Fossil fuels and greenhouse gases

Let us now start to look at one potential cause of the observed warming, namely the level of greenhouse gases in the atmosphere. We discussed in chapter 1 that the presence of such gases maintains the surface temperature at a level higher that it otherwise would be the current average surface temperature is about 15 C, whereas the radiative equilibrium temperature, which would hold if Earth had no atmosphere, is about -18 C. Thus, a rather obvious possible cause of the observed increase in...

Some personal remarks

In this last section, I would like to emphasize two aspects about global warming that do not, I think, get sufficient attention We need to think in terms of probabilities or Carbon dioxide (CO2) and a few other gases are greenhouse gases, meaning that they absorb and re-emit longwave radiation that is emitted from Earth's surface, maintaining the surface at a higher temperature than it would be in their absence (about 15 C as opposed to 18 C). Greenhouse gases, and in particular CO2, are added...

Flow in gyres

Suppose that the wind blows zonally across the ocean, with a stronger eastward wind to the north, as in figure 4.3. Away from coastal regions (where friction may be important) the forces present are the zonal wind force (which here we simply denote F W), the Coriolis force (fv and fu) and the pressure gradient force (9 9x and 9 9y, where pip), and we represent their balance mathematically as in the zonal and meridional directions, respectively. If there were no wind, the flow would be in...

CLiMate And Weather

What is the difference between climate and weather It is intuitively clear what the weather is it is the day-to-day state of the atmosphere at some location, usually with particular reference to such things as temperature, windiness, and precipitation. It is also intuitively clear that when we speak of climate we wish to average out all these day-to-day fluctuations and refer to some kind of average of the weather. But what precisely There is no ideal definition of climate, but a useful working...

Temperature

Schematic of temperature profiles before and after the addition of greenhouse gases. The total outgoing longwave radiation must remain the same because this radiation balances the incoming solar radiation, and so the emissions temperature, Te, stays the same. However, the emissions height must increase (from Zl to Z2) because of the increased absorptivity of the atmosphere. Hence, if the temperature gradient in the vertical remains similar, the surface temperature must increase....

And The ocEAN

Plurality should not be posited without necessity. Occam's Razor (attributed to William of Ockham) IN THIS FINAL CHAPTER WE DISCUSS A TOPIC OF GREAT current and likely future interest, namely global warming. In the first half of the chapter, we talk about warming quite generally what it is, what the evidence is for it, what the consequences might be, and what the level of uncertainty might be about future warming. In the second half of the chapter, we concentrate on the role and effects of and...

Notes to chapter

As argued by Czaja & Frankignoul (2002). 2. Few acronyms are pleasing to the ear, perhaps because they are forced words. Think how ugly the acronyms ENSO, QUANGO, SAT, NASDAQ, GIF, JPEG, and MS-DOS are. A few exceptions, notably radar and laser, do have a certain euphony and have become accepted as true words with no need for capitalization. 3. The name El Ni o was originally used by fishermen along the coasts of Ecuador and Peru to refer to a warm ocean current that usually appears around...

CT m T Td72b

In these equations, t is time, Tm and Td are the temperature anomalies of the mixed layer and deep ocean, respectively, F is the anomalous radiative forcing caused by greenhouse warming, and Cm and Cd are the heat capacities of the mixed layer and deep ocean, respectively. The parameters and X2 are exchange coefficients that determine the rate at which heat is transferred from the upper ocean to the atmosphere and from the upper ocean to the deep ocean, respectively. Although an exact solution...

Mainly the ocean

How the Ocean Works An Introduction to Oceanography. Princeton, N.J., Princeton Univ. Press. Discusses the ocean from a mechanistic point of view, including physical, chemical, and biological aspects. An Open University Course Team, 1998. The Ocean Basins Their Structure and Evolution. 2d ed. Oxford, U.K., Pergamon Press. An Open University Course Team, 2001. Ocean Circulation. 2d ed. Oxford, U.K., Pergamon Press. The Open University has a series of books on various aspects of...

How much

On average, both the atmosphere and the ocean transport heat poleward, and this transport is illustrated in figure 5.3. The total transport of the atmosphere plus the ocean may be determined fairly directly from satellite measurements. Over the whole planet, there is a balance between the incoming solar radiation and outgoing longwave radiation, and if there were no heat transport, the incoming solar radiation would equal the outgoing infrared radiation at each latitude a state of pure...

