Ocean–Atmosphere Basis for Seasonal Climate Forecasting

There are many phenomena of interest in the atmosphere and ocean, only some of which are relevant for seasonal forecasting. One way of identifying the processes likely to be active is through scale analysis which identifies the important terms in the governing equations and highlights the importance of geostrophic balance. Simple arguments for Rossby waves are given. These waves are important in both atmosphere and ocean as a means of transferring energy over large distances. When the waves are embedded in a westerly flow it is possible for the waves to be stationary, giving rise to the possibility of a coherent remote response. A possible source of stationary atmospheric Rossby waves could be the deep convection over parts of the equatorial oceans where the sea surface temperatures are high. These stationary wave trains may interact with mid-latitude phenomena such as the storm tracks, so changing the occurrence and preferred locations of storms. This is an example of interaction between weather and lower frequency climate changes. Other teleconnections are introduced, such as the link between the Indian summer monsoon and Mediterranean climate. The area of the world where the interaction between the atmosphere and ocean is strongest is in the tropics. It is important to understand how the upper equatorial ocean works and how it is connected to the subtropical thermocline. The connection of the tropics to the subtropics gives a possible mechanism for low frequency variability of ENSO. Various theories of ENSO are introduced in which the importance of equatorial Kelvin and Rossby/ planetary waves is highlighted. Simple models illustrate oscillatory behaviour in certain parameter regimes but damped oscillations in others. While these ideas are interesting in generating a framework within which to consider ENSO, the real test comes from the making of forecasts and determining by experience the limits of predictability.