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Etude par analyses spectrales de l'instabilité spatio-temporelle des téléconnexions basse-fréquences entre les fluctuations globales du secteur Atlantique et les climats de l'Europe du NW (1700-2010) et du Sahel ouest-africain (1900-2010)

Abstract : Since the industrial revolution, an anthropogenic forcing occurs over all time-scales of natural variability. This change resulted in significant variations of climate on the regional scale. Therefore, based on spectral analysis, this dissertation revisits the question of the spatiotemporal non-stationnarity of teleconnections between the global climatic variability in the Atlantic and the regional climates of NW Europe and West-Africa. The study of long-term climatic records back to the pre-industrial period reveals similar trends in temperatures of England and Northern France, contrary to trends of rainfalls. Multi- and interdecadal variabilities display several periods of enhanced amplitude for both temperature and rainfall that may be related to large-scale climate control. On these scales, the relationship between the Atlantic Multidecadal Oscillation (AMO) and temperatures is marked by phase changes over the 19th century. For rainfall, coherence with AMO is observed for scales at around 30-60-year, whereas coherence with the North Atlantic Oscillation (NAO) is detected on 50-80-year and interdecadal 16-23-year scales. However, NAO/rainfall teleconnections are highly unstable due to spatial fluctuations of NAO patterns. After the Little Ice Age, the NAO pattern seems shifted south-westwards in winter and spring. Moreover, the results of an analysis of North Atlantic sea-level pressure (SLPs) at these co-oscillation time scales highlight not only NAO regimes, but also other patterns explaining a non-negligible amount of variance. Over West-Africa, zonal contrasts are identified through a study of time-evolutions of Sahel rainfall variability. These patterns are more pronounced at quasi-decadal scale. The teleconnections with Atlantic sea-surface temperatures (SSTs), which depend on time-scales, also show east-west contrasts. During the 20th century, a synchronous Sahel rainfall teleconnection with North Atlantic and Tropical South Atlantic SSTs is rarely observed. In-phase relationship with North Atlantic SSTs only occurs in wet periods (e.g. 1950s) and across multi- and quasi-decadal scales. This teleconnection display a NW-SE pattern suggesting non-uniform modulations of meridional displacements of the ZCIT. The Tropical South Atlantic SSTs variability is often related to a dipole-type configuration between the in-phase rainfall along Gulf of Guinea and the phase opposition in the Sahel. Nevertheless, this teleconnection depends on the time-period, the time-scale, and the mean state of Pacific SSTs. The involved quasi-decadal anomalies of such zonal contrasts vary over time. In the 1950/60s, the western flank of West-Africa displays fluctuations of Harmattan flux, rising motions over sahelian region and of the subtropical sinking motions. In the 1970/80s, variations of the cross-equatorial Atlantic SST and pressure gradient can be related to modulations of monsoon fluxes, and more generally northward-southward shift of the ZCIT.
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https://theses.hal.science/tel-00843683
Contributor : Bastien Dieppois Connect in order to contact the contributor
Submitted on : Friday, July 12, 2013 - 2:27:13 PM
Last modification on : Tuesday, October 19, 2021 - 11:34:04 PM
Long-term archiving on: : Wednesday, April 5, 2017 - 10:14:29 AM

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  • HAL Id : tel-00843683, version 1

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B. Dieppois. Etude par analyses spectrales de l'instabilité spatio-temporelle des téléconnexions basse-fréquences entre les fluctuations globales du secteur Atlantique et les climats de l'Europe du NW (1700-2010) et du Sahel ouest-africain (1900-2010). Climatologie. Université de Rouen, 2013. Français. ⟨NNT : ⟩. ⟨tel-00843683⟩

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