The Atmosphere and Ocean

A Physical Introduction

Neil C. Wells

• Publisher: Wiley
• Year: 2011
• Print ISBN: 9780470694688, 9780470694695
• eBook ISBN: 9781119979845
• Edition: 3
• Topic: Physical Sciences
• Subtopic: Meteorology & Climatology

The Atmosphere and Ocean

The Atmosphere and Ocean is a fully revised and updated student friendly physical introduction to the atmosphere and ocean. Now in its Third Edition, the book continues to provide students with an accessible description of the atmosphere and ocean with emphasis on their physical properties and interdependence. Clearly structured throughout, the book demonstrates that the atmosphere and ocean are both subject to the influence of the Earth's rotation and therefore they have a common dynamical basis. The author clearly demonstrates the fundamental differences between the two environments and provides the reader with a much better understanding of the atmosphere and the ocean and an appreciation of their close interactive relationship. There have been many developments in the field over the past ten years and the latest edition of this highly successful textbook brings together new material on the ocean-atmosphere system and climate, the observed circulation of the atmosphere and ocean and radiation in the atmosphere and ocean.

• Fully revised and updated Third Edition of student friendly physical introduction to the atmosphere and ocean.
• Now includes new chapters on observed circulation of the atmosphere and ocean, energy flows in the ocean atmosphere system, modeling the ocean and atmosphere, the ocean atmosphere system and climate.
• Well structured and written in an authoritative yet accessible style suitable for 2nd and 3rd year students taking courses in meteorology, oceanography and related Earth Sciences or as an introduction for graduate students.
• Emphasis placed on physical properties and inter-dependence of the ocean and climate.

Chapter 1

The Earth within the Solar System

1.1 The Sun and its constancy

Any account of the Earth's atmosphere and ocean cannot be regarded as complete without a discussion of the Sun, the solar system and the place of the Earth within this system. The Sun supplies the energy absorbed by the Earth's atmosphere-ocean system. Some of Sun's energy is converted directly into thermal energy, which drives the atmospheric circulation. A small portion of this energy appears as the kinetic energy of the winds which, in turn, drives the ocean circulation. Some of the intercepted solar energy is transformed by photosynthesis into biomass, a large proportion of which is ultimately converted into heat energy by chemical oxidation within the bodies of animals and by the decomposition and burning of vegetable matter. A very small proportion of the photosynthetic process produces organic sediments which may eventually be transformed into fossil fuels. It is estimated that the solar radiation intercepted by the Earth in seven days is equivalent to the heat that would be released by the combustion of all known reserves of fossil fuels on the Earth. The Sun, therefore, is of fundamental importance in the understanding of the uniqueness of the Earth.

The Sun is a main sequence star in the middle stages of its life and was formed

years ago. It is composed mainly of hydrogen (75% by mass) and helium (24% by mass); the remaining 1% of the Sun's mass comprises the elements oxygen; nitrogen; carbon; silicon; iron; magnesium and calcium. The emitted energy of the Sun is

W and this energy emission arises from the thermonuclear fusion of hydrogen into helium at temperatures around

K in the core of the Sun.

In the core, the dominant constituent is helium (65% by mass) and the hydrogen content is reduced to 35% by mass as a direct result of its consumption in the fusion reactions. It is estimated that the remaining hydrogen in the Sun's core is sufficient to maintain the Sun at its present luminosity and size for a further 4 × 109 years. At this stage it is expected that the Sun will expand into a red giant and engulf all of the inner planets of the solar system (i.e. Mercury, Venus, Earth and Mars).

There exists a high-pressure gradient between the core of the Sun and its perimeter, and this is balanced by the gravitational attraction of the mass of the Sun. In the core, the energy released by the thermonuclear reaction is transported by energetic photons but, because of the strong absorption by peripheral gases, most of these photons do not penetrate to the surface. This absorption causes heating in the region outside the core. In contrast, the outer layers of the Sun are continually losing energy by radiative emission into space in all regions of the electromagnetic spectrum. This causes a large temperature gradient to develop between the surface and the inner region of the Sun. This large temperature gradient produces an unstable region and large scale convection currents are set up that transfer heat to the surface of the Sun. The convection currents are visible as the fine grain structure, or granules, in high resolution photos of the Sun's surface. It is thought that the convection currents have a three-tier structure within the Sun. The largest cells, 200 × 103 km in diameter are close to the core. In the middle tier the convection cells are about 30 × 103 km in diameter and at the surface they are 1 × 103 km. The latter cells have a depth of 2000 km. In each cell, hot gas is transported towards the cooler surface, whilst the return flow transports cooler gases towards the interior.

