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Global Guide to Tropical Cyclone Forecasting:
CHAPTER 5: SEASONAL FORECASTING

5.1 INTRODUCTION

Tropical cyclones typically do not evolve in climatologically fixed conditions but rather in environments that exhibit favourable weekly to seasonal scale deviations from the long-term background climatology. Although the role of the environment has been known and accepted in varying degrees by tropical cyclone specialists for many years, the full extent of this role is only now becoming better understood and appreciated.

Recent studies have contributed to improved understanding of how month-to-month and season-to-season variations of wind and thermodynamic factors are associated with large variations in seasonal tropical cyclone frequency and intensity. Examples include: Gray (1988), Gray and Sheaffer (1991), Landsea and Gray (1992), Gray et al. (1992), Joseph and Liebmann (1991), Hastenrath (1986, 1990) and Nicholls (1985, 1992). Previously, we had pictured the tropical cyclone and the weaker mesoscale systems which spawned them as primarily the products of rapidly varying local circulation characteristics that have a large random component and for which prediction was not possible more than a few days ahead. Although this is still true for individual mesoscale cyclone systems, it is not the case for the seasonal aggregate of systems; particularly in the case of the North Atlantic basin.

In this chapter we first examine some known relationships between the El Nino Southern Oscillation (ENSO) and tropical cyclone activity in each ocean basin (Section 5.2). Relationships with the Quasi-Biennial Oscillation (QBO) are discussed in Section 5.3, and the potential for intraseasonal prediction is covered in Section 5.4. Considerable success has been demonstrated with seasonal forecasts of tropical cyclones in the North Atlantic in recent years; the techniques and relationships are therefore described in detail in Section 5.5.

Contents Chapter 5.2