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furthermore, enhances the greenhouse warming of CO

in the troposphere through

substantial downward thermal emissions to the troposphere.

How changes of ozone change stratospheric and tropospheric radiative heating

depends on the amounts of overlying ozone and, for thermal effect, on pressure and

radiative upwelling depending on underlying temperatures.

Besides radiative processes, stratospheric climate is charact erized by its tempera-

ture and wind patterns and by the chemical composition of its trace gases. At

midstratosphere, temperature increases from winter pole to summer pole with an

accompanying eastward jet stream in the winter hemisphere extending upward from

the tropospheric jet steam. This wind conﬁguration allows planetary wave distur-

bances to propagate into the stratosphere, contributing signiﬁcant temporal and

longitudinal variabilities. Conversely, the westward jet, found in the summer strato-

sphere attenuates wave disturbances from below, and so is largely zonally

symmetric, changing only with the seasonal heating patterns.

6 THE CRYOSPHERE

The term cryosphere refers to the components of the climate system dominated by

water in its frozen phase, that is, in high latitudes and extratropical winter conditions.

Elements include snow, its distribution and depths, sea ice, its distribution and

properties, high-latitude ice caps, and temperate glaciers. The largest volume of

frozen water is stored in ice caps, and glaciers. This storage acts to remove water

from the oceans. How it changes with climate change is, hence, of interest for

determining changing sea levels.

Ice is highly reﬂective of sunlight, especially in crystal form. The loss of solar

heating because of this high albedo acts to substantially reduce high-latitude

temperatures especially in spring and early summer where near-maximum solar

radiation sees white snow-covered surfaces. This high albedo can be substantially

masked by cloud cover and, over land, tall vegetation such as conifer forests.

7 THE OCEAN

Oceans are a major factor in determining surface temperatures and ﬂuxes of water

into the atmosphere. They store, release, and transport thermal energy, in particular,

warming the atmosphere under wintertime and high- latitude conditions, and cooling

it under summer and tropical conditions.

How the oceans carry out these services depends on processes coupling them to

the atmosphere. At mospheric winds push the oceans into wind-driven circulation

systems. Net surface heating or cooling, evaporation, and precipitation determine

oceanic densities through controlling temperature and salinity, hence oceanic buoy-

ancy. This net distribution of buoyancy forcing drives ‘‘thermohaline’’ overturning of

the ocean, which acts to transport heat. Climate of the surface layers of the ocean

includes the depth to which waters are stirred by waves and net heating or cooling.

126 OVERVIEW: THE CLIMATE SYSTEM

Heating acts to generate shallow warm stable layers, while cooling deepens the

surface mixed layers. Under some conditions, convective overturning of cold

and=or high-salinity water can penetrate to near the ocean bottom.

REFERENCES

Grotjahn, R. (1993). Zonal average observations, Chapter 3, in Global Atmospheric

Circulations: Observations and Theories, New York, Oxford University Press.

Hartmann, D. L. (1994). Atmospheric temperature, Chapter l.2, in Global Physical

Climatology, San Diego, Academic Press.

Holton, J. R. (1992). The observed structure of extratropical circulations, Chapter 6.1, in An

Introduction to Dynamic Meteorology, San Diego, Academic.

Kiehl, J. T. and K. E. Trenberth (1997). Earth’s annual global mean energy budget, J. Clim.

78, 197–208.

Peixoto, J. P., and A. H. Oort (1992). Observed atmospheric branch of the hydrological cycle,

Chapter 12.3, Physics of Climate, New York, American Institute of Physics.

REFERENCES 127

CHAPTER 10

THE OCEAN IN CLIMATE

EDWARD S. SARACHIK

1 INTRODUCTION

Earth’s present climate is intrinsically affected by the ocean—the climate without the

ocean would be different in many essential ways: Without the evaporation of water

from the sea surface, the hydrological cycle would be different; without ocean heat

transport and uptake, the temperature distribution of the globe would be different;

and without the biota in the ocean, the total amount of carbon in the atmosphere

would be many time its current value. Yet, while we may appreciate the role of the

ocean in climate, the difﬁculty and expense of making measurements below the

ocean’s surface has rendered the vast volume of the ocean a sort of mare incognita.

