over the ocean during the summer or high sun season. During this season, the air over the ocean bodies remains relatively cool because of the slower heating of water relative to land surfaces. Over land, intensification takes place in the winter months. At this time, land cools off quickly, relative to ocean, forming large cold continental air masses.
The subpolar lows form a continuous zone of low pressure in the Southern Hemisphere at a latitude of between 50 and 70° ( Figures 3 and 4 ). The intensity of the subpolar lows varies with season. This zone is most intense during Southern Hemisphere summer ( Figure 4 ). At this time, greater differences in temperature exist between air masses found either side of this zone. North of subpolar low belt, summer heating warms subtropical air masses. South of the zone, the ice covered surface of Antarctica reflects much of the incoming solar radiation back to space. As a consequence, air masses above Antarctica remain cold because very little heating of the ground surface takes place. The meeting of the warm subtropical and cold polar air masses at the subpolar low zone enhances frontal uplift and the formation of intense low pressure systems.
In the Northern Hemisphere, the subpolar lows do not form a continuous belt circling the globe ( Figures 3 and 4 ). Instead, they exist as localized cyclonic centers of low pressure. In the Northern Hemisphere winter, these pressure centers are intense and located over the oceans just to the south of Greenland and the Aleutin Islands. These areas of low pressure are responsible for spawning many mid-latitude cyclones. The development of the subpolar lows in summer only occurs weakly ( Figure 4 - over Greenland and Baffin Island, Canada), unlike the Southern Hemisphere. The reason for this phenomenon is that considerable heating of the Earth's surface occurs from 60 to 90° North. As a result, cold polar air masses generally do not form. (2)
Most European windstorms are caused by extratropical cyclones (ETCs). These mid-latitude weather systems derive their energy from horizontal temperature contrasts between cold, polar air masses and warm, subtropical air masses. Because the temperature contrasts between these air masses are greatest during winter, the frequency and intensity of European windstorms peak during this season as well.
European windstorms differ dynamically from Atlantic hurricanes or IndoPacific typhoons, which are strong tropical cyclones . Tropical cyclones derive their energy from the vertical temperature contrast between the warm lower layer and cold upper reaches of the tropical atmosphere. Because the hurricane ultimately derives its energy from warm seawater, its winds quickly diminish when its eye moves over cooler water or land.
The horizontal temperature gradient that powers an extratropical cyclone can persist as the storm center moves over land. Thus wind speeds in these storms can occasionally remain high, or even increase, after landfall. Also, while a hurricane can sustain the same minimum central pressure for days, the energy that drives an extratropical cyclone rapidly decays as the air masses within it intermix—a single cyclone typically exists independently for three to five days.
Storms are steered by the polar jet stream, so the position of the jet stream defines the storm track. The polar jet stream and storm track move from month to month and from year to year, but storms moving along the storm track tend to reinforce the jet, and keep it from shifting. This is called eddy feedback. By reinforcing the jet, eddy feedback helps the jet and storm track "remember" their position over weeks to months, and thus enhances the persistence and predictability of mid-latitude variability. It is also responsible for the tendency of European windstorms to occur in series . Eddy feedback is not, however, sufficiently robust to provide persistence from year to year.
The storms that affect Europe are the eastern outliers of the ~200 disturbances that form in the north Atlantic each year and track eastward along with the jet stream. The track of the jet is affected by the position of the Azores high pressure cell and the Icelandic low. Shifts in the relative strength of these pressure cells control where a storm might make landfall in Europe.
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