Climate Change: Course Preview

Atmospheric Circulation and Global Climate Change

by Dr. Ed Mathez

This essay was developed for Week 1 of the AMNH online course Climate Change, part of Seminars on Science, a program of online graduate-level professional development courses for K-12 educators. Explore more sample resources...

The Dynamic Climate System
Figure 1: Earth's Atmosphere
The tropopause is an atmospheric boundary between the troposphere and the stratosphere. It's found at an altitude of 16 to 18 kilometers (52,000 to 59,000 feet) in the tropics, but drops to 8 to 10 kilometers (26,000 to 33,000 feet) in the polar regions. The drop is gentle from the equator to latitudes of about 40°N and 40°S, where it plunges about two kilometers because of circulation patterns in the troposphere, before continuing its gentle drop to the poles. Above the stratosphere is the mesosphere, which reaches to about 85 kilometers (50 miles) above the surface, and above that is the thermosphere, which constitutes only a minute fraction of the atmosphere but extends some 500 kilometers (300 miles) above the surface. ©AMNH

We think of climate as a system. The parts of this system are the atmosphere, the ocean, the biosphere (all living things), the cryosphere (all things frozen), and the solid Earth. All these parts interact both chemically and physically to produce a climate that is both complex and dynamic but also characterized by long periods of stability. Think of the parts as the organs in your body, each with a specific function, each dynamic, all interacting to keep the beast alive. If two organs are most central to the climate system-the brain and the heart, so to speak-they are the atmosphere and ocean. An understanding of the climate system requires some knowledge of the character and dynamics of the atmosphere and ocean and how they interact.

The atmosphere has stable layers

The lowest layer of the atmosphere-the one we live in-is the troposphere (Figure 1), which accounts for about 80% of its total mass. This is where weather occurs (where clouds exist, storms form and dissipate, and weather fronts advance) and also where most of the greenhouse gases are concentrated. The top of the troposphere reaches an altitude of 8 to 18 kilometers (26,000 to 59,000 feet), depending on location, and is overlain by the stratosphere (Figure 1). The stratosphere, in turn, extends to about 50 kilometers (30 miles) and contains most of the atmosphere's ozone (the molecule consisting of three atoms of oxygen, O3). By absorbing ultraviolet (UV) radiation, which is lethal to life, ozone makes life on the surface of Earth possible.

Without incoming solar radiation, the temperature of the atmosphere would simply decline steadily with altitude. However, because of how it is heated, the atmosphere organizes itself into distinctive layers. About half of the incoming solar radiation is absorbed by the ocean and land surfaces, which thus heat the troposphere from below. At the same time, the troposphere radiates energy outward. As a result, the troposphere decreases in temperature with increasing elevation (which is why mountain peaks are snow-capped). The overlying stratosphere is relatively warm because its ozone absorbs both solar UV radiation from above and, being a greenhouse gas, infrared radiation from below (Figure 1).

One consequence of how the troposphere and stratosphere are heated is that the region between the two is cold. This boundary layer is known as the tropopause and acts basically as a roof on the troposphere. For example, it prevents water vapor from migrating into the stratosphere, which would completely change the atmosphere's thermal structure. In short, the layered structure of the atmosphere is remarkably stable, which is why the weather stays mostly in the troposphere.

Two factors drive the circulation of both the atmosphere and the ocean

The first factor is that the planet is heated unevenly. Because Earth is a sphere, more solar energy falls on equatorial regions than on the poles (Figure 2). This uneven distribution generates winds that carry heat from the equator to the poles, and from the surface to the upper atmosphere. The winds also drive ocean surface currents, which are also guided by the positions of the continents and other factors.

Figure 2: Global Temperature Distribution
This data visualization is a composite of daily measurements of sea surface temperatures for May 2001. Reds are warm, purples are cold. Note the Gulf Stream, the current around the tip of southern Africa that carries warm Indian Ocean water into the South Atlantic, and the eddy currents near the equator. ©NASA / GSFC / MODIS

The second factor that drives circulation is the Coriolis effect, which refers to the deflection of fluids on a rotating sphere. In particular, Coriolis forces deflect air and water masses to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. To understand the phenomenon, imagine standing on the equator and holding a ball (Figure 4). Even though you are not moving, both you and the planet are rotating eastward. Now, consider a point directly to your north. It is also moving eastward but not quite as fast because the rotating circle to which it is anchored is smaller. Next, imagine that you throw the ball directly north. The ball still maintains its eastward equatorial velocity, although the Earth beneath it is not moving as fast. Thus, from the perspective of the Earth below, the ball appears to be moving north and also drifting eastward, or to the right if looking toward the pole.

