Vincent Moron

Anomalies de température de surface de la mer et précipitations tropicales : Synthèse de quelques travaux récents portant sur les précipitations au Sahel et dans le Nordeste

francaisLe cycle de leau aux latitudes tropicales depend en grande partie des etats de surface de la Terre via la convection profonde. A ce titre, les Temperatures de surface marine (TSM) sont determinantes. Leur etude montre une evolution a long terme significative : rechauffement sur le Pacifique depuis le milieu des annees soixante-dix, compatible avec des phases chaudes plus intenses du phenomene couple El Nino ? Oscillation australe ; anomalies du gradient meridien de temperature sur lAtlantique tropical depuis le debut des annees soixante-dix. La variabilite des TSM planetaires peut etre decomposee en modes propres qui definissent des aires de variabilite coherentes sur le Pacifique, lAtlantique et locean Indien. Ces modes de variabilite sont associes a des structures particulieres danomalies TSM qui presentent des connexions statistiques avec la pluviometrie tropicale periatlantique sur les regions du Nordeste bresilien et du Sahel africain. Afin de mieux documenter ces relations, des experiences complementaires ont ete menees a laide de deux modeles numeriques de circulation generale de latmosphere (LMD/CNRS et CNRM/Meteo-France) disponibles en France : lune exprime la reponse atmospherique a levolution des TSM observees sur la periode 1970-1988, lautre la reponse a lequilibre a des anomalies TSM idealisees. Les resultats montrent que les connexions TSM-pluviometrie tropicale sont une manifestation du forcage des TSM sur les cumuls de precipitations a lechelle regionale, forcage dans lequel les anomalies de gradient meridien des TSM dans lAtlantique et la variabilite TSM sur les trois bassins equatoriaux sont significativement impliques. Ces structures TSM generent des anomalies dans la circulation divergente de latmosphere, induisant une variabilite damplitude et de positionnement des branches verticales des cellules divergentes de types Hadley et Walker au-dessus du Nordeste en mars-avril et du Sahel en juillet-septembre. EnglishThe hydrological cycle in the tropics depends greatly on deep convection triggered by surface conditions, so Sea Surface Temperatures (SST) are decisive. The study of the SST shows a significant long term evolution: warming in the Pacific ocean since the mid-1970s consistent with the intensification of the El Nino - Southern Oscillation warm events); an anomaly in the meridional SST gradient in the tropical Atlantic during the post-1970 period. The global SST variability can be decomposed in eigenmodes which define areas of coherent variability in the Pacific, Atlantic and Indian oceans. These patterns of variability are associated with some particular structures of SST anomalies statistically related to the tropical rainfall around the Atlantic in the Nordeste (Brazil) and Sahel (Africa) regions. In order to better document these relationships numerical experiments have been performed with the two French atmosphere general circulation models (LMD/CNRS and CNRM/Meteo-France). The first investigates the response of the atmosphere to the evolution of the observed SST during the period 1970-1988, the second seeks the equilibrium response to a set of idealised SST anomalies. Results show that connections between SST and tropical rainfall at regional scale occur through the SST forcing on the rainfall amounts and that anomalies in the meridional SST gradients in the Atlantic and the variability in the three equatorial basins are significantly involved. These SST structures lead to anomalies in the divergent circulation of the atmosphere inducing anomalies in the amplitude and the position of the Hadley and Walker cells over Nordeste (March-April) and Sahel (July-tember).

On the spatial coherence of sub-seasonal to seasonal Indian rainfall anomalies

The spatial coherence of interannual variations of sub-seasonal to seasonal anomalies in Indian summer monsoon rainfall is investigated at 0.25° spatial resolution using various metrics, including estimates of the number of degrees of freedom, the spatial scale of daily wet “patches”, as well as relationships between local and regional-scale rainfall anomalies and the monsoon circulation. Spatial coherence of interannual rainfall variations is generally found to peak near monsoon onset, in late May–June over Monsoonal India, and again during the withdrawal stage in September–October. However, the spatial coherence and correlations between local rainfall and the regional-scale monsoon circulation decrease during the core phase of the monsoon between early July and late August, when the interannual variability of local-scale amounts tend to be more tightly related to mean daily intensity rather than to the frequency of wet days. The drop in spatial coherence during the core phase is related to increases in the number and intensity of wet “patches” of daily rainfall while their mean spatial extent remains similar throughout the monsoon season. The core phase, with large values of precipitable water, is characterized by very variable daily rainfall amounts across gridpoints, as a consequence of the near exponential distribution of daily rainfall. The interannual variations of sub-seasonal amounts are then dominated by very wet days, which tend to be rather randomly distributed. This contrasts with the onset and withdrawal phases where the mean of the exponential is smaller, and where interannual variations in the timing of regional-scale onset and end of the monsoon predominate. The early and late phases may thus be more potentially predictable than the core of the monsoon season.