And temperature

In this appendix, we give an elementary mathematical treatment of the relationship between heating and temperature. We will explain two things why the temperature range is smaller if a body has a larger heat capacity and why there is a lag between heating and temperatures. We model the system with the simple equation Here, S is the heating source, T is the temperature, and t, the time. The parameter C is the heat capacity of the system, and X is a constant that determines how fast the body...

Consequences and impacts of el nio

What are the consequences of El Ni o There are both local effects, arising because of the warm eastern Pacific, weakened trade winds, and shifted regions of convection, and distant effects, mostly arising because the atmosphere carries the signal of El Ni o far afield. We'll talk about the local effects first. El Ni o brings significantly warmer water to the eastern tropical Pacific, and this water spreads both north and south along the coast, giving a detectable signal in the SST as far north...

Putting it all together

Thus, to summarize, the meridional overturning circulation has two mechanistically distinct components a component maintained by mixing and a component maintained by wind, both responding to the surface buoyancy distribution. The two can exist side by side, and the overturning circulation in the Atlantic Ocean is schematically illustrated in figure 4.8. Some of the water that sinks in the North Atlantic moves across into the Southern Hemisphere and upwells in the ACC (enabled by the wind), and...

EoSTRoPHic BALAN

Let us now consider what forces and balances occur in the horizontal direction, for these forces give rise to the ocean currents and the great circulation patterns in the ocean. Away from regions where the direct effects of wind and friction are important (usually at the top and bottom of the ocean), the two dominant forces in the horizontal direction are the pressure gradient force and the Coriolis force if the flow is steady, these two forces almost balance each other. This balance is called...

More Effects of and on the ocean

Let's now consider how the ocean might affect global warming and how global warming might affect the ocean. One of the most important effects stems from the fact that global warming is a form of climate variability, albeit variability of a forced kind thus, just as with natural variability, the ocean damps the response in some fashion. There are other potential effects, so let's make a short list of them and then consider them one by one. 1. The damping effect will slow down the warming and...

Radiative Balance Solar radiation received

Above Earth's atmosphere, the amount of radiation, S, passing through a plane normal to the direction of the sun (e.g., the plane in the lower panel of figure 1.1) is about 1,366 W m2. (A watt is a joule per second, so this is a rate at which energy is arriving.) However, at any given time, half of earth is pointed away from the sun, so that on average Earth receives much less radiation than this. How much less Let us first calculate how much radiation earth receives in total every second. The...

Mainly The AtmoSPHERE

M. & Hobbes, P., 2006. Atmospheric Science An Introductory Survey. 2d ed. Burlington, Mass., Academic Press. Covers a wide range of topics at the advanced undergraduate level. Andrews, D. G., 2010. An Introduction to Atmospheric Physics. 2d ed. Cambridge, U.K., Cambridge Univ. Press. Written at the upper-division undergraduate level, with a couple of chapters on dynamics. Holton, J. R., 2004. An Introduction to Dynamic Meteorology. 4th ed. Burlington, Mass., Academic Press. A...

THE SLoWiNG oF GLoBal WarmiNG

Perhaps the most important and least ambiguous effect of the ocean is that it acts to slow down global warming in a certain sense, although the effect is rather subtle. Furthermore, the ocean is not likely to significantly affect the final equilibrium temperature that the planet will reach if, let us say, co2 levels eventually double before leveling off. So just what does the ocean do Let's try to explain it first just using words appendix A to this chapter provides a mathematical treatment of...

THE WiNDDRivEN ciRcuLATioN And The Great Gyres

To better understand how the processes described above produce an ocean circulation like that described in chapter 2, let us consider an idealized ocean, with much simplified geometry, and see if we can first understand how that works. Our idealized view of the gyres is illustrated in figure 4.1, which the reader will appreciate is an abstraction Figure 4.1. An idealized gyre circulation in a rectangular ocean basin in the Northern Hemisphere, showing the subtropical gyre (lower, typically...

The overturning circulation maintained by mixing

How do the above considerations apply to the circulation of the ocean For simplicity, we consider only the effects of temperature and not of salinity, and a schema of the circulation is given in the top panel of figure 4.6. The ocean, is, roughly speaking, a big basin of water for which the temperature of air just above the sea surface decreases with latitude. Air-sea exchange of heat heats or cools the water at the sea surface so that it has, approximately and on average, the temperature of...

The Oceans A Descriptive Overview

Persons attempting to find a motive in this narrative will be prosecuted persons attempting to find a moral in it will be banished persons attempting to find a plot in it will be shot. Mark Twain, Adventures of Huckleberry Finn WE NOW START TO LOOK AT THE OCEAN(S)1 IN A LITTLE more detail, albeit in a rather descriptive manner, as a precursor to the more mechanistic or dynamical description, or explanation, that we try to provide in chapter 4. That is, in this chapter we describe what's going...