Why is the ocean so important in Earth’s climate, and which of its properties are of

special signiﬁcance for climate? How have we learned about the ocean and its role in

climate, and what more do we need to know?

2 PROPERTIES OF THE OCEAN AND PROCESSES IN THE OCEAN

The ocean covers 70% of Earth’s surface to an average depth of about 4 km. The

mass of the ocean is 30 times and its heat capacity 120 times that of the atmosphere,

and the ocean contains 80 times the carbon dioxide stored in the atmosphere.

The density of the ocean is controlled both by its temperature and by its salt

content. Ocean density increases with salinity and decreases with temperature.

Unlike fresh water, which has a maximum density at 4



C (so that colder water

and ice ﬂoat on 4



C water and the temperature at the bottom of a nonfrozen lake

is 4



C), water saltier than 26 parts per thousand of water is continuously denser as

Handbook of Weather, Climate, and Water: Dynamics, Climate, Physical Meteorology, Weather Systems,

and Measurements, Edited by Thomas D. Potter and Bradley R. Colman.

ISBN 0-471-21490-6 # 2003 John Wiley & Sons, Inc.

129

the temperature is lowered, and the temperature a t the bottom of the ocean is

closer to 1



Since heat always diffuses from warm to cool temperatures, why does not the

temperature of the deep ocean eventually adjust and become the same as its surface

temperature? Cold water const antly sinks at high latitudes (both in the Northern and

Southern Hemispheres) and ﬁlls the deeper parts of the oceans with cold water so

that the water at depth is always cold, even when the surface temperature is very

warm. This circulation is called the thermohaline circulation.

About 7% of the ocean surface is covered by sea ice. Growth of sea ice radically

changes the nature of the ocean surface: Sea ice reﬂects solar radiation, thereby

preventing it from being absorbed by the surfa ce and blocks the transfer of heat and

moisture from the surface of the ocean to the atmosphere.

The average salinity in the global oceans is 34.7 parts per thousand salt to water

by weight. As the total amount of salt in the ocean is constant, changes in salinity

only occur because of additions and subtractions of fresh water. Salinity decreases as

rain falls on the ocean or river water enters the ocean, and it increases as water

evaporates from the surface of the ocean. As sea ice grows, it rejec ts salt into the

ocean thereby increasing its salinity. Similar ly, as ice melts, it dilutes the surrounding

ocean and lowers its salinity. A speciﬁc parcel of water can either increase or

decrease its salinity by mixing with parcels with different salinities.

3 HOW THE OCEAN INTERACTS WITH THE ATMOSPHERE TO

AFFECT THE CLIMATE

The ocean interacts with the atmosphere at (or very near) the sea surface where the

two media meet. Visible light can penetrate into the ocean several tens of meters, but

heat, moisture, and momentum, carbon dioxide, and other gases exchange directly at

the surface. Sea ice forms at the surface and helps to determine the local exchanges.

The basic problem of the ocean in climate is to explain these interchanges and to

determine those characteristics of the ocean that affect these exchanges.

The ocean may be considered to interact with the atmosphere in two distinct

ways: passively and actively. It interacts passively when the ocean affects the atmo-

sphere but does not change the essential manner in which the atmosphere is operat-

ing. An example of a passive interaction is the oceanic response to adding CO

to the

atmosphere where the ocean simply delays the greenhouse warming of the atmo-

sphere as heat and CO

enters the ocean.

Active interaction with the atmosphere produces results that would not otherwise

be there—an example is where the warming of the atmosphere reduces the thermo-

haline circulation and produces a climate reaction that could not have been obtained

without the essential interaction of the ocean. In particular, since the northern

branch, say, of the thermohaline circulation brings cold water from high latitudes

toward the equator, and since the water must be replaced by warm water that is

cooled as it moves northward, the net effect of the thermohaline circulation is to

transport heat northward and thereby warm the higher latitudes. As the atmosphere

130 THE OCEAN IN CLIMATE

warms, the water becomes less dense both by the effect of temperature and by

increased rainfall, a necessary concomitant of global warming. Since the atmosphere

sees a reduced north–south temperature gradient at the sea surface, it reacts funda-

mentally differently than if the thermohaline circulation were at full strength.

