CHAPTER 1

An Introduction tothe Climate System

Climate dynamics is the scientific study of how and why climate changes. Theintent is not to understand day-to-day changes in weather but to explain averageconditions over many years. Climate processes are typically associatedwith multidecadal time scales, and continental to global space scales, but onecan certainly refer to the climate of a particular city.

Climate dynamics is a rapidly developing field of study, motivated by the realizationthat human activity is changing climate. It is necessary to under standthe natural, or unperturbed, climate system and the processes of human-inducedchange to be able to forecast climate so that individuals and governments canmake informed decisions about energy use, agricultural practice, water resources,development, and environmental protection.

Climate has been defined as "the slowly varying aspects of the atmosphere/hydrosphere/lithosphere system." Other definitions of climate might also explicitlyinclude the biosphere as part of the climate system, since life on theplanet plays a well-documented role in determining climate. Anthropogenicclimate change is just one example, but there are others, such as the influenceof life on the chemical composition of the atmosphere throughout its 4.5billion–year life span.

The word climate is derived from the Greek word klima, which refers to theangle of incidence of the sun. This is a fitting origin because solar radiation isthe ultimate energy source for the climate system. But to understand climatewe need to consider much more than solar heating. Processes within the earthsystem convert incoming solar radiation to other forms of energy and redistributeit over the globe from pole to pole and throughout the vertical expansesof the atmosphere and ocean. This energy not only warms the atmosphere andoceans but also fuels winds and ocean currents, activates phase changes ofwater, drives chemical transformations, and supports biological activity. Manyinteracting processes create the variety of climates found on the earth.

A schematic overview of the global climate system is provided in Figure1.1. This diagram represents the climate system as being composed of fivesubsystems—the atmosphere, the hydrosphere, the biosphere, the cryosphere,and the land surface. It also depicts processes that are important for determiningthe climate state, such as the exchange of heat, momentum, and wateramong the subsystems, and represents the agents of climate change.

Figure 1.1 provides an excellent summary of the climate system, and it isuseful as a first-order, nontechnical description. At the other end of the spectrumis the Bretherton diagram, shown in Figure 1.2. This detailed, perhaps abit overwhelming, schematic was constructed to characterize the full complexityof climate. It is a remarkable and rich representation of the system, illustratingthe many processes that influence climate on all time scales. It coalesceshistorically separate fields of scientific inquiry—demonstrating that not onlyatmospheric science and oceanography are relevant to climate science but thatvarious subdisciplines of geology, biology, physics, and chemistry—as well asthe social sciences—are all integral to an understanding of climate.

This is a very exciting and critical time in the field of climate dynamics.There is reliable information that past climates were very different from today'sclimate, so we know the system is capable of significant change. We alsounderstand that it is possible for the system to change quickly. The chemicalcomposition of the atmosphere is changing before our eyes, and satellite-andearth-based observing networks allow us to monitor changes in climate fairlyaccurately.

Clearly, this one text on climate dynamics cannot cover the full breadthof this wide-ranging and rapidly developing field, but it provides the readerwith the fundamentals—the background needed for a basic understanding ofclimate and climate change, and a launchpad for reading the scientific literatureand, it is hoped, contributing to the profound challenge before humanityof managing climate change. With this fundamental understanding, science canaddress the questions, needs, and constraints of society in a reasonable anduseful way, and offer informed answers to guide society's behavior.

CHAPTER 2

The Observed Climatology

This chapter forms a concise atlas of the climate system. An overview of thesystem is presented using the variables and terminology commonly used tocharacterize climate. These terms are referenced in subsequent chapters as adeeper understanding of climate processes is developed.

Some features of the climate system are known accurately, while othersare known only approximately. Climate observations can be limited by insufficientspatial and temporal resolution, inadequate global coverage, or a lackof long-term records. Precipitation observations are a good example. Becauseof the high variability of precipitation over a wide range of space and timescales, the observing requirements for establishing a precipitation climatologyare demanding. Global measurements of precipitation or, more accurately,measurements of radiative fluxes that can be translated into rainfall rates havebeen available only since the beginning of the satellite era in the early 1970s.Pre-satellite coverage over vast regions of the oceans was particularly sparse,especially in regions where ships rarely traveled. Establishing a climatologyfor other variables, such as evaporation and soil moisture, is even morechallenging.

Many of the figures in this text were drawn using reanalysis products,which combine simulations using state-of-the-art numerical models with observations.To produce a reanalysis climatology, computer models are run tostimulate many decades, with observed fields incorporated into the model atthe time they were observed. This process is called four-dimensional data assimilation,for the three spatial dimensions plus time. (Data assimilation is alsoused routinely in generating weather forecasts.) Thus, the reanalysis product isnot pure observations but a blend of observations and computer model output.Reanalysis values of variables that are assimilated—for example, winds andtemperatures—are accurate. Other variables, however, are model-dependentoutput and may not be as reliable. Sometimes, as in the case of evaporation,the reanalysis product is the best information available with global coverage.For other variables, ground-based and satellite observations, if available, arepreferred to the reanalysis product.