The overturning circulation maintained by wind

The second mechanism that can lead to a deep overturning circulation relies, in its simplest form, on the presence of strong zonal wind blowing over the ocean surrounding Antarctica, as illustrated in the lower panel of figure 4.6. Unlike an ocean basin, the ocean surrounding Antarctica is effectively a channel, for it has no meridional boundaries and so no real gyres. Let's first look at the flow in this channel, and then look at how this flow affects the global overturning circulation. The...

A simple climate model

We are now in a position to make what is probably the simplest useful climate model of Earth, a radiationbalance or energy-balance model (EBM), in which the net solar radiation coming in to Earth is balanced by the infrared radiation emitted by Earth. A fraction, a, known as the albedo, of the solar radiation is reflected back to space by clouds, ice, and so forth, so that Net incoming solar radiation S0(1 a) 239 Wm2, (1.6) with a 0.3 (we discuss the factors influencing the albedo more below)....

Brief Introduction To Dynamics

Mathematics is the easiest bit in physics. Pierre-Gilles de Gennes, Les Objets Fragile WE NOW BEGIN OUR QUEST OF PROVIDING AN EXPLANAtion for how and why the ocean circulates the way it does and how and why it affects the climate. In this chapter, we'll explain some of the basic dynamical principles that determine the circulation in the next chapter, we'll apply these principles to the circulation itself. The current chapter is a little more mathematical than the others in this book, but it...

Composition

In today's climate the oceans are mainly liquid only about 2 of the water on the planet is frozen. Most of the frozen water is in the ice sheets of Antarctica (with 89 of the world's ice, and an average depth of about 2 km) and Greenland (8 , 1.5 km deep). The volume of sea ice, formed by the freezing of seawater, is far less than that of land ice because typically it is only a few meters thick. Its extent also varies considerably by season. However, the importance of land ice and sea ice for...

Seasurface slope and the geostrophic current

It may seem a little fantastical that the wind can produce a change in the sea level and that this in turn can produce the currents of the great gyres. However, we do not need a large change in the sea level to produce quite substantial flows, as we can see with a simple calculation. The geostrophic current is a balance between the Coriolis and pressure gradient forces, so that The pressure at a level below the surface is given by the weight of the fluid above it, so that where h is the height...

Climate variability from weeks to years

Climate is in the eye of the beholder. In this chapter we look at climate variability, and in particular climate variability that is associated in one way or another with the ocean. This condition is not very restrictive because nearly all forms of climate variability on timescales of months to decades are affected by, or even caused by, the ocean. Even in cases in which the underlying cause of the variability is nonoceanic, the ocean may modulate the variability and determine its tim-escale,...

Red herrings and straw men

Of course the models might be wrong and the temperature increase might come from natural causes natural variability in the climate system, such as we discussed in chapter 6. Thus, the observed warming might not be anthropogenic but rather might be related to our emergence from the Little Ice Age over the past century. This idea is not so much an argument as a speculation because no viable mechanism has been posited that could cause the warming seen over the course of the past century, except...

I

Sun's rays (Arrive parallel at Earth) Figure 1.1. Top The incoming solar radiation impinges on a disk of area na2 but is on average spread out over a sphere of area 4ua2. Bottom Variation of incoming solar radiation with latitude. A given amount of radiation is spread over a larger area at high latitudes than at low latitudes, so the intensity of the radiation is diminished, and thus high latitudes are colder than low latitudes. radius a, and the area of a sphere is Ana2. Thus, the average...

Has The ocEan Warmed

Let us begin our discussion of the ocean's role by asking a simple question Has the ocean interior itself warmed The ocean itself has in fact warmed over the years, as shown in figure 7.6, which shows an increase in the heat content from 1955. (There are not sufficient data before this time to extend the time series further into the past.) The heat content is defined as the heat capacity of seawater (which is almost a constant) multiplied by the change in temperature, integrated over the entire...

The ocean basins

The ocean covers about 70 of Earth's surface, and so has a total area of about 3.61 X 1014 km2. Currently, about two-thirds of Earth's land area is in the Northern Hemisphere, so that about 57 of the ocean is in the Southern Hemisphere, 43 in the Northern the Northern Hemisphere itself is 61 ocean, and the Southern Hemisphere is about 80 ocean. The ocean's average depth is about 3.7 km, but there are deep trenches where the depth reaches about 10 km. The volume of the ocean is approximately 1.3...

