Browsing by Author "Lengaigne, Matthieu"
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Item Restricted ENSO and greenhouse warming(Nature Research, 2015-08-17) Cai, Wenju; Santoso, Agus; Wang, Guojian; Yeh, Sang-Wook; An, Soon-II; Cobb, Kim M.; Collins, Matt; Guilyardi, Eric; Jin, Fei-Fei; Kug, Jong-Seong; Lengaigne, Matthieu; McPhaden, Michael J.; Takahashi, Ken; Timmermann, Axel; Vecchi, Gabriel; Watanabe, Masahiro; Wu, LixinThe El Nino/Southern Oscillation (ENSO) is the dominant climate phenomenon affecting extreme weather conditions worldwide. Its response to greenhouse warming has challenged scientists for decades, despite model agreement on projected changes in mean state. Recent studies have provided new insights into the elusive links between changes in ENSO and in the mean state of the Pacific climate. The projected slow-down in Walker circulation is expected to weaken equatorial Pacific Ocean currents, boosting the occurrences of eastward-propagating warm surface anomalies that characterize observed extreme El Nino events. Accelerated equatorial Pacific warming, particularly in the east, is expected to induce extreme rainfall in the eastern equatorial Pacific and extreme equatorward swings of the Pacific convergence zones, both of which are features of extreme El Nino. The frequency of extreme La Nina is also expected to increase in response to more extreme El Ninos, an accelerated maritime continent warming and surface-intensified ocean warming. ENSO-related catastrophic weather events are thus likely to occur more frequently with unabated greenhouse-gas emissions. But model biases and recent observed strengthening of the Walker circulation highlight the need for further testing as new models, observations and insights become available.Item Open Access Impact de la variabilité climatique sur l’hydrologie du bassin amazonien(Université Pierre et Marie Curie, 2009-01-13) Espinoza, Jhan Carlo; Janicot, Serge; Lengaigne, MatthieuLe bassin amazonien est le plus important bassin versant du monde par sa taille d’environ 6 000 000 km2 (≈ 5% des terres émergées) et son débit moyen annuel de 209 000 m3/s (Molinier et al., 1996). Il s’étend sur 7 pays, le Brésil, où se trouve 63% de sa superficie, le Pérou (16%), la Bolivie (12%), la Colombie (6%), l’Equateur (2%), le Venezuela et la Guyana (1%). Il abrite la plus grande forêt tropicale de la planète et un des plus riches écosystèmes connus (Turner, 2001). Pour ces raisons, compte tenu des enjeux liés au changement climatique et à la déforestation, la communauté scientifique internationale a déployé des efforts considérables dans les dernières années pour mieux comprendre les mécanismes climatiques dans cette région. Cependant les impacts du climat sur l’hydrologie restent un sujet peu abordé alors que se éveloppent de grands équipements le long de rivières (barrages hydroélectriques, hydrovias ou voies fluviales, etc) et que surviennent des événements extrêmes comme la seca de 2005. Ce manque est plus important encore dans les pays andins (Pérou, Bolivie, Colombie, Equateur) à cause principalement des difficultés d’accès aux données in situ et de la complexité géographique de cette région. Une vision globale du rôle du climat sur l’hydrologie du bassin amazonien faisant donc défaut, nous nous proposons de contribuer à combler cette lacune.Item Restricted Increased frequency of extreme La Niña events under greenhouse warming(Nature Research, 2015) Cai, Wenju; Wang, Guojian; Santoso, Agus; McPhaden, Michael J.; Wu, Lixin; Jin, Fei-Fei; Timmermann, Axel; Collins, Mat; Vecchi, Gabriel; Lengaigne, Matthieu; England, Matthew H.; Dommenget, Dietmar; Takahashi, Ken; Guilyardi, EricThe El Niño/Southern Oscillation is Earth’s most prominent source of interannual climate variability, alternating irregularly between El Niño and La Niña, and resulting in global disruption of weather patterns, ecosystems, fisheries and agriculture1,2,3,4,5. The 1998–1999 extreme La Niña event that followed the 1997–1998 extreme El Niño event6 switched extreme El Niño-induced severe droughts to devastating floods in western Pacific countries, and vice versa in the southwestern United States4,7. During extreme La Niña events, cold sea surface conditions develop in the central Pacific8,9, creating an enhanced temperature gradient from the Maritime continent to the central Pacific. Recent studies have revealed robust changes in El Niño characteristics in response to simulated future greenhouse warming10,11,12, but how La Niña will change remains unclear. Here we present climate modelling evidence, from simulations conducted