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An Introduction to the Global Circulation of the Atmosphere
By David Randall PRINCETON UNIVERSITY PRESS
Copyright © 2015 Princeton University Press
All rights reserved.
ISBN: 978-1-4008-6652-6
CHAPTER 1
Perpetual Motion
The atmosphere circulates. The circulation is global in extent (see fig. 1.1). The circulating mass consists of "dry air" and three phases of water. Energy and momentum are carried with the air but evolve in response to various processes along the way. Many of those same processes add or remove moisture.
The circulation is sustained by thermal forcing, which ultimately comes from the Sun. On the average, the Earth absorbs about 240 W m-2 of incoming or "incident" solar energy, of which roughly 2% is converted to maintain the kinetic energy of the global circulation against frictional dissipation. Additional, "primordial" energy leaks out of the Earth's interior but at the relatively tiny rate of 0.1 W m-2 (Sclater et al., 1980; Bukowinski, 1999). The thermal forcing of the global circulation is strongly influenced by the circulation itself—for example, as clouds form and disappear. The interactions between the circulation and the heating are fascinating but complicated.
Averaged over time, the global circulation has to satisfy various balance requirements: for example, the infrared radiation emitted at the top of the atmosphere must balance the solar radiation absorbed, precipitation must balance evaporation, and angular momentum exchanges between the atmosphere and the ocean–solid Earth system must sum to zero. We will discuss the global circulation from this classical perspective. We will also supplement this discussion with descriptions and analyses of the many and varied but interrelated phenomena of the circulation, such as the Hadley and Walker circulations, monsoons, stratospheric sudden warmings, the Southern Oscillation, subtropical highs, and extratropical storm tracks. In addition, we will discuss the diabatic and frictional processes that maintain the circulation, and the ways in which these processes are affected by the circulation itself.
The circulations of energy and water are closely linked. It takes about 2.5 × 106 J of energy to evaporate 1 kg of water from the oceans, and the same amount of energy is released when the water vapor condenses to form a cloud. The energy released through condensation drives thunderstorm updrafts that in one hour or less can penetrate a layer of the atmosphere 10 or even 20 km thick. The cloudy outflows from such storms reflect sunlight to space and block infrared radiation from the warm surface below. Shallower clouds cast shadows over vast expanses of the oceans. One of the aims of this book is to give appropriate emphasis to the role of moisture in the global circulation of the atmosphere.
It is conventional and useful, although somewhat arbitrary, to divide the atmosphere into parts. For purposes of this quick sketch, we will divide the atmosphere vertically and meridionally, only briefly mentioning the longitudinal variations. Let's start at the bottom.
Most of the solar radiation that the Earth absorbs is captured by the surface rather than within the relatively transparent atmosphere. Several processes act to transfer the absorbed energy upward from the ocean and land surface into the lower portion of the atmosphere.
The layer of air that is closely coupled with the Earth's surface is, by definition, the planetary boundary layer, or PBL. The top of the PBL is often very sharp and well defined (see fig. 1.2). The depth of the PBL varies considerably in space and time, but a ballpark value to remember is 1 km. The air in the PBL is turbulent, and the turbulence is associated with rapid exchanges or "fluxes" of sensible heat (essentially temperature), moisture, and momentum between the atmosphere and the surface. These exchanges are produced by the turbulence, and also promote the turbulence, through mechanisms that will briefly be discussed later. The most important exchanges are of moisture, upward into the atmosphere via evaporation from the surface, and of momentum, via friction. The latent heat associated with the surface moisture flux is a key source of energy for the global circulation, and surface friction strongly influences the ocean currents.
Above the PBL is the free troposphere. Because the troposphere includes the PBL, we add the adjective "free" to distinguish the part of the troposphere that lies above the PBL. The free troposphere is characterized by positive static stability, which means that buoyancy forces resist vertical motion. The depth of the troposphere varies strongly with latitude and season.
A turbulent process called entrainment gradually incorporates free-tropospheric air into the PBL. Over the oceans, entrainment is, with a few exceptions, relatively slow but steady. Over land, entrainment is promoted by strong daytime heating of the surface, which helps generate turbulence. As a result, the turbulent PBL rapidly deepens during the day. When the Sun goes down, the processes that promote turbulence and entrainment are abruptly weakened, and the PBL reorganizes itself into a much shallower nocturnal configuration, leaving behind a layer of air that was part of the PBL during the afternoon. This diurnal deepening and shallowing of the PBL acts as a kind of "pump" that captures air from the free troposphere and adds it to the PBL starting shortly after sunrise, modifies the properties of that air during the day through strong turbulent exchanges with the surface, and then releases the modified air back into the free troposphere at sunset. This diurnal pumping is one way that the PBL exerts an influence on the free troposphere.
