Interactions océan-atmosphère au sein des cyclones tropicaux du pacifique sud : processus et climatologie

Cette thèse apporte une meilleure compréhension des interactions océan-atmosphère au sein d'évènements extrêmes que sont les cyclones tropicaux. L'étude de la réponse océanique aux cyclones et de sa rétroaction a permis de souligner l'importance de la dynamique océanique et atmosphérique et tend à contredire les estimations extrêmes faites précédemment à partir de modèles théoriques simplifiés. L'impact des cyclones sur le climat est probablement surestimé dans les études qui négligent les processus d'advection et la ré-émergence d'anomalies océaniques en surface durant l'hiver. La rétroaction négative du refroidissement de surface induit par les cyclones est également surestimée dans les études théoriques en raison des fortes hypothèses faites sur les échelles de temps impliquées dans le processus d'intensification des cyclones. De même, la structure océanique à grande et moyenne échelle est souvent négligée (par exemple dans les indices de cyclogenèse), alors qu'elle module fortement les mécanismes de couplage. Enfin, l'utilisation de modèles à méso-échelle et de simulations à long terme produisant un grand nombre d'événements est essentielle afin de séparer un mécanisme robuste d'effets anecdotiques.Tropical cyclone (TC)-ocean interactions are essential for cyclone formation and evolution. Surface cooling is observed in the cyclone wake and is expected to exert a negative feedback to the storm intensity. This thesis provide a quantification of the ocean response and its feedback using a coupled regional model of the southwest Pacific developed for present climate long-term simulations at mesoscale resolution which are requested to separate robust features from anecdotic effects. The results highlight the neglected role of three-dimensional dynamics in the ocean and the atmosphere and tend to contradict the extreme estimations made from simple theoretical models. Previous estimates that neglect the upwelling process and ocean warm anomaly re-emergence by winter entrainment overestimate the local heat uptake by the ocean. The intensity distribution of TCs is significantly affected by the cold wake but the feedback of SST cooling to storm intensity is of moderate amplitude, compared with theoretical models based on thermodynamic arguments. Actually, our analyses contradict the direct thermodynamic control of TC intensification by surface moisture fluxes in favor of a storm-scale dynamic control. In addition, regional oceanography has a large impact on coupling. It is stronger in the Coral Sea that has shallow mixed layer and numerous eddies but extremely weak in the warm pool that has deep mixed layer, thick barrier layer and no mesoscale activity

Diagnosing tropical cyclone intensity variations from the surface wind field evolution

Tropical cyclone (TC) intensity fluctuations remain a challenge for TC forecasters. Occurring through a wide range of processes, such as vortex contraction, eyewall replacements, or emission of vortex Rossby waves, they are inherently multiscale, transient, and asymmetric. In a recent study, estimates of surface wind field inner-core properties from high-resolution satellite observations were spotted as valuable for the improvement of intensity variations statistical predictability. The present study evaluates how the temporal evolution of the vortex structure, at scales ranging from O(1km) to vortex-wide, further provides insights on the modulation of intensity. The study is based on a set of seven realistic TC simulations with one-kilometer grid spacing. The surface wind field structure is studied through an original set of descriptors which characterize the radial profile, the azimuthal asymmetries, and their spectral distribution. While radial gradients evolve concurrently with intensity, the azimuthal variability of the inner-core shows a stronger connection with shorter-scale intensity modulation. The increase of high wavenumber asymmetries distributed around the ring of maximum winds is shown to precede phases of rapid (re-)intensification by 5-6h, while the concentration of asymmetry in wavenumbers 1 and 2 leads to intensity weakening. A machine learning classification finally highlights that the classification of intensification phases (i.e. intensification or weakening) can be improved by at least 11% (thus reaching ∼75%) when accounting for the evolution of the radial wind gradient and the variance distribution among scales in the ring of maximum wind, relative to the sole use of vortex-averaged parameters