Our present models of greenhouse warming have the ocean acting in both the

active and passive modes—active when warming leads to a slowed thermohaline

circulation and passive when heat and CO

simply enter the ocean surface and is

therefore lost to the atmosphere. Another example of active interaction is El Nin˜o, a

phenomenon that would not exist were it not for the active interaction of the atmo-

sphere and the ocean (see the chapter by Trenberth). The ocean also has been

inferred (by examining the composition of ancient ice stored in the Greenland and

Antarctic ice sheets) to have, and probably actively take part in causing, climatic

variability on time scales anywhere from decades to a few thousand years, a type of

variability not seen on Earth since it emerged from the last glacial maximum some

18,000 years ago.

4 MEASURING THE OCEAN

The ocean is remarkably poorly measured. While the global atmosphere is constantly

probed and analyzed for the purposes of weather prediction, until recently no such

imperative existed for the ocean. Our ability to measure the ocean is severely limited

basically by the inability of radiation to penetrate very far into the ocean—this

requires direct in situ measurements of the interior of the ocean. The world’s oceano-

graphic research ﬂeet is small and incapable of monitoring the breadth and depth of

the world’s ocean, although valuable research measurements are constantly being

taken at selected places. As a result of ocean observations, we know the basic path-

ways of water in the ocean, we have a good idea of the transports by the ocean, we

have some idea of the basic mechanisms for much of the major ocean currents, and

we have a good idea of how the exchanges at the surface are accomplished. Yet we

cannot measure vertical velocity (it is far too small), and so we remain completely

ignorant of the processes by which the interior of the ocean affects the surface.

Similarly, we are ignorant of the basic processes of mixing and friction in the

ocean, both basic to being able to model the ocean for climate purposes.

Major oceanographic programs have been conducted in the last d ecade (the

World Ocean Circulation Experiment, WOCE) and (the Tropical Ocean–Global

Atmosphere, TOGA), and while they have taught us much about the ocean circula-

tion and El Nin˜o, respectively, the basic lesson is that, unless we can make contin-

uous long-term measurements beneath the surface of the ocean, it will forever

remain unknown territory.

5 MODELING THE OCEAN

Because the ocean is poorly measured, and because much of what we need to know

about the past and predict about the future cannot be directly known, only inferred,

5 MODELING THE OCEAN 131

models have played a particularly important role in the development of oceano-

graphy and, in particular, the role of the ocean in climate.

The basic tool of climate studies is the coupled model, where the various compo-

nents of the climate system—the atmosphere, ocean, cryosphere, biosphere, and

chemosphere—are simultaneously and consistently coupled. The ocean component

of such a coupled model exchanges its heat, fresh water, and momentum with the

atmosphere at the sea surface. The test of the successful coupling of the atmosphere

and ocean is the correct simulation of the time-varying sea surface temperature and

surface winds, both of which are relatively easy to measure: Directly by ship or

mooring, remotely by satellite, or by a combination of the two.

The development of off-line ocean-only models requires the heat, momentum,

and freshwater forcing from the atmosphere to be known. Since precipitation and

evaporation, in particular, are so poorly measured over the ocean, it is a continual

struggle to know whether errors in the ocean model are due to errors in the model

itself or errors in the forcing of the ocean by the atmosphere.

Ocean models themselves are relatively simple in concept: The known equations

of water and salt are discretized and time stepped into the future. The discretization

process requires a trade-off between ﬁne resolution for accuracy and the need to

simulate over long periods of time, which, because of limited computer resources,

requires coarser resolution. While the equation of state of seawater relating density

to salt, temperature, and pressure cannot be written down simply, it has, over the

course of time, become known to high accuracy.

What makes ocean modeling difﬁcult is the speciﬁcation of those mixing

processes that unavoidably cannot be resolved by whatever resolution is chosen.

We are beginning to understand that enhanced small-scale mixing occurs near

bottom topography and near boundaries: Purposeful release experiments, where a

dye is released and then followed in time to see how the dye cloud evolves, has

revealed this to us. Larger scale mixing, where parcels are interchanged because of

the large-scale circulation (but still unresolved by the ocean models) itself is more

problematic, but recent advances in parameterizing these unresolved mixing effects

have shown promise.