Hydro-climatology of the Lower Rhône Valley. historical flood reconstruction (AD 1300–2000) based on documentary and instrumental sources

ABSTRACT From the HISTRHONE database we extracted 1483 hydro-meteorological events from AD 1300 to 2000 that occurred in the Lower Rhône Valley, France. Daily heights of the Rhône River at Beaucaire and Arles are also available, from 1816 and 1829, respectively. A total of 517 floods were divided into three categories and a synthetic frequency severity index (FSI) was computed. Running averages of 11 and 31 years show a succession of poor and rich flood fluctuations. Extreme floods tripled in the second half of the period (1650–2000). Singular spectrum analysis isolates a dominant irregular component (main positive anomalies in 1450–1580, around 1700, late 18th century, and most of the 20th century). We focus on the 17th century, with rare flooding events between two secular so-called “hyper phases”, i.e. frequent and/or severe floods. We also recorded 173 episodes of ice in the river, during the Little Ice Age.

Daily Weather Types in February–June (1979–2016) and Temperature Variations in Tropical North Africa

AbstractThis study investigates to what extent weather types (WTs) computed over Tropical North Africa and the tropical North Atlantic (40°W–40°E, 0°–30°N) are relevant for documenting intraseasonal and interannual temperature variability in Tropical North Africa (West of 37°E, 2°–27°N). Nine WTs are extracted using clustering analysis of the daily anomalies of sea-level pressure and low-level 925-hPa winds from two reanalyses (NCEP-DOE and ERAInterim) from 1979 to 2016. The analyses are carried out separately for February–March and April–June, when temperatures reach their annual peak across most of the region. The WT patterns mix the effect of different multiscale phenomena including the extra-tropical Rossby waves traveling on the northern edge of the domain (and partly related to the North Atlantic Oscillation), the Madden-Julian Oscillation and the Kelvin waves in the subequatorial zone. For eachWT, warm (cold) minimum (TN) and maximum (TX) daily temperature anomalies tend to be systematically locate...

Les interactions d'échelle au sein du système climatique : l'exemple de l'impact des phases chaudes de l'El Niño Oscillation Australe en Indonésie

Scale interactions in climate system: case study of the impact of warm phase of El Niño Southern Oscillation in Indonesia This paper analyses the spatio-temporal modulation of the impact of El Niño Southern Oscillation on Indonesian rainfall. The regional-scale rainfall anomaly is spatially uniform across monsoonal Indonesia from September to November, with large delays of the onset of austral summer monsoon associated with anomalous large-scale subsidence, enhanced ESE winds and cold sea surface temperatures (SST) anomalies across Indonesia seas. During the core of the austral summer monsoon (i.e. in December-March), the regional-scale low-level easterly anomaly now reduces the intensity of usual WNW austral summer monsoon. The local SST warms and the quiescent regional-scale winds are able to enhance the diurnal cycle between sea and mountainous islands. Negative rainfall anomalies across most of the Indonesian sea contrast now with positive rainfall anomalies over mountains and particular faces of islands, and western Indonesia.

Réponse à Marcel Leroux

• Tout article scientifique, surtout de synthèse, nécessite de rappeler les travaux antérieurs ayant vraiment traité le sujet, en l’occurrence les relations entre anomalies de température de surface de la mer (TSM) et anomalies pluviométriques autour du bassin Atlantique. Sur cet aspect, les références annexées à l’article remontent aux années 77-78 (Hastenrath et Heller, 1977 ; Lamb, 1978 a et b), soit dix, dix-sept et dix-huit ans avant les écrits de M. Leroux rappelés dans sa lettre (Leroux, 1988, 1995 et 1996), écrits qui, de plus, ne traitent pas directement de ces questions.