Maps of climate variables use latitude and longitude as coordinates with anequidistant cylindrical projection. Keep in mind that the area of middle andhigh latitudes is falsely large in this projection. In reality, half the surface areaof the globe lies between 30°N and 30ºS latitude, whereas in the figures thisregion occupies only one-third of the area. Vertical profiles of climate variablesare also shown, averaged globally or over certain regions using area weightingto correctly account for the decreasing distance between meridians (linesof constant longitude) away from the equator. Another useful way to displayclimate variables is as the zonal mean, in which the variable is averaged overall longitudes and displayed in the latitude/height plane.

The international system of units (SI) is used as reviewed in Appendix A.

2.1 The Atmosphere

We begin our description of the atmosphere with air pressure. Pressure is definedas "force per unit area" and is expressed in SI units of pascals (abbreviatedPa). Pressure is simply the weight of the overlying mass, m, per unit area:

p [equivalent to] mg/area [??] Pa~[newton/m²] =(kg · m)/s²/m² = kg/m · s², (2.1)

where g is the acceleration due to gravity. Figure 2.1 shows the global distributionof surface pressure in units of hectapascals (hPa), where 100 Pa = 1 hPa.This figure is not helpful for learning about the atmosphere, however, becausesurface topography dominates the distribution. Surface pressure is lowest overthe highest mountains, and high and uniform over the oceans, because the overlyingair column is thinner (less massive) at higher elevations. Consequently,surface pressures in the Himalayan Mountains and over Antarctica drop below600 hPa but are close to 1000 hPa everywhere over the world's oceans.

Figure 2.1 demonstrates the close connection between pressure and elevation.Pressure is often used as a vertical coordinate in describing the atmosphere,replacing elevation, z. The average relationship between p and z inthe earth's atmosphere, typical of large space and time scales, is in Figure 2.2.Note that p is not a linear function of z, that is, p ≠ az + b, where a and b areconstants. Instead, pressure decreases exponentially with height.

In the figures that follow, atmospheric variables are presented on surfaces ofconstant pressure, or isobars, instead of surfaces of uniform elevation. Figure2.2 can be used to estimate the altitude of any pressure surface. Where topographyextends up into the atmosphere, certain pressure levels may not exist.For example, since the surface pressure over Antarctica is 700 hPa or lower(see Fig. 2.1), there is no 900 hPa surface. Such regions may be specified as havingmissing data, or the data may be extrapolated to fill in the missing values.

When pressure is substituted for height as an independent variable, thenthe height of the pressure level becomes a dependent variable. (Recall that independentvariables are the coordinate axes, and dependent variables describethe system. For the atmosphere, temperature and wind speeds are examples ofdependent variables.) It is common to use geopotential height, Z, as the independentvariable instead of height, z, where

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (2.2)

In Eq. 2.2, g₀ is the acceleration due to gravity at the surface of the earth.Because the gravitational attraction between two bodies depends on the distancebetween them, the acceleration due to gravity decreases with increasingheight—or decreasing pressure—in the atmosphere. At the earth's surface,g = g₀ = 9.81 m/s². At 10 km elevation, g = 9.77 m/s². This 0.4% reduction in gwithin the lower atmosphere is relatively small, so g can be taken as constantin Eq. 2.2. With this assumption, geopotential height, Z, can be interpreted asthe elevation, z, of a pressure level.

Geopotential height is closely related to the gravitational potential energy,Φ, given by

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (2.3)

(Recall that work, which is a form of energy, is "force × distance".) Then Φevaluated at some altitude z is the work that was done against gravity to lift aunit mass of air from the surface to that altitude. Equivalently, it is the "potential"energy that would be extracted if the unit mass were to fall to the surface.In common practice, Φ is referred to simply as the geopotential.

The annual mean geopotential height climatology at 900 and 200 hPa isdisplayed in Figure 2.3. Note the following:

• The 900 hPa surface is roughly 800 m from the surface in the subtropics(near 30°N and 30°S) and slopes down closer to the surface at higherlatitudes and near the equator.

• Geopotential height surfaces at 200 hPa are more than 12 km from thesurface near the equator and slope down below 11 km at higher latitudes inboth hemispheres.

• At 900 hPa, the highest geopotential heights are preferentially located overthe oceans.

• The equatorial trough is the region of relatively low geopotential heightsdeep in the tropics.

• Poleward of the subtropical highs in both hemispheres, geopotential heightsdecrease through the midlatitudes all the way to the poles. (Geopotentialheight values at 900 hPa over East Antarctica are not realistic because thetopography rises above this level.)

• The presence of the continents disturbs the zonal (east/west) uniformity ofthe geopotential height lines.