Disentangling the mesoscale ocean‐atmosphere interactions

International audienceIn the decades, the use of scatterometer data allowed to demonstrate the global ubiquity of the ocean mesoscale thermal feedback (TFB) and current feedback (CFB) effects on surface winds and stress. Understanding these air-sea interactions is of uttermost importance as the induced atmospheric anomalies partly control the ocean circulation and thus can influence the Earth climate. Whether the TFB and CFB effects can be disentangled, and whether satellite scatterometers can properly reveal them, remain rather unclear. Here, using satellite observations and ocean-atmosphere coupled mesoscale simulations over 45 degrees S to 45 degrees N, we show that the CFB effect can be properly characterized and unraveled from that due to the TFB. We demonstrate that the TFB can be unambiguously characterized by its effect on the stress (and wind) divergence and magnitude. However, its effect on the wind and stress curl is contaminated by the CFB and thus cannot be estimated from scatterometer data. Finally, because scatterometers provide equivalent neutral stability winds relative to the oceanic currents, they cannot characterize adequately the CFB wind response and overestimate the TFB wind response by approximate to 25%. Surface stress appears to be the more appropriate variable to consider from scatterometer data

Observations of tropical cyclone inner-core fine-scale structure, and its link to intensity variations

Tropical Cyclone (TC) internal dynamics have emerged over recent decades as a key to understand their intensity variations, but are difficult to observe, as they are sporadic, multi-scale, and occur in areas of very strong wind gradients. The present work aims at describing the internal structure of TCs, as observed with newly available satellite synthetic aperture radars (SARs) wind products, and at evaluating relations between this structure and the TC life cycle. It is based on a unique dataset of 188 SAR high-resolution (1 km) images, containing 15 to 47 by intensity category. An extraction method is designed to retrieve and characterize, the TC radial profile, its azimuthal degree of asymmetry, and the energy distribution in the eyewall and maximum wind areas. Vortex contraction and sharpening of the eyewall wind radial gradient with increasing TC intensity are observed, as well as a symmetrization of energy distribution around the vortex. Eyewall high wave number structures show a dependence on the life cycle phase, supporting previous findings discussing the vortex rapid evolution with onset and propagation of eyewall mesovortices and associated vortex Rossby wave generation. A machine learning approach finally highlights that the eye shape and eyewall radial wind gradient fine-scale dynamics have the potential to improve the statistical prediction of TC intensity variations, compared to the sole use of vortex averaged parameters and synoptic information. The high-resolution radial and azimuthal coverage provided by SARs make these acquisitions a very valuable tool for TC research and operational application

Western and Central tropical Pacific rainfall response to climate change: Sensitivity to projected Sea Surface Temperature patterns

Rainfall projections from the Coupled Model Intercomparison Project (CMIP) models are strongly tied to projected Sea Surface Temperature (SST) spatial patterns through the “warmer-gets-wetter” mechanism. While these models consistently project an enhanced equatorial warming they however indicate much more uncertain changes in zonal SST gradients. That translates into large uncertainties on rainfall projections. Here, we force an atmospheric model with synthetic SSTs whose zonal SST gradient changes span the range of CMIP5 uncertainties in the presence and in the absence of the robust equatorially enhanced warming. Our results confirm that projected rainfall changes are dominated by the effect of circulation changes, which are tied to SST through the “warmer-gets-wetter” mechanism. We show that SPCZ rainfall changes are entirely driven by the uncertain zonal SST gradient changes. The western equatorial Pacific rainfall increase is largely controlled by the robust enhanced equatorial warming for modest zonal SST gradient changes. However, for larger values, the effect of the zonal SST gradient change on rainfall projections becomes dominant due to nonlinear interactions with the enhanced equatorial warming. Overall, our study demonstrates that uncertainties in the zonal SST gradient changes strongly contribute to uncertainties in rainfall projections over both the South Pacific Convergence Zone and western equatorial Pacific. It is thus critical to reduce these uncertainties to produce more robust precipitation estimates

Unveiling the global influence of tropical cyclones on extreme waves approaching coastal areas

