CHAPTER 10 (Moran and Morgan, 1997) This unit focuses upon the planetary scale atmospheric circulation regime, the largest scale of atmospheric motion in terms of both space and time. Specifically, characteristic distances are approximately km or the circumference of the earth and extend from seasons to years. We will examine the prevailing wind regimes and the semipermanent pressure cells first from an observation basis and then attempting to explain why these patterns exist. Attention is also focused upon the upper tropospheric jet streams that encircle the globe in mid latitudes. STUDY NOTES CHAPTER 10 Figure 10.1 -- Study this four-panel figure, which develops a simple planetary-scale atmospheric circulation regime for the earth. Start with panel (A), which shows a simple, thermally direct circulation cell that would result if the earth were not rotating. Note that in this simple circulation cell, warm air rises from the equator and cold sinks over the poles. Next, visualize in panel (B) what happens to this simple cell if the earth were slowly rotating. This simple regime follows the explanation proposed by George Hadley in 1735. Now compare this panel with panels C and D, which show a more realistic view of the prevailing wind and semi-permanent pressure belts at the earth's surface. Figure 10.2 -- Locate the major, semi-permanent pressure features on the world charts for the months of January (Panel A) and July (Panel B). Specifically, look for the large high pressure cells located over the subtropical oceans (near the latitudes of 30 degrees North and South), the low pressure cells particularly near 60 degrees North latitude and a trough of lower pressure near the tropics between the subtropical highs. Based upon your application of the "hand-twist" model, how would the near-surface winds spiral around these features? Recall that south of the equator, the circulation around pressure features is reversed from that of the Northern Hemisphere. Compare your description with that appearing in panels C and D in Figure 10.1. Table 10.1 -- Spend some time becoming thoroughly acquainted with the locations of the semi-permanent pressure systems, the prevailing wind regimes and the names associated with these features. Figure 10.3 -- Compare the features appearing in this satellite image and accompanying legend with the major planetary atmospheric circulation features appearing in Figure 10.4. Can you detect similarities between these two figures? Figure 10.4 -- Study this diagram noting the positions of the major features of both the surface pressure and wind fields. Compare with Table 10.1 and panels C and D of Figure 10.1. Also note that a scalloped line, identified as the "polar front" is also drawn in subpolar latitudes. This scalloped line represents the wavy nature of the boundary that separates tropical air from polar air. A subpolar cyclone often would be located where the polar front makes an excursion poleward. Figure 10.5 -- Study the vertical and horizontal motions within a vertical cross-section of the tropical Hadley circulation cell within the northern and southern hemispheres. Compare this diagram with the panels in Figure 10.1. Specifically, these cells do not extend as originally proposed by Hadley in Panel B, but only to the subtropics, conforming to the more realistic surface pressure and wind regimes appearing in the bottom panels of Figure 10.1. Figure 10.6 -- Spend a moment inspecting the schematic of an upper-level flow pattern with waves in the mid latitude westerly flow. Note that the troughs represent regions where the flow tends to move equatorward around the block letter L, while the ridge represents a region associated with a poleward excursion of the flow around the block letter H. Figure 10.7 -- Study the north-south vertical cross-section of the troposphere in the Northern Hemisphere. A simple model with three segments is shown extending from the North Pole to the equator. Figure 10.8 -- Inspect the seasonal variation in the position of the Intertropical Convergence Zone (ITCZ). Note that the presence of continental landmasses has a significant influence upon the north-south excursion of the ITCZ, especially in July, the summer season of the Northern Hemisphere. Figure 10.9 -- Look at the bar charts depicting the mean monthly precipitation at two stations at approximately the same latitude: Sacramento, CA and Washington, DC. Using the description in the text, note the differences between these two locations and consider the causes for these differences associated with the planetary scale circulation regime. Figures 10.10, 10.11 and 10.13 provide a series of different upper air flow patterns, ranging from zonal, to meridional and to a situation marked with blocking highs and cut-off lows. Figure 10.12 is an example of a highly amplified meridional flow pattern that persisted through much of the 1976-77 winter. Figure 10.15 is an example of a "blocking" regime where a warm anticyclone remained entrenched over much of the continental United States during the hot, dry summer of 1988. The other diagrams in this section are self-explanatory. Figure 10.17 -- Study this figure that represents a vertical cross-section of the troposphere and low stratosphere in the north-south (meridional) direction. Notice from the nearly flat isotherm pattern, the lower troposphere has a relatively uniform temperature in the horizontal direction from the tropics to mid latitudes, where the polar front marks a major transition to colder air in the polar cap region. Note the geographic location of the upper tropospheric polar front and subtropical jet streams. In this figure these two jets would be going into the page, since from the thermal wind relationship, the colder air is to the left of the upper-level winds. Finally note that in the vicinity of the tropical tropopause, the -70 degree Celsius isotherm indicates colder air over the equator than at poleward locations at that same 15 - 20 km altitude. This reversal in the north-south temperature gradient in the stratosphere from that of the lower troposphere helps explain why the strongest winds in the polar front and subtropical jet stream appear immediately under the tropopause. Figure 10.18 -- Look at the map view schematic of a jet streak, noting an elongated region near the innermost isotach where the wind speeds are the highest. The