Landsea, C. W., 2000: El Niño-Southern Oscillation and the seasonal predictability of tropical cyclones. In press in El Niño : Impacts of Multiscale Variability on Natural Ecosystems and Society, edited by H. F. Diaz and V. Markgraf.
Perhaps the most dramatic effect that El Niño has upon the climate system is in changing tropical cyclone characteristics around the world. This chapter reviews how tropical cyclone frequency, intensity and areas of occurrence are altered in all of the cyclone basins by the phases of El Niño-Southern Oscillation (ENSO). In addition to ENSO, other global (such as the stratospheric Quasi-Biennial Oscillation) and local factors (such as sea surface temperature, monsoon intensity and rainfall, sea level pressures and tropospheric vertical shear) can also help modulate tropical cyclone variability. Understanding how these various factors relate to tropical cyclone activity can be challenging due to the fairly short (on the scale of only tens of years) record of reliable data. Despite this limitation, many of the factors that have been linked to tropical cyclones - the foremost of which being ENSO - have substantial lead relationships and can be utilized to provide seasonal forecasts of tropical cyclones. Details of methodologies that have been developed for the North Atlantic, Northwest Pacific, South Pacific and Australian basin tropical cyclones are presented as well as the real-time forecasting performance of Atlantic hurricanes as issued by Prof. William Gray.
Tropical cyclones are the costliest and deadliest natural disasters around the world, as the approximate 300,000 death toll in the infamous Bangladesh Cyclone of 1970 and the $26.5 billion (U.S.) in damages due to Hurricane Andrew in the Southeast United States can attest (Holland 1993, Hebert et al. 1996). Pielke and Pielke (1997) show that hurricane property losses - exceeding that due to earthquakes by a factor of four - account for 40% of all insured losses in the United States for the period 1984 to 1993. Understanding and being able to predict how both tropical cyclone frequencies and intensities vary from year to year is obviously a topic of great interest to meteorologists, public and private decisionmakers and the general public alike. A review of multidecadal scale tropical cyclone variations and possible "greenhouse warming" effects has been covered in Landsea (1998). This chapter will explore the role that the El Niño-Southern Oscillation and other phenomena have upon tropical cyclones around the world and what progress has been made in utilizing such information to provide seasonal forecasting of these storms.
"Tropical cyclone" is the generic term for a non-frontal synoptic scale low-pressure system that develops over tropical or sub-tropical waters with organized convection and a well-defined cyclonic surface wind circulation. Its energy source is primarily derived from evaporation and sensible heat flux from the sea in the presence of high winds and lowered surface pressure. These energy sources are tapped through condensation and fusion in convective clouds concentrated near the cyclone's "warm-core" center (Holland 1993). Tropical cyclones with maximum sustained surface winds of less than 18 ms-1 are called "tropical depressions". Once the tropical cyclone reaches winds of at least 18 ms-1 they are typically called a "tropical storm" and assigned a name. Names are decided upon by representatives from countries in the basins affected at annual World Meteorological Organization regional meetings (Neumann 1993). If winds reach 33 ms-1, they are then called: a "hurricane" (the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E); a "typhoon" (the Northwest Pacific Ocean west of the dateline); a "severe tropical cyclone" (the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E); a "severe cyclonic storm" (the North Indian Ocean); and a "tropical cyclone" (the Southwest Indian Ocean) (Neumann 1993). Additionally, the category of "intense (or major) hurricane" has been utilized for the Atlantic basin for those tropical cyclones obtaining winds of at least 50 ms-1, which corresponds to a category 3, 4 or 5 on the Saffir-Simpson hurricane intensity scale (Simpson 1974, Hebert et al. 1996).
It should be pointed out that such definitions are quite arbitrary ones and that nearly all intensity wind values at the surface are an estimation (by satellite pictures) or an extrapolation (from aircraft reconnaissance downward to the surface). Thus by the nature of the tropical cyclone, by the limited data available and by the way that meteorologists have defined intensity thresholds, the strength of individual tropical cyclones can be difficult to pinpoint with certainty. Also, changes in observational platforms available to monitor tropical cyclones can produce as much or greater change in the cyclone record as can actual climate fluctuations. Studies of interannual (and especially interdecadal) changes of tropical cyclones must carefully consider both the relative arbitrariness of the intensity record of the storms and the dependency of intensity on the observations available.