Three Late Summer/Early Autumn Cases of Tropical–Extratropical Interactions Causing Precipitation in Northwest Africa

Knippertz, Peter; Fink, Andreas H.; Reiner, Andreas; Speth, P.

doi:10.1175/1520-0493(2003)131<0116:tlseac>2.0.co;2

Cited by 65 publications

(67 citation statements)

References 33 publications

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“…Specifically, given the relatively high latitude (15°N−25°N) of the projected changes in filtered meridional wind variance, it is possible that an increase in midlatitude trough activity could be contributing to the robust signal (43). However, given the projected decrease in storm activity along the equatorward side of the midlatitude storm track in the Northern Hemisphere, and the projected poleward shift in the location of storm tracks in the Northern Hemisphere (44), we expect the contribution from an increase in summertime midlatitude troughs over West Africa to be small.…”

Section: Resultsmentioning

confidence: 99%

Projected changes in African easterly wave intensity and track in response to greenhouse forcing

Skinner¹,

Diffenbaugh

2014

Proc. Natl. Acad. Sci. U.S.A.

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Significance African easterly waves (AEWs) are ubiquitous weather disturbances over the tropical Atlantic basin during the summer season. However, despite the critical influence of AEWs on Sahel rainfall, transport of Saharan dust, and initiation of Atlantic hurricanes, little is known about how global warming influences AEW activity. We use an ensemble of general circulation models to investigate the response of AEWs to elevated greenhouse gas concentrations. We find that increases in regional temperature gradients and the strength of convergence and uplift along the Intertropical Front of Africa lead to increases in the strength of AEWs. Given the relationships that are observed in the current climate, changes in AEW strength could influence climate over both West Africa and the larger Atlantic basin.

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Section: Resultsmentioning

confidence: 99%

Projected changes in African easterly wave intensity and track in response to greenhouse forcing

Skinner¹,

Diffenbaugh

2014

Proc. Natl. Acad. Sci. U.S.A.

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“…In this section, we provide evidence that the LLAT criterion detects the low pressure associated with the heating in the low levels, and not that linked with mid-latitude depressions which are known to intrude over North Africa on a regular basis (Knippertz et al 2003).…”

Section: Relationship With Surface Pressurementioning

confidence: 95%

“…This strong anticyclonic circulation acts to reduce the advection of cool air from the Atlantic, and warm air is effectively ''recycled'' by the south-westerlies: therefore the anticyclone favours the increase of the temperature in the low troposphere, that in turn generated the dynamical dilatation of the atmospheric layer between 925 and 700 hPa, which was detected by our method. Ultimately, this led to the formation of a tropical plume (as identified by Knippertz et al 2003) around 23 April, associated with a negative anomaly of geopotential at 300 hPa over Morocco, a large cloud band oriented south-west to north-east over West Africa and a large daily rainfall anomaly over , the SHL onset occurred on 18 June, close to the climatological value. As discussed in Drobinski et al (2007), the anticyclone climatologically centred above Libya was located above Tunisia in 2006, which led to an enhanced southwestward flow east of the Hoggar with respect to the climatology.…”

Section: Wahl Detection In 2006mentioning

confidence: 99%

Seasonal evolution of the West African heat low: a climatological perspective

Lavaysse¹,

Flamant²,

et al. 2009

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The West African heat low (WAHL), a region of high surface temperatures and low surface pressures, is a key element of the West African monsoon system. In this study, we propose a method to detect the WAHL in order to monitor its climatological seasonal displacement over West Africa during the period 1979-2001, using the European Centre for Medium-range Weather Forecast (ECMWF) ERA-40 reanalyses. The low-level atmospheric thickness (LLAT), a variable defined as the difference of geopotential heights at 700 and 925 hPa, is used to detect the dilatation of these levels generated by an increase of the temperature. We define grid points with 10% highest values of the LLAT as the WAHL. We show that our method reliably positions the WAHL over areas of high surface temperatures and low surface pressures, and that it is effective at detecting heat lows. In the course of the year,

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“…It is found below 6 hPa in the direct research area but reaches values above 10 hPa (thick line) over the North Atlantic. However, the variability in the North Atlantic does not significantly influence the precipitation south of the high Atlas (Knippertz et al, 2003b). …”

Section: Calculation Of Cwtsmentioning

confidence: 99%

“…Rainfall there is more often associated with depressions off the Moroccan coast, steering moist air onto the African continent that might be released owing to forced lifting at the southern slopes of the Atlas Mountains. In several cases, moisture supply originates from tropical latitudes and is advected in the middle troposphere above the dry boundary layer northward along the western coast of Africa (Knippertz, 2003;Knippertz et al, 2003b). Further east, the so-called Sharav cyclones play an increasingly important role in generating precipitation (Alpert and Ziv, 1989;Egger et al, 1995).…”

Section: Introductionmentioning

confidence: 99%

Downscaling of current and future rainfall climatologies for southern Morocco. Part I: Downscaling method and current climatology

Huebener

Kerschgens

2007

Intl Journal of Climatology

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Abstract:The use of a statistical-dynamical downscaling approach to obtain a high-resolution rainfall climatology for a subtropical region in southern Morocco is analysed. The statistical part of the downscaling uses Circulation Weather Types (CWTs) as a measure for near-surface wind fields calculated from sea-level pressure (SLP) data. The daily CWTs are correlated with daily rainfall data at three different climate stations in the region for the period 1978-1997 and systematic differences are discussed. Results are viewed for one extreme dry year (1984) and one extreme wet year (1989) to show the limits of purely statistical downscaling.Dynamical downscaling is realized for representative days to enable statistical-dynamical downscaling. Comparison of statistically-dynamically downscaled rainfall with measured rainfall for the year 2002 and climatological results show that the method is capable of capturing the relevant mechanisms triggering rainfall in the area. For extreme dry (wet) years, rainfall is overestimated (underestimated) by the method and errors might occur even for normal years. However, application to a climatology calculated for the period 1958-1997 via analysing daily SLP fields from NCEP Reanalyses gives satisfactory results. For sufficiently long periods, the method is well capable of producing a reliable high-resolution rainfall climatology. We will therefore apply the method to climate change simulations in part II of this paper.

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Three Late Summer/Early Autumn Cases of Tropical–Extratropical Interactions Causing Precipitation in Northwest Africa

Cited by 65 publications

References 33 publications

Projected changes in African easterly wave intensity and track in response to greenhouse forcing

Projected changes in African easterly wave intensity and track in response to greenhouse forcing

Seasonal evolution of the West African heat low: a climatological perspective

Downscaling of current and future rainfall climatologies for southern Morocco. Part I: Downscaling method and current climatology

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