CiRCULATiOn Changes And A ThermohaliNe Shutdown

One possibility that is raised from time to time is that global warming will bring about a slowdown or cessation of the meridional overturning circulation, sometimes called a thermohaline shutdown.11 Is such a thing likely First recall that in chapter 4 we divided the ocean circulation up into a quasi-horizontal circulation that is primarily wind driven (the gyres) and an overturning circulation. With warming, the winds may change in 1000 1900 2100 2300 3000 4000+ Figure 7.7. Schema of a...

Basics Of Climate

The climate's delicate, the air most sweet. William Shakespeare, A Winter's Tale To appreciate the role of the ocean in climate, we need to have a basic understanding of how the climate system itself works, and that is the purpose of this chapter. Our emphasis here is the role of the atmosphere we don't pay too much attention to the oceans as we'll get more of that (lots more) in later chapters and we assume for now that the climate is unchanging. So without further ado, let's begin.

APPENDix B FLoW iN An Ekman Layer

Most large-scale flow in the ocean is in geostrophic balance, meaning that, away from the direct influence of the wind and so beneath the Ekman layer, the pressure gradient force is balanced by the coriolis force. Mathematically we have, as in equation 3.11, where ug and vg are the fluid speeds in the zonal and meridional directions, respectively. The first of this pair of equations is the momentum balance in the zonal direction (with x the distance toward the east) the Coriolis force -fv...

Climate And The Oceans

PRINCETON UNIVERSITY PRESS Princeton & Oxford Copyright 2012 by Princeton University Press Published by Princeton University Press 41 William Street, Princeton, New Jersey 08540 In the United Kingdom Princeton University Press 6 Oxford Street, Woodstock, Oxfordshire 0X20 1TW press.princeton.edu All Rights Reserved ISBN 978-0-691-14467-2 (cloth) ISBN 978-0-691-15028-4 (pbk) Library of Congress Cataloging-in-Publication Data Vallis, Geoffrey K. Climate and the oceans Geoffrey K. Vallis. p. cm....

Satellite measurements

Given that there are possible errors in the direct measurements of temperature, it is useful to compare them with satellite measurements, which provide a completely independent record, less influenced by urbanization issues. Of course, satellites have not been taking measurements for as long as the surface record has existed, and they too are subject to their own errors difficulties both in calibration and in accounting for the fact that the orbit of a satellite tends to decay over time,...

The Ocean Circulation

This is a court of law, not a court of justice. The climate in general and the oceans in particular are complicated systems, and if one is not careful it is easy to lose sight of the forest for the trees. For that reason, a useful philosophy is to begin with an austere picture of the phenomenon at hand and then gradually add layers of complexity and detail. The first picture will be a simplification, but if it is based on sound scientific principles, then it will provide a solid foundation for...

Ocean influence on climate variability

How might the ocean affect climate variability Indeed, might it even effect climate variability The ocean has one unambiguous influence on midlatitude climate variability and a number of more ambiguous influences. The unambiguous influence stems from the fact that the heat capacity of the ocean is much greater than that of land, as we discussed in chapter 5. Thus, changes in surface temperatures over land, especially in locations far from the ocean, are much larger than the changes over the...

Weekly variability and the weather

On daily and weekly timescales, the main mechanism that causes variations in the temperature and wind is simply the familiar weather. The mechanism that gives rise to weather resides in the atmosphere, and it is the consequence of a fluid instability called baroclinic instability. This instability can be thought of as a type of con-vective instability in which if a fluid is heated from below it expands, becomes lighter than its surroundings, and therefore rises. Baroclinic instability has a...

S the oceans overall role in climate

The coldest winter I ever spent was a summer in San Francisco. The OCEAN PLAYS a number of roles in our present climate, and in this chapter we discuss two of the most important 1. The ocean moderates the climate by taking in heat when the overlying atmosphere is hot, storing that energy and releasing heat when the atmosphere is cold. 2. The ocean redistributes heat in the large-scale ocean circulation. In addition, the ocean generally has a lower albedo than land, so that if all the ocean were...

The observed global temperature record

Since the late nineteenth century, the observed average surface temperature has been increasing, as shown in figure 7.1. The data from land comes from about 4,000 stations distributed widely over the the globe, although naturally enough there are more stations in North America and Europe and fewer stations in such places as Antarctica, Greenland, Siberia, and the Sahara Desert. The actual measurements typically are taken twice daily Figure 7.1. The instrumental record of global average surface...