acceleration of the air parcels upon entering this jet streak from the left produces a region where upper level horizontal divergence and convergence take place, as noted by the letters D and C, respectively. Deceleration of the air upon exiting the front of the core (to right of the figure) also produces convergence and divergence patterns. Figure 10.19 -- Consider this diagram, with a series of panels over a three day span, showing the relative position of the jet streak with respect to the upper-level trough. This trough moves eastward across the country. The location of the jet streak has important implications for the development and maintenance of surface low pressure systems. Figure 10.20 -- Compare the winter (blue arrow) and summer (red arrow) positions of the polar front jet over the continental United States. Take a moment to consider why this seasonal change occurs. How would these seasonal changes affect the movement of surface storms? Figure 10.21 and 10.22 -- Look at both diagrams, as if they were slightly different perspectives of the same wave pattern in the upper-tropospheric westerly wind flow. In Figure 10.21, note that the map view of the wind flow within the wave produces regions of horizontal convergence and divergence as winds alternately decelerate and accelerate as they move through the wave pattern. Using this same pattern, note that the 3-dimensional view of this upper-level flow in Figure 10.22 can be linked with the surface pressure features. Specifically, note how a surface low can be found under the east limb of an upper-level trough where horizontal divergence occurs. Likewise, a surface high is often found under the region of convergence located near the west limb of the upper trough. Figure 10.23 -- This block diagram shows upwelling within a large body of water caused by prevailing winds. Note how the flow of surface water away from the coast would be replaced by bottom waters that move up to the surface. Figure 10.24 -- Spend a moment inspecting the map view showing the observed abnormal weather patterns that would occur in the three-month northern winter (December, January and February) during an El Niņo year. Note that while this diagram was based upon information gathered during El Niņo years prior to the intense 1997-1998 episode, the pattern for the 1997-1998 winter is remarkably similar. Read the Special Topic (Singularities) on page 236 -- Have you experienced any of the singularities described in this topic? Read the Weather Fact (Defining Drought) on page 240. Read the Special Topic (El Niņo of 1982-83) on pages 248 and 249. While the 1982-83 El Niņo has been considered a benchmark when describing an El Niņo, the stronger 1997-1998 El Niņo appears destined to become the new benchmark. Skim the first half and read the second half of the Mathematical Note (The Polar Front and the Midlatitude Jet Stream) on pages 250 and 251. The first half of this Note presents the mathematical equation that describes the hydrostatic balance relationship, or in other words, how differences in air density can affect the pressure changes with height in the atmosphere. This portion is intended for the mathematically inclined. In the second half of the Note, a verbal description is given explaining the existence of the upper tropospheric jet stream found over the polar front. As mentioned, this explanation describes what meteorologists identify as the "thermal wind" relationship, which is essentially a vertical wind shear above a region of strong horizontal temperature contrast. Inspect Figure 2, which is a vertical cross-section through a portion of the atmosphere where a warm air column (to the right) lies next to a cold air column (to the left). Even if the surface pressure (not shown) were the same form north to south, an increase in the horizontal pressure gradient will occur as you would ascend through the atmosphere. This increased pressure gradient results from the horizontal temperature differences. Specifically, this cross-section shows the slope of each successive constant pressure surface, identified as a dashed blue line, becomes increasingly steeper from north to south. CHAPTER 10 (Moran and Morgan, 1997) PLANETARY-SCALE CIRCULATION This and the next five chapters are concerned with the genesis and characteristics of a variety of weather systems. We examine these systems in order of decreasing spatial scale beginning with the largest scale, the global or planetary circulation. Semipermanent pressure systems, wind belts, and the intertropical convergence zone (ITCZ) are principal features of planetary-scale circulation. We show how these features are interrelated and how they change with the seasons. Special consideration is given the westerlies of midlatitudes because midlatitude weather is a major focus of this text. Hence, we describe wave patterns in the westerlies and properties of the jet stream with the objective of demonstrating how smaller scale weather systems are linked to the planetary circulation. CHAPTER OBJECTIVES After reading this chapter, the student should be able to: identify the principal components of the planetary-scale circulation. describe the linkage between the subtropical anticyclones and the trade winds and the midlatitude westerlies. compare and contrast the planetary winds at the surface with those in the mid and high troposphere. describe the seasonal changes in the principal components of the planetary-scale circulation. contrast weather patterns associated with zonal flow with those associated with meridional flow. explain the association between a blocking circulation pattern and weather extremes. describe the linkage between the polar front and the midlatitude jet stream. explain the role of the jet stream and upper-air troughs in the development of synoptic-scale cyclones. describe the connection between sea-surface temperature anomalies and changes in the planetary circulation. 10 Planetary-Scale Circulation 226 Idealized Circulation Pattern 227 Pressure Systems and Wind Belts 228 Upper-Air Westerlies 234 El Niņo 245 Conclusions 247 Special Topic: Singularities 236 Weather Fact: Defining Drought 240 Special Topic: El Niņo of 1982-83 248 Mathematical Note: The Polar Front and the Midlatitude Jet Stream 250 Key Terms 247 Summary Statements 247 Review Questions 248 Questions for Critical Thinking 249 Selected Readings 251