Figure 7.32 shows the actual surface circulation of Earth, averaged over 50 years of climate records (1951-2000). Typical circulation patterns are shown in this figure for January and July. The circulation patterns observed here differ somewhat from the three-cell model in Figure 7.31 (section, Models of Global Circulation). These differences are caused primarily by two factors. First, the Earth's surface is not composed of uniform materials. The two dominant surface materials are water and land. These two materials behave differently during heating and cooling, leading to less uniform latitudinal pressure zones. The second factor influencing actual circulation patterns is elevation. Elevation tends to intensify pressure centers as altitude increases. This situation is especially true of high pressure systems.

    

            Figure 7.32 displays average surface wind patterns for both January and July. The Intertropical Convergence Zone (ITCZ) is identified in these figures by an orange line that connects several tropical low pressure centers. The formation of this band of low pressure results from solar heating and the convergence of the trade winds. In January, the Intertropical Convergence Zone is found mainly south of the equator (Figure 7.32A). During this month, 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. The bends found in the Intertropical Convergence Zone 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 than the ocean.  

            During July, the Intertropical Convergence Zone is generally found north of the equator (Figure 7.32B). This shift in position occurs because the Sun is now at its highest altitude in the Northern Hemisphere. The most significant 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 in Northwestern Africa, the Middle East, and South Asia to rapidly warm, creating four thermally created low pressure centers, which become 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 7.32A and 7.32B). The extreme movement of the ITCZ in this part of the world also helps to intensify the development of a regional wind system called the Asian monsoon.

            In reality, the Subtropical High Pressure Zone does not form a uniform area of high pressure stretching around the world. Instead, the system consists of several localized anticyclonic cells of high pressure. These systems are located roughly between 20° and 40° of latitude and are labeled with the letter H (Figures 7.32A and 7.32B). The subtropical high pressure systems develop because 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 ocean bodies remains relatively cool because water heats more slowly than land surfaces. Overland, intensification takes place in the winter months. At this time, the land cools more quickly than the ocean, forming large cold continental air masses.

            The Subpolar Lows form a continuous zone of low pressure at latitudes between 50 and 70° in the Southern Hemisphere (Figures 7.32A and 7.32B). The location and strength of the Subpolar Lows vary with season. This zone is most intense during the Southern Hemisphere summer (Figure 7.32A). At this time, greater temperature differences exist between air masses on either side of this zone. North of the Subpolar Low belt, summer heating warms subtropical air masses. South of this zone, the ice-covered surface of Antarctica reflects much of the incoming solar radiation to space. Consequently, air masses over Antarctica remain cold because very little ground heating occurs. 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. Instead, they exist as localized cyclonic centers of low pressure. In the Northern Hemisphere winter, these pressure centers are strongest and are located over the oceans between Greenland and Iceland and south of the Aleutian Islands (Figure 7.32A). These areas of low pressure are responsible for spawning many mid-latitude cyclones during the winter season. The development of the Subpolar Lows in summer occurs only weakly (Figure 7.32B) over northeastern Russia, Alaska, between Greenland and Iceland, and southeastern Baffin Island, Canada, unlike in the Southern Hemisphere. The reason for this phenomenon is that the Earth's land surface is heated considerably from 60 to 80° North during summer. As a result, the temperature gradient between land and ocean surfaces is slight, and this does not intensify the surface pressure systems here. 

FIGURE 7.32  January (A - top image) and July (B - bottom image) surface wind patterns for the Earth's surface averaged from 1951-2000. Arrows indicate the dominant wind direction. See legend for wind speed. The Intertropical Convergence Zone (ITCZ) is shown as an orange line.  Image Copyright: Michael Pidwirny. Original Image Source: Climate Reanalyzer (https://climatereanalyzer.org), Climate Change Institute, University of Maine, USA. The dataset used is NOAA 20th Century Reanalysis Version 2.