What are the effects

What are the gross effects of the ocean heat transport on the climate The main effect is simply that the high latitudes, especially the high latitudes in the Northern Hemisphere, are warmer than they would be if the oceans were not present. How much warmer is a question that we cannot answer with armchair reasoning. We would need to perform detailed calculations with comprehensive climate models of the type used to predict the weather or used for global warming experiments. One such set of...

What mechanisms

Oceanic heat transport is mainly effected by the large-scale circulation, with some transport by mesoscale eddies, mainly in the ACC. As we discussed in chapter 4, there are two distinct aspects to this circulation the wind-driven gyres and upper ocean circulation, and the meridional overturning circulation. Let's see how each of these transport heat. The wind-driven gyres, especially the subtropical wind-driven gyres, are a major factor in the heat transport in both hemispheres. If we consider...

The Antarctic circumpolar current

At high latitudes, the Southern Hemisphere has a qualitatively different geometry than the Northern Hemisphere the other continents do not connect to Antarctica, and so the oceans are free to circulate all the way around the globe, forming the Antarctic Circumpolar Current (ACC). The flow is predominantly zonal (east-west) and in that sense resembles that of the atmosphere. An enormous transport of water is sustained over great lengths, comparable to that of the great gyres. The strength of...

Coriolis force

In this section, we give a mathematical, although elementary, derivation for the magnitude of the Coriolis force. Coriolis force for a body moving zonally Consider a disk rotating with an angular velocity X, so that at a radius r from the axis of rotation the disk has a tangential velocity ud Xr. Suppose that an observer is sitting on the disk and is thus stationary in the disk's frame of reference. As we saw in the previous section, in the rotating frame there is a centrifugal force of...

EL Nio And The SouTHERN oSciLLATioN

In the next few sections we describe the phenomenon (and what a phenomenon it is ) known as El Ni o, or sometimes as El Ni o and the Southern Oscillation (ENSo). The Southern oscillation is the atmospheric part of the phenomenon, and El Ni o, the oceanic component ENSO is the combination. However, unless we need to be particularly precise, we often just use the term El Ni o, for this has a pleasant euphony lacking in the acronym ENSO.2 El Ni o is the largest and most important phenomenon in...

What Makes The Ocean CiRCULATE

As we discussed in chapter 2, it is useful to think of the large-scale ocean circulation as having two main components a quasi-horizontal circulation consisting of the gyres and other surface-enhanced currents, and a deeper overturning circulation, the meridional overturning circulation. What makes the ocean go around this way What drives the ocean, if anything Bypassing the ambiguous term drive, there are three main distinct physical phenomena that lead to the circulation of the ocean 1 1. The...

THE ovErturnng Crculaton

The other main component of the ocean circulation is the meridional overturning circulation (MOC), circulation essentially occurring in the meridional plane. There are two rather distinct aspects to this circulation, but they each have a common feature, namely the sinking of dense water at high latitudes and its subsequent rise to the surface elsewhere. Thus, in general the overturning circulation may be regarded as being buoyancy enabled in the sense that without buoyancy gradients at the...

What is El

Every few years the temperature of the surface waters in the eastern tropical Pacific rises quite significantly. The strongest warming takes place between about 5 S and 5 N, and from the west coast of Peru (a longitude of about 80 W) almost to the dateline, at 180 W, as illustrated in figure 6.3. The warming is significant, with a difference in temperature up to 6 C from an El Ni o year to a non-El Ni o year. The warmings occur rather irregularly, but typically the interval between warmings...

The global circulation and the conveyor belt

The ocean contains two major interhemispheric basins, containing the Pacific and the Atlantic oceans, as well as the smaller, primarily Southern Hemisphere, basin containing the Indian ocean, all connected via the Antarctic Circumpolar Current (ACC). Wind-driven gyres exist in all the major basins, and the Atlantic has a robust overturning circulation. How does it all fit together There is no universally accepted picture, and certainly no quantitative theory. Rather, observations and numerical...

The Pressure Force

A fluid, either a gas or a liquid, is composed of molecules in motion. The collective motion of the molecules gives rise to the flow of the fluid the winds of the atmosphere and the currents of the ocean even when there is no such organized flow, the molecules are moving, but their motion is more random. In a gas at room temperature, the molecules are moving at typical speeds of 450-500 m s-1. These molecules naturally collide with each other (in fact, a typical distance a molecule travels...