In addition, moisture and energy are carried upward from the PBL into the free troposphere by several mechanisms. Throughout the tropics and the summer-hemisphere middle latitudes the most important of these mechanisms is cumulus convection. Cumulus clouds typically grow upward from the PBL. The updrafts inside the clouds carry PBL air into the free troposphere, where it is left behind when the clouds decay (see fig. 1.3). One of the effects of this process is to remove air from the PBL and add it to the free troposphere.
Frontal circulations also can carry air from the PBL into the free atmosphere, essentially by "peeling" the PBL from the Earth's surface, like the rind from an orange, and lofting the detached air toward the tropopause. This process is especially active in the middle latitudes in winter.
Figure 1.4 shows somewhat idealized observed midlatitude vertical distributions of temperature, pressure, density, and ozone mixing ratio, from the surface to the 70 km level. In the lowest 12 km, the troposphere, the temperature decreases (almost) monotonically with height. The troposphere is cooled radiatively, because it emits infrared radiation much more efficiently than it absorbs solar radiation. The net radiative cooling is balanced mainly by the release of the latent heat of water vapor as clouds form and precipitate.
The upper boundary of the troposphere is called the tropopause. The height of the tropopause varies from 17 km or so in some regions of the tropics to about half that near the poles. Above the tropopause, the temperature becomes uniform with height and then begins to increase with altitude in the region known as the stratosphere. The temperature increase is due to the absorption of solar radiation by ozone, which is created in the stratosphere by photochemical processes. Without ozone there would be no stratosphere. The summer-hemisphere stratosphere is almost devoid of active weather and has warm air over the pole. The winds of the summer-hemisphere stratosphere are predominantly easterly; that is, they blow from east to west. In contrast, the winter-hemisphere stratosphere experiences much more active weather, mainly owing to waves propagating upward from the troposphere below; has very cold air over the pole; and experiences strong westerly winds. During winter, the polar stratosphere is occasionally disturbed by sudden stratospheric warmings, which are dramatic changes in temperature (and wind) that occur sporadically in the Northern Hemisphere and much less frequently in the Southern Hemisphere.
Even though the stratosphere is very dry, its moisture budget is quite interesting. It receives small amounts of moisture from the troposphere and also gains some moisture through the oxidation of methane. The upper boundary of the stratosphere, called the stratopause, occurs near the 1 hPa (~50 km) level. In this book we focus mainly on the tropospheric circulation. We discuss selected aspects of the stratospheric circulation, but we do not discuss the portion of the atmosphere that resides above the stratopause.
For meteorological purposes, the tropics can be defined as the region from about 20° S to 20° N. Although the tropical temperature and surface pressure are remarkably uniform in space and temporally monotonous, the winds and rainfall are quite variable. In many parts of the tropics deep cumulus and cumulonimbus clouds—that is, thunderstorms—produce lots of rain and transport energy, moisture, and momentum vertically, essentially continuing the job begun closer to the surface by the turbulence of the PBL. The convective clouds often produce strong exchanges of air between the PBL and the free troposphere, in both directions: positively buoyant PBL air "breaks off" and drifts upward to form the cumuli, while negatively buoyant downdrafts associated with the evaporation of falling rain can inject free-tropospheric air into the PBL. In the convectively active parts of the tropics, the air is slowly rising in an area-averaged sense.
The mean flow in the tropical PBL is easterly. This is the trade wind regime. The tropical temperature and surface pressure distributions are generally very flat and monotonous, for simple dynamical reasons (discussed in chapter 3) that are connected to the smallness of the Coriolis parameter in the tropics. The tropical moisture and wind fields are more variable than the temperature, however. The tropics is home to a variety of distinctive traveling waves and vortices that organize the convective clouds on scales of hundreds to thousands of kilometers. Finally, the tropics is dominated by powerful and very large-scale monsoon systems that extend into the subtropics and even middle latitudes.
The tropical atmosphere acquires the angular momentum of the Earth's rotation from the continents and oceans. The global atmospheric circulation carries the angular momentum to higher latitudes, where it is "put back" into the continents and oceans.
The tropics is home to some circulation phenomena that do not occur in higher latitudes. Most famously, tropical cyclones produce tremendous amounts of rainfall and strong winds. They are relatively small in scale and highly seasonal. In contrast, monsoons are driven by seasonally varying continental-scale land-sea contrasts. The Madden-Julian Oscillation, or MJO, is a powerful tropical weather system that influences rainfall across about half of the tropics. El Niño, La Niña, and the Southern Oscillation, collectively known as ENSO, make up a strong, quasi-regular oscillation of the ocean-atmosphere system, with a period of a few years (Philander, 1990). In an El Niño, the sea-surface temperatures warm in the eastern tropical Pacific, while in a La Niña they cool. The Southern Oscillation is a shift in the pressure and wind fields of the tropical Pacific region that occurs in conjunction with El Niño and La Niña. The tropical stratosphere features an amazing periodic reversal of the zonal (i.e., west-to-east) wind, called the Quasi-Biennial Oscillation, or QBO. Its period is slightly longer than two years.