The distribution of geopotential heights has strong seasonal dependence, particularlyat lower levels. To represent seasonal changes in this field and others,climatologies for the December, January, and February mean, designated DJF, areused to display Northern Hemisphere winter and Southern Hemisphere summer.For the opposite season, June, July, and August averages are displayed, and denotedas JJA. Seasonal ranges are quantified by plotting the difference DJF-JJA.

DJF and JJA geopotential heights at 900 hPa are shown in Figures 2.4a andb, respectively, and their difference is in Figure 2.4c.

• Geopotential heights are higher over the oceans than over the land in thehemisphere experiencing summer; the opposite is the case in the winterhemisphere.

• Seasonal differences in geopotential heights are greater over land surfacesthan over the oceans.

• The subtropical highs centered over the oceans that were noted in the annualmean (Fig. 2.3) are primarily a summer feature in the Northern Hemisphere.

• In the Southern Hemisphere, geopotential heights are more zonally uniformin the winter (JJA), and located slightly closer to the equator.

• The monsoon regions of the world—southeastern Asia, northern Africa,tropical South America, the southwestern United States, and Australia—arecharacterized by low geopotential heights in the summer.

Geopotential height distributions at 200 hPa are displayed in Figure 2.5.The lines of constant geopotential height are much more zonally uniform thanat the 900 hPa level, but some effects of continentality are still discernible.

• The low-level monsoon lows at 900 hPa are overlain by regions of highgeopotential heights. This means that the distance between the 900 hPa and200 hPa levels, known as the thickness, is greater in these regions.

• North of about 45°N in DJF there is a wavy pattern in the geopotentialheight lines, with two wave cycles encircling the globe. This pattern bringslow heights to lower latitudes on the east coasts of Asia and North America,along with high geopotential height gradients. These are the storm tracks ofthe Northern Hemisphere, running southwest to northeast over the NorthPacific and Atlantic Oceans.

• The most pronounced seasonal differences at this level occur overnortheastern Asia (Siberia) and, to a lesser degree, northern North America.

Figure 2.6 portrays the annual mean air temperature climatology at 900hPa. Note the following features:

• Annual mean air temperatures over land tend to be warmer than over theoceans at the same latitude. (Note how the isotherms curve poleward overthe continents in both hemispheres.)

• The lowest annual mean surface air temperatures on the planet are locatedover Antarctica, where they are about 60 K colder than the warmesttemperatures over northern Africa.

• Surface air temperatures on the eastern sides of the Atlantic and PacificOcean basins are cooler than over the western sides of the basins in bothhemispheres. (Note how the isotherms dip equatorward over the easternsides of the Pacific and Atlantic Ocean basins.)

• Continentality, defined as the effects of land/sea distributions on climatevariables, is in evidence in the temperature field, as it was for thegeopotential height distributions shown above.

• The thermal equator marks the latitude of maximum temperature. Note thatit is not necessarily located on the geographical equator and tends to favorthe Northern Hemisphere.

• Meridional temperature gradients, indicated by the density of the isothermsin the north/south direction, are greater in middle and high latitudes than inthe tropics.

Seasonal air temperatures at 900 hPa and their differences are displayed inFigure 2.7.

• Seasonality, defined here as differences in climate variables between DJF andJJA, is larger over the continents than over the oceans, and greater at highlatitudes than at low latitudes.

• Seasonality is much more pronounced in the Northern Hemisphere thanin the Southern Hemisphere in association with the distribution of thecontinents, except over Antarctica.

• Air temperatures over land are warmer than temperatures over the oceansin the summer hemisphere, and generally cooler in the winter hemisphere.Recall from Figure 2.6 that the annual mean air temperature is warmer overland than over oceans.

• The east–west temperature gradients over the tropical oceans are in placethroughout the year.

Temperature profiles provide more detail about the vertical structure ofthe atmosphere. Figure 2.8 shows the globally and annually averaged temperatureprofile through the height of the earth's atmosphere. The regionof the atmosphere from the surface to roughly 12 km is termed the troposphere,which is derived from the Greek root meaning "the turning or changingsphere." This region is where weather systems exist and is the focus ofour study of climate dynamics. Temperature decreases with height, z, in thetroposphere, from a globally averaged surface air temperature of 287.5 K toabout 218 K at the top of the troposphere, which is known as the tropopause.Pressure decreases from about 1000 hPa at the surface (the globally averagedvalue is generally taken as 1013 hPa) to about 200 hPa at the tropopause.Almost all the atmosphere's water resides in the troposphere, as does about80% of its mass.

The stratosphere lies above the tropopause, extending to about 48 km or 1hPa, and is capped by the stratopause. It is a vertically stable, stratified region —henceits name—in which temperature increases with altitude. The mesosphere("middle sphere") stretches from the stratopause to about 80 km, with temperatureagain decreasing with height. The region of transition to inter planetaryspace, above 80 km, is the thermosphere.