Ekman Layers

In the final topic of this chapter, we consider what happens to water in the upper few tens of meters when the wind blows over it. The water moves, and hence the Coriolis force acts, but because there is an additional force coming from the wind, geostrophic balance cannot exactly hold. So what does happen The problem was first considered by Vagn Walfrid Ekman, a Swedish oceanog-rapher, at the beginning of the twentieth century, at the suggestion of Fridtjof Nansen, the Norwegian explorer and...

Sealevel RiSE

An almost certain consequence of global warming on the ocean is that sea level will rise, if only because as water warms it expands. In the oceans, the only way that an increased volume of the ocean can be accommodated is by an increase in sea level, and sea level has indeed risen over the past several decades, as illustrated in figure 7.6. Sea level is estimated to have risen about 20 cm since records began in the late nineteenth century, and it rose at about 2 mm per year over the last half...

The Ekman and windinduced geostrophic flows

We show first that the wind does indeed induce a geo-strophic flow that has the same sense as the wind itself. The mean winds are to the east in midlatitudes and to the west in the tropics and, as we showed in the section in chapter 3 on Ekman layers, there is a flow in the upper Figure 4.2. Production of gyres by winds. The winds blowing as shown induce a converging Ekman flow, causing the sea level to increase in the center, thus giving rise to a pressure gradient. This gradient in turn...

Ocean structure and circulation

The aspect of the ocean that perhaps most affects the climate is the temperature distribution at the surface the MfE WE 180 1 W 60W 0- Figure 2.2. The annual average temperature at the ocean surface, in degrees centigrade. Adapted from World Ocean Atlas, 2009 of the National Oceanic and Atmospheric Administration (http www.nodc .noaa.gov OC5 WOA09 pr_woa09.html). MfE WE 180 1 W 60W 0- Figure 2.2. The annual average temperature at the ocean surface, in degrees centigrade. Adapted from World...

Ocean eddies

We are all familiar with the fact that the weather differs from the climate, the difference arising because the atmospheric flow is unsteady, and we talk more about this in chapter 6. The same applies to the ocean, only more so the large-scale currents in the ocean are almost all unstable, rather like a river flowing over rapids, and tend to break up into smaller mesoscale eddies, as illustrated in the lower panel of figure 2.6. The resulting eddies are the oceanic analogue of atmospheric...

Appendix A Mathematics Of The Twobox Model

Here we give a mathematical description of the two-box model of the ocean, illustrated in figure 7.9. The evolution equations of the two boxes are Cm dTm F miTm m (Tm Td), (7.2a) Figure 7.9 A simple two-box model of the ocean, with a mixed layer at a temperature Tm and a deep ocean layer at a temperature Td, and exchanges of heat between the components as shown. Figure 7.9 A simple two-box model of the ocean, with a mixed layer at a temperature Tm and a deep ocean layer at a temperature Td, and...

HYDRoSTATic BALANcE

Let us now consider what the balance of forces is in the vertical direction. A moment's thought indicates that there must be a pressure gradient in the vertical direction, because without one there would be nothing to hold up the fluid. A piece of fluid, be it a piece of air or a piece of seawater, is not weightless, so the force of gravity acts on it, pulling it down toward Earth. If the fluid parcel is stationary, then there must be a force in the opposite direction that balances gravity, and...

Greenhouse effect

Earth is covered with a blanket of gas made mainly of nitrogen, oxygen, carbon dioxide, and water vapor. This blanket is essential to life on Earth, for (at least) two reasons 1. We breathe the air, taking in the oxygen and breathing out carbon dioxide. Similarly, plants use sunlight together with the carbon dioxide in the atmosphere to photosynthesize and create organic compounds. 2. The atmosphere absorbs the infrared or longwave radiation emitted by Earth's surface and re-emits it back to...

Global Warming iN Context The Past Millennium

When discussing global warming, a fair question to ask is, How does the temperature increase of the past century compare with other periods in the past It is a difficult question to answer precisely because we only have direct measurements of surface temperature for the past 140 years or so, although a few individual records go back further there is a record of measured temperatures in central England from 1659, for example. However, the use of various proxies enables the construction of the...

The centrifugal force

In this section, we derive an expression for the centrifugal force on a body moving in a circle of radius r with an angular velocity Q, as illustrated in figure 3.5. Let us first consider what forces are involved in a body that is moving in a circle with a uniform speed. Newton's first law of motion says that if a body is left to its own devices it will either remain stationary or move in a straight line with a constant speed. If a body is undergoing uniform circular motion (for example, a...

