A significant number of tropical cyclones move into the midlatitudes and transform into extratropical cyclones. This process is generally referred to as extratropical transition (ET). During ET a cyclone frequently produces intense rainfall and strong winds and has increased forward motion, so that such systems pose a serious threat to land and maritime activities. Changes in the structure of a system as it evolves from a tropical to an extratropical cyclone during ET necessitate changes in forecast strategies. In this paper a brief climatology of ET is given and the challenges associated with forecasting extratropical transition are described in terms of the forecast variables (track, intensity, surface winds, precipitation) and their impacts (flooding, bush fires, ocean response). The problems associated with the numerical prediction of ET are discussed. A comprehensive review of the current understanding of the processes involved in ET is presented. Classifications of extratropical transition are described and potential vorticity thinking is presented as an aid to understanding ET. Further sections discuss the interaction between a tropical cyclone and the midlatitude environment, the role of latent heat release, convection and the underlying surface in ET, the structural changes due to frontogenesis, the mechanisms responsible for precipitation, and the energy budget during ET. Finally, a summary of the future directions for research into ET is given.

The objective of this study is to understand how interactions with upper-tropospheric troughs affect the intensity of tropical cyclones. The study includes all named Atlantic tropical cyclones between 1985 and 1996. To minimize other factors affecting intensity change, times when storms are over subcritical sea surface temperatures (Յ26ЊC) or near landfall are removed from the sample. A trough interaction is defined to occur when the eddy momentum flux convergence calculated over a 300-600-km radial range is greater than 10 (m s Ϫ1) day Ϫ1. The trough interaction cases are separated into four composites: (i) favorable superposition [tropical cyclone intensifies with an upper-tropospheric potential vorticity (PV) maximum within 400 km of the tropical cyclone center], (ii) unfavorable superposition, (iii) favorable distant interaction (upper PV maximum between 400 and 1000 km from the tropical cyclone center), and (iv) unfavorable distant interaction. For comparison, two additional composites are created: (v) favorable no trough, and (vi) unfavorable no trough. Tropical cyclones over warm water and away from land are more likely to intensify than weaken after an interaction with an upper-level trough; 78% of superposition cases and 61% of distant interaction cases deepened. In the favorable superposition composite, intensification begins soon after a small-scale upper-tropospheric PV maximum approaches the storm center. As in previous studies, the PV maximum subsequently weakens, most likely due to diabatic heating, and never crosses the center and reverses the deepening. In the favorable distant interaction composite, the upper PV maximum remains well to the west of the tropical cyclone center, and intensification is not due to superposition. Strong upper-level divergence occurs downshear of the center, and an upper-level jet is located poleward of the maximum divergence. The center of the intensifying tropical cyclone is located in the right entrance region of the jet, where upward motion is favored. It is argued that the tropical cyclone and upper-level jet develop in a coupled fashion. In the unfavorable distant interaction composite, weakening is attributed to a slightly larger and stronger upper PV maximum than occurs in the favorable distant interaction composite, which induces about 5 m s Ϫ1 more vertical wind shear over the tropical cyclone center. The fairly subtle PV changes that bring about this increase in vertical shear may help account for the difficulty in forecasting tropical cyclone intensity change during distant trough interactions. The no-trough composites have dramatically smaller azimuthal asymmetries than those involving trough interactions. The major distinguishing factor between deepening and filling storms in the no-trough composites is the magnitude of the vertical wind shear.

El Niño-Southern Oscillation (ENSO) is a natural, coupled atmospheric-oceanic cycle that occurs in the tropical Pacific Ocean on an approximate time scale of 2-7 years. ENSO events have been shown in previous studies to be related to regional extremes in weather (e.g., hurricane occurrences, frequency and severity of tornadoes, droughts, and floods). The teleconnection of ENSO events to extreme weather events means the ability to classify an event as El Niño or La Niña is of interest in scientific and other applications. ENSO is most often classified using indices that indicate the warmth and coolness of equatorial tropical Pacific Ocean sea-surface temperatures (SSTs). Another commonly used index is based on sea-level pressure differences measured across the tropical Pacific Ocean. More recently, other indices have been proposed and have been shown to be effective in describing ENSO events. There is currently no consensus within the scientific community as to which of many indices best captures ENSO phases. The goal of this study is to compare several commonly used ENSO indices and to determine whether or not one index is superior in defining ENSO events; or alternatively, to determine which indices are best for various applications. The response and sensitivity of the SST-based indices and pressure-based indices are compared. The Niño 4 index has a relatively weak response to El Niño; the Niño 1+2 index has a relatively strong response to La Niña. Analysis of the sensitivity of the indices relative to one another suggests that the choice of index to use in ENSO studies is dependent upon the phase of ENSO that is to be studied. The JMA index is found to be more sensitive to La Niña events than all other indices. The SOI, Niño 3.4, and Niño 4 indices are almost equally sensitive to El Niño events and are more sensitive than the JMA, Niño 1+2, and Niño 3 indices.

Since 1991, the Florida Division of Forestry has been making seasonal fire severity forecasts based on a relationship between area burned in Florida and El Niño–Southern Oscillation (ENSO). The present study extends the original analysis on which these forecasts are based and attempts to augment it with the addition of other patterns of climate variability. Two atmospheric teleconnection patterns, the North Atlantic Oscillation and Pacific–North American pattern, are examined as potential indicators of seasonal and monthly area burned in Florida. Although ENSO was the only climate index to show a significant correlation to area burned in Florida, the Pacific–North American pattern (PNA) is shown to be a factor influencing fire season severity although the relationship is not monotonic and therefore not revealed by correlation analysis.

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