International audienceAbstract Tropical and extra-tropical storms generate extreme waves, impacting both nearby and remote regions through swell propagation. Despite their devastating effects in tropical areas, the contribution of tropical cyclones (TCs) to global wave-induced coastal risk remains unknown. Here, we enable a quantitative assessment of TC’s role in extreme waves approaching global coastlines, by designing twin oceanic wave simulations with and without realistic TC wind forcing. We find that TCs substantially contribute to extreme breaking heights in tropical regions (35-50% on average), reaching 100% in high-density TC areas like the North Pacific. TCs also impact remote TC-free regions, such as the equatorial Pacific experiencing in average 30% of its extreme wave events due to TCs. Interannual variability amplifies TC-induced wave hazards, notably during El Niño in the Central Pacific, and La Niña in the South China Sea, Caribbean Arc, and South Indian Ocean coastlines. This research offers critical insights for global risk management and preparedness

Unveiling the global influence of tropical cyclones on extreme waves approaching coastal areas

International audienceAbstract Tropical and extra-tropical storms generate extreme waves, impacting both nearby and remote regions through swell propagation. Despite their devastating effects in tropical areas, the contribution of tropical cyclones (TCs) to global wave-induced coastal risk remains unknown. Here, we enable a quantitative assessment of TC’s role in extreme waves approaching global coastlines, by designing twin oceanic wave simulations with and without realistic TC wind forcing. We find that TCs substantially contribute to extreme breaking heights in tropical regions (35-50% on average), reaching 100% in high-density TC areas like the North Pacific. TCs also impact remote TC-free regions, such as the equatorial Pacific experiencing in average 30% of its extreme wave events due to TCs. Interannual variability amplifies TC-induced wave hazards, notably during El Niño in the Central Pacific, and La Niña in the South China Sea, Caribbean Arc, and South Indian Ocean coastlines. This research offers critical insights for global risk management and preparedness

StormR: An R package to quantify and map the tropical storms and cyclones’ winds characteristics

International audienceStormR is an R package allowing the easy extraction of storm track data from a provided database and the generation of surface wind fields (speed and direction) as reconstructed from storm track data and a parametric cyclone model. Then StormR allows us to compute three summary statistics (the maximum sustained wind speed, the power dissipation index, and the duration of exposure to winds reaching a given wing speed along the cyclone life span) and to plot the results. We suggest to use the IBTrACS (International Best Track Archive for Climate Stewardship) database as input (Knapp et al., 2010, 2018). This database provides a fairly comprehensive record of tropical storms and cyclones with a 3-hours temporal resolution since 1841. However any storm track data can be used as long as the mandatory fields are provided

StormR: An R package to quantify and map the tropical storms and cyclones’ winds characteristics

StormR is an R package allowing the easy extraction of storm track data from a provided database and the generation of surface wind fields (speed and direction) as reconstructed from storm track data and a parametric cyclone model. Then StormR allows us to compute three summary statistics (the maximum sustained wind speed, the power dissipation index, and the duration of exposure to winds reaching a given wing speed along the cyclone life span) and to plot the results. We suggest to use the IBTrACS (International Best Track Archive for Climate Stewardship) database as input (Knapp et al., 2010, 2018). This database provides a fairly comprehensive record of tropical storms and cyclones with a 3-hours temporal resolution since 1841. However any storm track data can be used as long as the mandatory fields are provided. Storm track data can be extracted using a specified point location, a user defined spatial polygon shapefile, a country or a cyclone basin name. The main functions of the StormR package allow us to generate wind speed and direction fields as reconstructed from storm track data and a parametric cyclone model. Different models and models combinations can be chosen by the user. By default the spatial resolution is set to 2.5 min (~4.5 km at the equator), but a finer spatial resolution of 30 s (~1 km at the equator) and coarser spatial resolutions of 5 min (~9 km at the equator) or 10 min (~18.6 km at the equator) can be set. The temporal resolution is set to 1 hour by default but finer resolutions of 45 min, 30 min, or 15 min can be set. Once wind speed is generated for each cell or specific location and each time step, StormR functions can compute summary statistics on wind speed over the lifespan of a storm. Summary statistics encompass the maximum sustained wind speed, the power dissipation index or total power dissipated by a tropical storm (Emanuel, 1999, 2005), and the duration of exposure to winds reaching defined speed thresholds. By default the duration of exposure is computed for each Saffir-Simpson Hurricane Scale threshold values for tropical cyclone categories, i.e., 33, 43, 50, 58, and 70 .−1 (Simpson, 1974), but can be defined by the user