Implications for global warming

Earlier we noted that one possible cause of the warming over the past century is natural variability, with one candidate mechanism being a variation in the ocean that releases heat to the atmosphere, thus causing atmospheric warming. This is a very plausible conjecture because the ocean certainly has the potential to vary on the timescale of decades to centuries however, the empirical evidence is against it. If the ocean were to have given up its heat to the atmosphere, then it would have...

The lag in the seasons

The observant reader noted in figure 5.1 that not only is the seasonal cycle more muted in San Francisco, but also that the maximum temperatures occur later in the season, in September. This again is an effect of the large heat capacity of the system, as a simple argument shows. Suppose that a system is heated externally (e.g., by the sun) and is cooled by the effects of longwave radiation and that the cooling is proportional to the temperature itself. If the system has a very small heat...

Western iNTENSiFicATioN

We have now explained the underlying reason for gyres, but we have not explained one of their most important and indeed obvious aspects the gyres are not symmetric in the east-west direction. Thus far, our explanation would lead to gyres that look like those in the left panel of figure 4.3, whereas in fact the gyres look more like those in the right-hand panel, with intense western boundary currents, of which the Gulf Stream in the western North Atlantic is the most famous example to Americans...

PoSSiBLE cAuSES of GLoBal WarmiNG The likely culprit

Greenhouse Effect Temperature Profile

The atmosphere contains greenhouse gases that absorb and re-emit longwave (infrared) radiation, thus warming the surface. If the greenhouse gas concentration increases, we can therefore expect temperatures to rise. Thus, a reasonable hypothesis is that the increase in anthropogenic greenhouse gases, and in particular CO2, over the past several decades is responsible for the increase in temperature. In chapter 1 we constructed a very simple mathematical model of this process we now present a...

THE MoDERATiNG iNFLuENcE of the ocean

Zero Annual Amplitude

Perhaps the most obvious effect that the ocean has on climate is its moderating effect on extremes of temperature, both diurnally (i.e., the day-night contrast) and annually (the seasonal cycle). We focus on the effects on the annual cycle because these tend to be on a larger scale and more befitting a book with climate in the title, but much the same principles and effects apply to the diurnal cycle. First we take a look at the observations to confirm that there is a moderating influence from...

Differential rotation and Earths sphericity

Finally, let us consider the effect of Earth's sphericity on the Coriolis force. The rotation axis of Earth is a line between the North Pole and the South Pole, and the Co-riolis force always acts in a direction perpendicular to this line. We are primarily interested in the horizontal deflection of fluid parcels by the Coriolis force because in the vertical direction, gravitational effects dominate and the Coriolis force is relatively small. The horizontal deflection is caused mainly by the...

Westward drift

In this section, we give a slightly different explication of why the boundary current is in the west. It is not really a different explanation because the cause is still differential rotation, but here we think about it quite differently. We'll see that the effect of differential rotation is to make patterns propagate to the west, and hence the response to the wind's forcing piles us in the west and produces a boundary current there. We noted already that the component of Earth's rotation in...

The Planet Earth

Earth is a planet with a radius of about 6,000 km, moving around the sun once a year in an orbit that is almost circular, although not precisely so. Its farthest distance from the sun, or aphelion, is about 152 million km, and its closest distance, perihelion, is about 147 million km. This ellipticity, or eccentricity, is small, and for most of the rest of the book we will ignore it. (The eccentricity is not in fact constant and varies on timescales of about 100,000 years because of the...

Magnitude of the Coriolis force

There is a simple way we can calculate the Coriolis force on a body moving zonally (i.e., in the east-west direction) if we already know the form of the centrifugal force. For simplicity, we first suppose that Earth is a flat disk, with the axis of rotation perpendicular to the disk and passing through the disk's center, which is then analogous to the North Pole. Gravity points down into the disk. (We consider the effects of sphericity in the next section until then, Earth is flat.) Let Earth's...

Centrifugal Force

Suppose that you are riding in a train that starts to go around a bend rather quickly. You feel like you are being thrust outward toward the side of the car, and if you are really going quickly around a tight curve, you might have to hang onto something to stay put. The outward force that you are feeling is commonly known as centrifugal force. Strictly speaking, it is not a force at all (we'll explain that cryptic comment later), but it certainly feels like one. What is going on One of the most...

