Chapter 1 Introduction

The Controls on Australian Climate

A complex web of physical, chemical and biological processes, interacting over a range of time scales within the total earthatmosphere climate system, determines the characteristics and behaviour of the earth’s climate (Figure 1.1). Particularly important roles are played by vertical and horizontal exchange processes in the atmosphere and oceans and by the global cycles of energy, water and carbon.

The main processes that determine the general characteristics of the earth’s climate are represented in Figure 1.1. However, two important influences that are not depicted, the shape and the rotation rate of the earth, play a key role in determining its geographical distribution. If the earth were spherical but not rotating, the incoming solar energy (shown schematically at the top left of Figure 1.1) which is greatest in the equatorial latitudes would be balanced by outgoing terrestrial (infrared) radiation (shown schematically at the top right of Figure 1.1) in the higher latitudes. In this case, a large thermally driven meridional circulation cell would be established in the atmosphere to carry excess heat from the equator to the poles. A similar circulation cell would also develop in the oceans. In reality, however, the rotation of the earth causes the meridional circulation in the atmosphere to break down into a number of smaller cells as shown in Figure 1.2 with a region of strong descent, relatively high pressure and low rainfall in the middle latitudes.

Figure 1.1 The components of the global climate system consisting of the atmosphere (including the troposphere and stratosphere), the geosphere (which includes the solid earth (lithosphere), the oceans, rivers and inland water masses (hydrosphere), and the snow, ice and permafrost (cryosphere)) and the biosphere (the transition between them within which most plant and animal life exists and most living and dead organic matter (biomass) is to be found). The main physical processes that take place within the climate system, and thus exert an influence on the patterns of global climate, are also indicated.

Australia’s position relative to the region of large scale descent at the poleward edge of the Southern Hemisphere Hadley Cell (one of the two large meridional cells shown in Figure 1.2) and the associated belt of eastward migrating high pressure systems, is responsible for the generally arid climate over much of the continent. The zonally aligned belt of low rainfall is, however, somewhat modified by the orographic influence of the Great Dividing Range on Australia’s eastern seaboard.

Another major influence on Australia’s climate is associated with the year to year variability of the tropical east-west or ‘Walker ’ circulation (top Figure 1.3) which is closely anchored to the distribution of the continents and the patterns of ocean temperature over the globe.

Figure 1.2 The main features of the general circulation of the atmosphere showing a typical daily pattern of surface pressure systems and (in greatly exaggerated vertical scale) the zonally averaged meridional (left) and zonal (right) circulation.

Figure 1.3 The Walker Circulation shown in a typical year (top) and compared with an El Niño year (bottom).

The strong high level westerly winds of the sub-tropical jet streams shown on the right in Figure 1.2 occur near the poleward limit of the cells. The east-west tropical Walker Circulation is represented schematically in top Figure 1.3. In normal seasons (top Figure 1.3), air rises over the warm western Pacific and flows eastwards in the upper troposphere to subside in the eastern Pacific high pressure system and then flow westward in the surface layers across the tropical Pacific Ocean. Weaker cells also exist over the Indian and Atlantic Oceans. In so-called El Niño years (bottom Figure 1.3), this pattern is disrupted, the central and eastern Pacific Ocean warms and the main area of ascent, with associated cloud and high rainfall, moves to the central Pacific. Such events are frequently associated with increased subsidence and a period of drought over eastern Australia. The opposite occurs in years when the main area of ascent is shifted to the west rather than east and the socalled La Niña brings above average rainfall to many parts of Australia.