A 17-yr “climatology” of tropical-system activity, track, size, and 24-h intensity change in the southwest Indian Ocean (SWIO) is developed and analyzed in comparison with other intensively studied basins such as the North Atlantic Ocean. A first formulation of the empirical maximum potential intensity of SWIO tropical systems is also proposed, along with the climatology of sea surface temperatures from September to June. Systems with a 34-kt (1 kt = 0.514 m s−1) wind radius that does not exceed 46 km are considered to be very small or midget systems, on the basis of the 5th percentile of storm size distribution. Using the 95th percentile of overwater intensity changes, rapid intensification (RI) is statistically defined by a minimum increase of 15.4 m s−1 day−1 in the maximum 10-min mean surface wind speed (VMAX). This value is similar to the 30-kt threshold commonly used in the North Atlantic basin for 1-min sustained wind speeds. Rapid decay (RD) can be statistically defined by a minimum weakening of 13.9 m s−1 day−1, although the spread in the 5th percentile of intensity changes among the different intensity classes indicates that it is not as appropriate to use a unique RD threshold for all systems. It is shown that 43% of all tropical systems and all very intense tropical cyclones (VMAX ≥ 59.6 m s−1) underwent RI at least once during their lifetimes. It is highlighted that systems have a greater propensity to intensify rapidly for an initial intensity between 65 and 75 kt. Statistics indicate that operational intensity forecast errors are significantly greater at short range for RI cases while track errors are reduced.

This study is part of the efforts undertaken to resolve the ''bad trough/good trough'' issue for tropical cyclone (TC) intensity changes and to improve the prediction of these challenging events. Sensitivity experiments are run at 8-km resolution with vortex bogusing to extend the previous analysis of a real case of TC-trough interaction (Dora in 2007). The initial position and intensity of the TC are modified, leaving the trough unchanged to describe a realistic environment. Simulations are designed to analyze the sensitivity of TC prediction to both the variety of TC-trough configurations and the current uncertainty in model analysis of TC intensity and position.Results show that TC intensification under upper-level forcing is greater for stronger vortices. The timing and geometry of the interaction between the two cyclonic potential vorticity anomalies associated with the cutoff low and the TC also play a major role in storm intensification. The intensification rate increases when the TC (initially located 128 northwest of the trough) is displaced 18 closer. By allowing a gradual deformation and equatorward tilting of the trough, both scenarios foster an extended ''inflow channel'' of cyclonic vorticity at midlevels toward the vortex inner core. Conversely, unfavorable interaction is found for vortices displaced 38 or 48 east or northeast. Variations in environmental forcing relative to the reference simulation illustrate that the relationship between intensity change and the 850-200-hPa wind shear is not systematic and that the 200-hPa divergence, 335-350-K mean potential vorticity, or 200-hPa relative eddy momentum fluxes may be better predictors of TC intensification during TC-trough interactions.