The buoyancy or Archimedean force

The force due to buoyancy is one of the most familiar forces occurring in a fluid and, rather famously, was known to Archimedes. It is the force that, among other things, allows objects to flow in water. The Archimedes principle is often stated as Any object, partially or wholly immersed in a fluid, experiences an upward force equal to the weight of the fluid displaced by the object. Let's see why this is so. Consider a container of still water and focus attention on a particular piece of water...

What are the important greenhouse gases

The most important greenhouse gases in Earth's atmosphere are water vapor and carbon dioxide, followed by ozone, nitrous oxide, methane, and various other gases (see table 1.1). Water vapor (and clouds) and carbon dioxide have the dominant effect, although their respective levels are maintained in different ways. The level of C02 is almost constant throughout the atmosphere, and year by year its level is maintained by a balance between emissions (e.g., respiration by animals and bacteria) and...

The general damping of climate variability by the ocean

N> t only does the ocean provide a moderating influence on the march of the seasons, but it also can provide a moderating influence on the variability of climate on other timescales too. We talk more about the mechanisms that give rise to climate variability in the next chapter, but for now let us just suppose that the climate system excluding the ocean is able to vary on multiple timescales, from days to years. Then, just as the ocean is able to damp the seasonal variability, the ocean damps...

The mechanism of El

Thermocline During Nina

The mean state of the atmosphere and the ocean Before discussing what processes conspire to produce El Ni o events, let us discuss what the mean state of the atmosphere and ocean are, beginning with the atmosphere. The trade winds throughout the equatorial region blow predominantly from higher latitudes toward the equator, and from the east to the west. The low-level convergence at the equator forces the air to rise and then, several kilometers above the surface, move poleward, sinking in the...

THE CORiOLiS FORCE

Another apparent force is caused by Earth's rotation, one that only arises when bodies are in motion relative to the rotating Earth, and this force is known as the Coriolis force after the French engineer and scientist Gaspard-Gustave Coriolis (1792-1843). It turns out to be much more important than the centrifugal force for currents and winds, although its effects are rather subtle. The sphericity of Earth is not important in the coriolis force itself, and until we get to the section on...

The overturning circulation and the vertical structure

The meridional overturning circulation (MOC) of the ocean is the name commonly given to the circulation in the meridional (i.e., north-south and up-down in water depth) plane. Horizontal variations in this circulation can be important, but let us put them aside for now. This circulation comes about as a consequence of various factors the temperature gradient between equator and pole, the temperature difference between high northern latitudes and high southern latitudes, the winds, especially...

Loss Of Sea iCE

Sea ice is formed by the freezing of seawater and so is to be found at high latitudes in both the Northern and Southern hemispheres, especially in their respective winters and springs. Sea ice is important to the climate system because it has a higher albedo than seawater (and therefore reflects back more solar radiation to space), because it affects the way the atmosphere and ocean exchange heat and water vapor, and because when sea ice forms, salt is extruded into the ocean, and when it...

A simple radiation model

Let us put aside the spatial variation of solar radiation for a while and try to obtain an estimate of the average surface temperature on Earth, given the average solar radiation coming in at the top of the atmosphere. Solar radiation causes Earth's surface to warm and emit its own radiation back to space, and the balance between incoming and outgoing radiation determines the average temperature of Earth's surface and of the atmosphere. To calculate the temperature, we need to know a few pieces...

The horizontal structure and the ocean currents

A schematic of the main horizontal ocean currents at the ocean surface is shown in figure 2.3. In most regions of the world, these currents extend a few hundred meters into the ocean, with the exception of the equatorial region. Here there is a shallow westward-flowing surface current and a more substantial eastward flow beneath (as well as narrow countercurrents on either side of the westward flow, not shown in the schematic). At first glance, the circulation seems complicated, especially as...

Torques and interior flow

If the wind stress acting on the ocean varies with latitude as we see that it does in figure 4.3 then the wind provides a torque that tends to spin the ocean. In a steady state, not only do the forces on the ocean have to balance but so do the torques otherwise the ocean would spin faster and faster. The torques on the ocean are provided by the wind, by friction, and by the Coriolis force (the pressure gradient does not provide a torque).2 Integrated over the entire ocean basin, the wind torque...

A simple mathematical model of the greenhouse effect

Let us now construct a simple mathematical model illustrating the greenhouse effect. our purpose in doing so is to see somewhat quantitatively, if approximately, whether the atmosphere might warm the surface up to the observed temperature. Let us make the following assumptions 1. The surface and the atmosphere are each characterized by a single temperature, Ts and Ta, respectively. 2. The atmosphere is completely transparent to solar radiation. 3. Earth's surface is a blackbody. 4. The...