Climate System Dynamics and Modelling

OUTLINE

This first chapter describes the main components of the climate system as well as some processes that will be necessary to understand the mechanisms analysed in the chapters that follow. Complementary information is available in the Glossary for readers not familiar with some of the notions introduced here.

Introduction

Climate is traditionally defined as a description in terms of the mean and variability of relevant atmospheric variables such as temperature, precipitation and wind. Climate thus can be viewed as a synthesis or aggregate of weather. This implies that portrayal of the climate in a particular region must contain an analysis of mean conditions, of the seasonal cycle and of the probability of extremes such as severe frost, storms and so on. In accordance with the standard of the World Meteorological Organisation (WMO), thirty years is the classic period for performing the statistics used to define climate. This is well adapted for studying recent decades because it requires a reasonable amount of data along with a good sample of the different types of weather that can occur in a particular area. However, when analysing the more distant past, such as the last glacial maximum around 21,000 years ago, climatologists are often interested in variables which are characteristic of longer time intervals. As a consequence, the thirty-year period proposed by the WMO should be considered more as a practical indicator than as a norm that must be followed in all cases. This definition of climate as representative of conditions over several decades should not, of course, obscure the fact that climate can change rapidly. Nevertheless, a substantial time interval is needed to observe a difference in climate. In general, the smaller the difference between two periods, the longer is the time required to confidently identify any climate changes between those periods.

We also must take into account the fact that the state of the atmosphere used in the preceding definition of climate is influenced by numerous processes involving not only the atmosphere but also the oceans, sea ice, vegetation and so on. Climate is therefore now defined with increasing frequency in the wider sense of a description of the climate system.