The subtropical portion of each hemisphere is roughly the region between 20° and 30° from the equator. In many parts of the subtropical troposphere, the air is sinking in large anticyclonic circulation systems called, appropriately enough, subtropical highs. The subsidence suppresses precipitation, which is why the major deserts of the world are found in the subtropics. Surface evaporation is very strong over the subtropical oceans, which have extensive systems of weakly precipitating shallow clouds. The subtropical upper troposphere is home to powerful subtropical jets, which are westerly currents that are particularly strong in the winter hemisphere (see fig. 1.5).
The tropical rising motion and subtropical sinking motion can be seen as the vertical branches of a "cellular" circulation in the latitude-height plane. This Hadley circulation transports energy and momentum poleward, and it transports moisture toward the equator. The Hadley circulation interacts strongly with the monsoons.
The region that we call the middle latitudes extends, in each hemisphere, from about 30° to 70° from the equator. There the surface winds are primarily westerly. The midlatitude free troposphere is filled with vigorous weather systems called baroclinic eddies, which have scales of a few thousand kilometers and grow through a process in which warm air shifts upward and poleward and is replaced by colder air that descends as it slides toward the equator (see fig. 1.6). These storms transport energy and moisture poleward and upward, primarily in the winter but also to some extent during summer. They transport westerly momentum both meridionally and downward. The downward momentum flux drives currents in the oceans and rustles the leaves on trees. The energy of the storms is derived from horizontal temperature differences that can be sustained only outside the tropics. The storms produce massive cloud systems and heavy precipitation.
On the average, the polar troposphere is characterized by sinking motion and radiative cooling to space. The North Pole is in the Arctic Ocean, which is covered with sea ice (see fig. 1.7) and is often blanketed by extensive cloudiness, while the South Pole is in the middle of a dry, mountainous continent (see fig. 1.8). Near the surface, the polar winds tend to be easterly, but weak.
The polar regions and middle latitudes are home to prominent annular modes, which by definition are visible even when the data are averaged over all longitudes. The annular modes fluctuate on a variety of timescales, almost uniformly in longitude. They are seen in both the stratosphere and troposphere, and make major contributions to the variability of the global circulation.
Overall, the atmosphere cools radiatively, and this cooling is balanced primarily by the release of latent heat, which in turn is made possible by surface evaporation. The net flow of energy is upward and toward the poles, carried by thunderstorms and the Hadley circulation in the tropics and by baroclinic eddies in middle latitudes. The energy escapes to space via infrared radiation at all latitudes, but especially in the subtropics.
This book is organized as follows. Chapter 2 provides an overview of the upper and lower boundary conditions on the global circulation. At the "top of the atmosphere," the observed pattern of radiation implies net poleward energy transports by the atmosphere and ocean together. Lower boundary conditions include the distributions of oceans, continents, and mountains; the pattern of sea-surface temperature and the directly related pattern of sea-surface saturation vapor pressure; the heat capacity of the surface; the distribution of vegetation on the land surface; and the distributions of sea ice, continental glaciers, and ice sheets. These lower boundary conditions strongly affect the flows of energy and moisture across the Earth's surface. The chapter closes with a brief overview of the vertically integrated energy and moisture budgets of the surface and atmosphere, and the connections between them.
Chapter 3 introduces some basic aspects of the global circulation from an observational perspective, starting with the global distribution of atmospheric mass, then progressing to winds, temperature, and moisture. The amount of interpretation is deliberately kept to a minimum in the chapter.
While chapter 3 focuses on the observations, chapter 4 presents a brief but intensive review of theory to be used later in the book, beginning with a review of the dynamics of fluid motion on a rotating sphere. Angular momentum conservation is derived, and energy transports and transformations are discussed in detail. The key subject of potential vorticity is introduced near the end of the chapter. The quasi-geostrophic approximation and the shallow water equations are also presented.
Chapter 5 discusses how the zonally averaged circulation is influenced by sources and sinks of moisture, energy, and momentum. The chapter also takes a first look at the effects of eddies that transport dry air, moisture, energy, and momentum along isentropic surfaces, that is, on which the entropy is uniform.
(Continues...)
Excerpted from An Introduction to the Global Circulation of the Atmosphere by David Randall. Copyright © 2015 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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