What do typical patterns of SST fluctuation look like? Figure 1.3 shows, for the tropical Pacific, the climatological normal (30-year average) SST observed during October-December (Fig. 1.3a), the average SST observed during the five warmest October-December seasons in the Nino3 curve (Fig. 1.3b), and (in Fig. 1.3c) the difference between the patterns in Fig. 1.3b and Fig. 1.3a, that is, the mean SST anomaly observed during the five warmest October-December seasons as measured in the Nino3 region. The pattern in Fig. 1.3c describes the mean SST anomaly observed during El Niño events, though as can be seen from Fig. 1.2, the warmings associated with El Niño events are never exactly the same. (More details and references on El Niño can be found elsewhere). Although El Niño is the leading pattern of SST variation on year-to- year timescales, there are many other ways in which SST can vary, though some of these are found themselves to be related to the El Niño patterns in the Pacific. For example, in the tropical Atlantic, Fig. 1.3d shows the climatological July-September SST, Fig. 1.3e shows the SST during the five warmest July-September seasons in a region close to and just south of the Equator, and Fig 1.3f shows the anomalous SST in those years. This pattern is known to be a leading way in which tropical Atlantic SST varies from year to year.

SST anomaly patterns often persist for a few months to seasons, and some patterns persist for even longer. This brings a degree of predictability to some atmospheric climate anomalies based solely on (i) assuming the SST anomalies will persist into the period being forecast, (ii) knowing the atmospheric climate patterns that the SST patterns are likely to generate.

The problem is complicated because the atmosphere (winds, surface air temperature) also impacts the ocean. This first lecture focuses on the impact of SST on the atmosphere - establishing this is a major step toward a degree of climate predictability on seasonal to interannual (SI) timescales, for the reasons outlined in the above paragraph.