for the Coupled Model Intercomparison Project phase 5 (ref. 13), for a near doubling in the frequency of future extreme La Niña events, from one in every 23 years to one in every 13 years. This occurs because projected faster mean warming of the Maritime continent than the central Pacific, enhanced upper ocean vertical temperature gradients, and increased frequency of extreme El Niño events are conducive to development of the extreme La Niña events. Approximately 75% of the increase occurs in years following extreme El Niño events, thus projecting more frequent swings between opposite extremes from one year to the next.Item Restricted Large-scale circulation patterns and related rainfall in the Amazon Basin: a neuronal networks approach(Springer, 2011-02-11) Espinoza, Jhan Carlo; Lengaigne, Matthieu; Ronchail, Josyane; Janicot, SergeThis study describes the main circulation patterns (CP) in the Amazonian Basin over the 1975–2002 period and their relationship with rainfall variability. CPs in the Amazonian Basin have been computed for each season from the ERA-40 daily 850 hPa winds using an approach combining artificial neural network (Self Organizing Maps) and Hierarchical Ascendant Classification. A 6 to 8 cluster solutions (depending on the season considered) is shown to yield an integrated view of the complex regional circulation variability. For austral fall, winter and spring the temporal evolution between the different CPs shows a clear tendency to describe a cycle, with southern wind anomalies and their convergence with the trade winds progressing northward from the La Plata Basin to the Amazon Basin. This sequence is strongly related to eastward moving extra tropical perturbations and their incursion toward low latitude that modulate the geopotential and winds over South America and its adjoining oceans. During Austral summer, CPs are less spatially and temporally organized compared to other seasons, principally due to weaker extra tropical perturbations and more frequent shallow low situations. Each of these CPs is shown to be associated with coherent northward moving regional rainfall patterns (both in in situ data and ERA-40 reanalysis) and convective activity. However, our results reveals that precipitation variability is better reproduced by ERA-40 in the southern part of the Amazonian Basin than in the northern part, where rainfall variability is likely to be more constrained by local and subdaily processes (e.g. squall lines) that could be misrepresented in the reanalysis dataset. This analysis clearly illustrates the existing connections between the southern and northern part of the Amazonian Basin in terms of regional circulation/rainfall patterns. The identification of these CPs provide useful information to understand local rainfall variability and could hence be used to better understand the influence of these CPs on the hydrological variability in the Amazonian Basin.Item Restricted Revisiting wintertime cold air intrusions at the east of the Andes: propagating features from subtropical Argentina to Peruvian Amazon and relationship with large-scale circulation patterns(Springer, 2013) Espinoza, Jhan Carlo; Ronchail, Josyane; Lengaigne, Matthieu; Quispe, Nelson; Silva Vidal, Yamina; Bettolli, María Laura; Avalos, Grinia; Llacza, AlanThis study investigates the spatial and temporal characteristics of cold surges that propagates northward along the eastern flank of the Andes from subtropical to tropical South America analysing wintertime in situ daily minimum temperature observations from Argentina, Bolivia and Peru and ERA-40 reanalysis over the 1975–2001 period. Cold surges usually last 2 or 3 days but are generally less persistent in the southern La Plata basin compared to tropical regions. On average, three to four cold surges are reported each year. Our analysis reveals that 52 % of cold episodes registered in the south of La Plata basin propagate northward to the northern Peruvian Amazon at a speed of around 20 m s⁻¹. In comparison to cold surges that do not reach the tropical region, we demonstrate that these cold surges are characterized, before they reach the tropical region, by a higher occurrence of a specific circulation pattern associated to southern low-level winds progression toward low latitudes combined with subsidence and dry condition in the middle and low troposphere that reinforce the cold episode through a radiative effect. Finally, the relationship between cold surges and atmosphere dynamics is illustrated for the two most severe cold intrusions that reached the Peruvian and Bolivian Amazon in the last 20 years.