Figure 1.2 illustrates the concept of climate anomalies. It shows time series of sea-surface temperature (SST) averaged for a region in the eastern tropical Pacific. The normal annual cycle of temperatures is shown in Fig 1.2a. The actual observed temperature in each month from 1950 is shown in blue (Fig. 1.2b), along with the repeating normal annual cycle (in red). Periods of several consecutive seasons when the temperature is above and below the normal annual cycle are clear. Plotting the difference between the actual temperature and the normal temperature shows the time series of SST anomalies (Fig. 1.2c). Indeed, such substantial SST fluctuations are now understood to occur naturally in the climate system on timescales from seasons to decades and beyond. Furthermore, it has been shown that these fluctuations in SST force changes in seasonal climate patterns extending around the globe.