The atmosphere is dynamic

The Coriolis forces interact with forces created by the uneven heating of Earth's surface to form a set of distinctive global wind circulation patterns specific to latitude. These give us the weather we experience every day. These circulation patterns also reflect motions due to convection, the process by which light matter rises and dense (heavy) matter sinks (Figure 3). Density is commonly determined by temperature, i.e., warm air is less dense than cold air, which is why warm air rises. Thus, in the atmosphere convection results mainly from the uneven heating of ocean and land surfaces. Temperature-driven, or thermal convection, is a fundamental process of heat transport in the atmosphere and ocean, and indeed in the solid planet as well.

Hadley cycles

Hadley Circulation
Figure 3: Hadley Circulation
Warm air rises at the equator, moves partway to the poles, sinks and then returns to the equator. On the surface this produces belts of westward-blowing winds known as the Trade Winds. ©Aerospace Corporation

As noted, more solar heat reaches the surface at the equator than at the poles. The result is that warm equatorial air flows toward the poles and cool air toward the equator as a means of evening out the heat. This convective motion is known as Hadley circulation, named for the George Hadley (1685-1768), the English mathematician who in 1735 first recognized that the spherical Earth must be unevenly heated. Hadley circulation starts when warm air near the equator rises, creating the upward limb of the Hadley cells (Figure 3). The downward limbs exist at about the 20° to 30° north and south latitudes, where they form two zones of high pressure on either side of the equator (Figure 4). Because the sinking air is dry, most of the world's great deserts are located in these regions. Some of the downward-flowing air is directed poleward, but most flows back toward the equator, completing the circuit. At the same time, the Coriolis effect deflects the Hadley cells, causing their upper poleward limbs to deflect eastward and their lower returning limbs to flow southwestward. The latter form the trade winds (Figure 4), which were well known to mariners in the days when ships depended on the winds for their power.

The upward limb of Hadley circulation, which is where the Northern Hemisphere and Southern Hemisphere trade winds meet near the equator, is known as the Intertropical Convergence Zone. Here, the large-scale upward motion of air results in low surface air pressure, high evaporation rate of water from the ocean, high rainfall, and the transfer of substantial amounts of heat from the ocean surface to the atmosphere.

Global Wind Patterns
Figure 4: Global Wind Patterns
This illustration shows idealized surface and global wind patterns. The red arrows show the general direction of surface winds. Note the Hadley, Ferrel, and polar cells, as well as the equatorial Intertropical Convergence Zone and the polar front. ©AMNH

Ferrel and polar cells

The Coriolis effect breaks up simple Hadley circulation to create a set of mid-latitude, weaker, and much less stable circulation patterns called Ferrel cells (Figure 4). These cells are caused by eddy circulations, or small swirls in the opposite direction from the main flow, which progress from west to east. In Ferrel cells, cold air masses rise at north and south latitudes of about 50° and sink at latitudes of about 30° to 40°. They transport heat and moisture poleward, and result in surface winds from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere. This gives rise to a parade of transient eddies that are responsible for weather disturbances. (These are what we see on continent-wide weather maps.)

Near the poles, cold dry air descends in vortices to create high-pressure regions in which the downward-flowing air diverges toward the equator. These high-pressure regions are the polar cells. The Coriolis force directs the air westward in a surface wind system called the polar easterlies, which encounter the Ferrel cell westerlies along the polar front, creating a zone of unstable weather.

Jet streams

These are high-speed, horizontal rivers of wind some hundreds of kilometers wide and several kilometers thick. The polar jet streams (Figure 5), with peak winds of about 300 kilometers (186 miles) per hour, undulate back and forth near the polar fronts at an altitude of about 10 kilometers (32,000 feet); they are in essence the upper tropospheric manifestations of the polar fronts. The slow undulations affect weather patterns between latitudes of about 45° and 60° and influence the paths of the weather fronts we hear about in daily forecasts.

A Powerful Stream
Figure 5: A Powerful Stream
This satellite image shows smoke from a fire being entrained in the polar jet stream and transported across the continent. Note the smoke trail dipping southward near Minnesota to form a haze over the southeastern U.S. The clouds below the haze display Ferrel cell eddy circulation. ©NASA / GSFC / SeaWiFS

Without the atmosphere, there would be no climate. Incoming solar energy warms the atmosphere, which in turn acts as an insulating blanket to keep Earth warm. Along with the ocean, movements of the atmosphere (winds) distribute heat around the planet. The atmosphere also connects the different parts of the climate system. For example, the atmosphere transports water from ocean to land, and it is an important pathway for carbon to move between land and ocean. The importance of these connections in determining climate will become apparent in Essay 1.3.

This essay is condensed from Climate Change: The Science of Global Warming and Our Energy Future (2009) by Edmond A. Mathez


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