Almost all of the solar radiation emitted into space, approximately 99.9%, originates from the visible disc of the Sun, known as the photosphere. The photosphere is the region of the Sun where the density of the solar gas is sufficient to produce and emit a large number of photons, but where the density of the overlying layers of gas is insufficient to absorb the emitted photons. This region of the Sun has a thickness of approximately 500 km but no sharp boundaries can be defined. The radiative spectrum of the Sun, when fitted to a theoretical black body curve, gives a black body temperature of 6000 K, although the effective temperature, deduced from the total energy emitted by the Sun, gives a lower temperature of 5800 K.

The photosphere is not uniform in temperature. The lower regions of the photosphere have temperatures of 8000 K, whilst the outer regions have temperatures of 4000 K. Furthermore, the convection cells produce horizontal temperature variations of 100 K between the ascending and descending currents of solar gas. Larger convection cells also appear in the photosphere and they have diameters of 30 000 km. They appear to originate from the second tier of convection within the Sun. The appearance of sunspots gives rise to horizontal variations of 2000 K within the photosphere. The inner regions of the sunspots have black body temperatures of 4000 K. Sunspots have diameters of 10 000–150 000 km and they may last for many weeks. It is thought that the ‘sunspot’ causes a localised suppression of the convection and therefore leads to a reduction in the transfer of heat into the photosphere from the interior. However, although the sunspot features are dramatic, they occupy less than 1% of the Sun's disc and therefore the effect on the luminosity of the Sun is small.

Beyond the photosphere lies the chromosphere where the temperature decreases to a minimum of 4000 K at 2000 km above the photosphere and then increases sharply to a temperature of 106 K at a height of 5000 km in the region of the corona. However, because of the low density of the gas in this region, the radiation emitted from the chromosphere and the corona only amount to 0.1% of the total radiation from the Sun.

These different temperature zones of the Sun can be observed in the solar spectrum (Figure 1.1). The visible and infra-red radiation emitted from the photosphere follow reasonably closely the black body curve for a temperature of 5800 K. However, substantial deviations from the theoretical curve occur in the X-ray and radio wavebands, and lesser deviations occur in the ultraviolet spectrum. The high temperature of the corona is responsible for the intense X-ray band, whilst the high radio frequency energy is associated with the solar wind and solar activity. However, the energy in these wavebands is a negligible fraction of the total emitted energy and therefore these very variable regions of the spectrum have little direct influence on the total solar energy received on the Earth. The depletion of energy in the ultraviolet spectrum is the result of emission at a lower temperature than the photosphere and therefore probably originates in the temperature minimum of the lower chromosphere. Recent observations have shown that the ultraviolet energy is not constant and shows considerable variability in the short-wave part of the spectrum amounting to 25% of its average value at 0.15 μm, and 1% at 0.23 μm. Again, the amount of energy in this band is relatively small and contributes less than 0.1% of the total energy. Furthermore, satellite observations of the solar spectrum since 1978 have demonstrated a 0.1% variation of the solar output over the 11 year sunspot cycle. These variations are rather small when compared to those produced by orbital variations of the Earth around the Sun.

Figure 1.1 Solar spectrum and blackbody curve: energy distribution of the Sun and a black body at 5800 K. Reproduced, with permission, from Allen, C.W., 1958, Quarterly Journal of the Royal Meteorological Society, 84: page 311, figure 3

1.2 Orbital variations in solar radiation

Let S be the total solar output of radiation in all frequencies. At a distance, r, from the centre of the Sun, imagine a sphere of radius r on which the flux of radiation will be the same (assuming the radiation from the Sun is equal in all directions). If the flux of radiation per unit area at a distance r is given by Q(r), then the total radiation on the imagined sphere is

.

In the absence of additional energy sou...

Table of contents

• Cover
• Title Page
• Copyright
• Series Foreword
• Preface to the Third Edition
• Chapter 1: The Earth within the Solar System
• Chapter 2: Composition and Physical Properties of the Ocean and Atmosphere
• Chapter 3: Radiation, Temperature and Stability
• Chapter 4: Water in the Atmosphere
• Chapter 5: Global Budgets of Heat, Water and Salt
• Chapter 6: Observations of Winds and Currents
• Chapter 7: The Influence of the Earth's Rotation on Fluid Motion
• Chapter 8: Waves and Tides
• Chapter 9: Energy Transfer in the Ocean-Atmosphere System
• Chapter 10: Mathematical Modelling of the Ocean and Atmosphere
• Chapter 11: Atmosphere-Ocean Interaction
• Chapter 12: Climate Change
• Problems
• Glossary
• General Reading
• Further Reading and References
• Figure Sources
• Appendices
• Index
• Colour plate