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The latest version of the atmospheric general circulation model ARPEGE‐Climat was used to perform 5‐member ensemble simulations for both present and RCP8.5 scenario climates (mid‐21st century). The rotated/stretched configuration enables a local horizontal resolution of less than 15 km over the tropical North Atlantic basin. Moreover, a tracking algorithm was used to extract tropical cyclones (TCs) simulated by the model. Through an Eulerian approach, this paper focuses on the relationships between TCs and rainfall over three French islands in the West Indies. Although the model underestimates the occurrence of TCs over this latitude band, especially in September, precipitation rates during TC days are realistic. Indeed, the model shows a good capacity to reproduce different relationships between island rainfall and TC characteristics such as proximity and intensity. In addition to rainfall distribution, the TC contribution to annual cumulative rainfall is also well captured by the model. We used three different series characterizing precipitation at the island scale to underline that the model overestimates the area that is impacted by TC rainfall. According to the simulations, the number of minor TCs tends to decrease in the future (about −15%) over the study domain (50–70°W × 10–25°N) despite a +1.6°C over ocean warming. In contrast, no trend was detected in the number of major hurricanes. Except for annual precipitation that decreases significantly in the future (about −15%), no significant change was detected in the relationships between TC properties and island rainfall. [ABSTRACT FROM AUTHOR]

The new CNRM-CM6 release of the CNRM/CERFACS atmospheric general circulation model has been used in a rotated/stretched configuration that allows a local horizontal resolution of less than 15 km over the tropical North Atlantic basin. Sea surface temperatures (SST) arise from a previous lower resolution simulation of the Coupled Model Intercomparison Project-5 exercise and corrected through a quantile–quantile method. Moreover, five-member ensemble simulations have been performed for both present and RCP8.5 scenario climates. For validation purposes, another five-member ensemble simulation has been performed with prescribed observed SST. Tracking of tropical cyclones (TCs) in these simulations reveals that the intensity of the simulated TCs are quite realistic and may reach the strongest hurricane ever observed, allowing to distinguish between TC categories in the analysis. Although the model tends to underestimate the occurrence of TCs over low latitudes, the realism of simulated TCs has nevertheless improved compared to previous versions of the model, due to both increased resolution and changes in the parameterizations used in the model. Changes observed in the simulations between present and future climates confirm previous results stating that there is no clear change in the overall number of TCs but an increase in the intensity of major hurricanes as well as an increase of rainfall in all TC categories. A new result suggests that TC activity response to climate warming may be significantly different from 1 month of the hurricane season to another. In our simulations we observe a robust decrease of TCs in the tropics in July while August and September experience a large increase of TCs over the mid-latitudes. Finally, we find a relation between a large increase in TC activity near the African coast and changes in the African atmospheric dynamics and rainfall in September. [ABSTRACT FROM AUTHOR]

This paper evaluates the performances of four cyclogenesis indices against observed tropical cyclone genesis on a global scale over the period 1979-2001. These indices are: the Genesis Potential Index; the Yearly Genesis Parameter; the Modified Yearly Convective Genesis Potential Index; and the Tippett et al. Index (J Clim, ), hereafter referred to as TCS. Choosing ERA40, NCEP2, NCEP or JRA25 reanalysis to calculate these indices can yield regional differences but overall does not change the main conclusions arising from this study. By contrast, differences between indices are large and vary depending on the regions and on the timescales considered. All indices except the TCS show an equatorward bias in mean cyclogenesis, especially in the northern hemisphere where this bias can reach 5°. Mean simulated genesis numbers for all indices exhibit large regional discrepancies, which can commonly reach up to ±50%. For the seasonal timescales on which the indices are historically fitted, performances also vary widely in terms of amplitude although in general they all reproduce the cyclogenesis seasonality adequately. At the seasonal scale, the TCS seems to be the best fitted index overall. The most striking feature at interannual scales is the inability of all indices to reproduce the observed cyclogenesis amplitude. The indices also lack the ability to reproduce the general interannual phase variability, but they do, however, acceptably reproduce the phase variability linked to El Niño/Southern Oscillation (ENSO)-a major driver of tropical cyclones interannual variations. In terms of cyclogenesis mechanisms that can be inferred from the analysis of the index terms, there are wide variations from one index to another at seasonal and interannual timescales and caution is advised when using these terms from one index only. They do, however, show a very good coherence at ENSO scale thus inspiring confidence in the mechanism interpretations that can be obtained by the use of any index. Finally, part of the gap between the observed and simulated cyclogenesis amplitudes may be attributable to stochastic processes, which cannot be inferred from environmental indices that only represent a potential for cyclogenesis. [ABSTRACT FROM AUTHOR]

The Weather Research and Forecast model at 1/3⅓°° resolution is used to simulate the statistics of tropical cyclone (TC) activity in the present climate of the South Pacific. In addition to the large-scale conditions, the model is shown to reproduce a wide range of mesoscale convective systems. Tropical cyclones grow from the most intense of these systems formed along the South Pacific convergence zone (SPCZ) and sometimes develop into hurricanes. The three-dimensional structure of simulated tropical cyclones is in excellent agreement with dropsondes and satellite observations. The mean seasonal and spatial distributions of TC genesis and occurrence are also in good agreement with the Joint Typhoon Warning Center (JTWC) data. It is noted, however, that the spatial pattern of TC activity is shifted to the northeast because of a similar bias in the environmental forcing. Over the whole genesis area, 8.2 ±± 3.5 cyclones are produced seasonally in the model, compared with 6.6 ±± 3.0 in the JTWC data. Part of the interannual variability is associated with El Niño--Southern Oscillation (ENSO). ENSO-driven displacement of the SPCZ position produces a dipole pattern of correlation and results in a weaker correlation when the opposing correlations of the dipole are amalgamated over the entire South Pacific region. As a result, environmentally forced variability at the regional scale is relatively weak, that is, of comparable order to stochastic variability (±±1.7 cyclones yr−−1), which is estimated from a 10-yr climatological simulation. Stochastic variability appears essentially related to mesoscale interactions, which also affect TC tracks and the resulting occurrence. [ABSTRACT FROM AUTHOR]