6 THE FUTURE OF THE OCEAN IN CLIMATE

It is clear that the ocean is a crucial component of the climate system. Since so much

of what is not known about the past and future of the climate system depends on

active interactions with the ocean, it is clear that we have to learn more about

its essential processes. How to go about learning about the ocean is the difﬁcult

question.

Direct measurements are very expensive, and satellites, while giving a global look

at the entire ocean, see only its surface. Designs are currently underway for a Global

Ocean Observing System (GOOS), but the cost of implementing such a system in

toto is prohibitive, even if shared among the wealthier countries of the world.

132 THE OCEAN IN CLIMATE

It is likely that a combi nation of studies, perhaps conducted for entirely different

purposes, will advance the ﬁeld most rapidly. In particular, the advent of the

El Nin˜o–Southern Oscillation (E NSO) prediction, which requires subsurface

ocean data as initial conditions, has made almost permanent the Tropical Atmo-

sphere-Ocean (TAO) array in the tropical Paciﬁc, giving an unprecedented and

continuous view of a signiﬁcant part of the tropical ocean. We may extend the

reasoning to say that, where predictability is indicated and shows societal or

economic value, the measurement systems to produce the initial data will almost

certainly be implemented. The promise of predicting climate from seasons to a few

years will expand the ocean-observing system considerably. Additional expansions

will come from resource monitoring, pollution monitoring, and various types of

monitoring for national security purposes. While monitoring for security has tradi-

tionally meant the data is classiﬁed, once taken, data can eventually reach the

research arena—the vast amount of Soviet and U.S. data that was declassiﬁed

after the end of the cold war has shown this.

Observations can be also combined with models to give ‘‘value-added’’ observa-

tions. Data at individual points in the ocean exist without reference to neighboring

points unless models are used to dynamically interpolate the data to neighboring

points using the equation of motion of a ﬂuid. This so-called four-dimensional data

assimilation is in the process of development and shows promise as a powerful way

of optimally using the ocean data that can be taken.

Models can also be compared with other models. While this might seem sterile,

ﬁne-resolution models can be used to develop parameterizations of large-scale

mixing for use in coarse-resolution ocean models that can be run the long times

needed to participate in coupled model simulations of climate. Advances in compu-

ter power will ultimately allow successive reﬁnements in resolution so that ﬁner

scale resolution models can be run directly.

We close by reemphasizing the crucial role that the ocean plays in climate and

climate variability and the necessity to know more about the ocean for all aspects of

the climate problem.

6 THE FUTURE OF THE OCEAN IN CLIMATE 133

CHAPTER 11

PROCESSES DETERMINING LAND

SURFACE CLIMATE

GORDON BONAN

1 INTRODUCTION

Energy is continually ﬂowing through the land–atmosphere system. As the sun’s

radiation passes through the atmosphere, some of it is absorbed, primarily by water

vapor and clouds, and some is reﬂected back to space by clouds and particles

suspended in the air. The remainder reaches Earth’s surface, where it is either

absorbed or reﬂecte d upwards. The solar radiation absorbed by the surface provides

the warmth needed to maintain life and is used in biological activities such as

photosynthesis. In turn, it provides energy to warm the atmosphere. Its surface

emits radiation in the infrared waveband in proportion to its temperature raised to

the fourth power. Most of this longwave radiation is absorbed by water vapor,

clouds, carbon dioxide, and other gases in the atm osphere, heating the atmosphere.

Without this heating, Earth’s effective temperature would be 33



C cooler than it

is now.

The solar and longwave radiation absorbed by Earth’s surface constitute the net

radiation at the surface. This energy is either stored or returned to the atmosphere as

sensible or latent heat. Objects that absorb radiation become warmer than their

surroundings and lose some of that energy by convection. For example, heat will

normally be lost from a warm surface to the cooler air surrounding it. The transfer of

this heat determines the temperature of air and is called sensible heat because it can

be felt. Heat is also lost from the surface by evapotranspiration. Evapotranspiration

determines the amount of water in the atmosphere, but it also cools the surface

because the change of water from liquid to gas requires a large amount of heat,

Handbook of Weather, Climate, and Water: Dynamics, Climate, Physical Meteorology, Weather Systems,

and Measurements, Edited by Thomas D. Potter and Bradley R. Colman.

ISBN 0-471-21490-6 # 2003 John Wiley & Sons, Inc.

135