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Lisa Hannak, Peter Knippertz, Andreas H. Fink, Anke Kniffka, and Gregor Pante

form underneath the core of the nocturnal low-level jet (LLJ). This jet depends on the monsoonal north–south pressure gradient and the acceleration due to ceasing turbulent friction during the night, typically creating maximum wind speeds a few hundred meters above ground in the early morning hours ( Parker et al. 2005 ; Lothon et al. 2008 ; Abdou et al. 2010 ; Gounou et al. 2012 ; Schrage and Fink 2012 ). Using high-resolution simulations Schuster et al. (2013) show that the LLJ transports

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M. Issa Lélé, Lance M. Leslie, and Peter J. Lamb

and suggested that the widespread reduction in rainfall during the drought years of 1972/73 in the Sahel is associated with the disappearance of the 850-hPa trough near 8°N and a weakening of the easterly jet above it. This is an indication that the westerly flow in the surface–850-hPa layer is an important moisture source for the Sahel. Lamb (1983) investigated the interannual and intraseasonal variability of the monsoon layer thickness and moisture content over WA. That study found that sub

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Kerry H. Cook and Edward K. Vizy

the reanalysis. The direction of the low-level flow in boreal summer ( Fig. 4e ) is opposite to that in boreal winter ( Fig. 4a ). The Somali jet is fully formed in both the reanalysis and the 20C simulation ( Fig. 4f ), placing southeasterly flow over East Africa in the Southern Hemisphere and southwesterly flow over the Horn of Africa. Winds extend into the continental interior over Kenya and Tanzania, and also flow through the Turkana Channel (see Fig. 2 ). Flow converges from the west and the

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Elinor R. Martin and Chris Thorncroft

of TCs in the future. However, the stationarity of seeds assumption has not been evaluated. AEWs are westward-moving systems with wavelengths of 2000–4000 km, periods of 2–10 days and peak amplitudes close to the level of the African easterly jet (AEJ; 600–700 hPa). They are triggered by upstream convection and develop through mixed baroclinic–barotropic processes along the AEJ (e.g., Burpee 1972 ; Kiladis et al. 2006 ; Thorncroft et al. 2008 ). The coastal enhancement of AEW activity and

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Gang Zhang, Kerry H. Cook, and Edward K. Vizy

westerly jet in Sahel rainfall variations . J. Climate , 25 , 2880 – 2896 , doi: 10.1175/JCLI-D-11-00394.1 . Rienecker , M. M. , and Coauthors , 2011 : MERRA: NASA’s Modern-Era Retrospective Analysis for Research and Applications . J. Climate , 24 , 3624 – 3648 , doi: 10.1175/JCLI-D-11-00015.1 . Saha , S. , and Coauthors , 2010 : The NCEP Climate Forecast System Reanalysis . Bull. Amer. Meteor. Soc. , 91 , 1015 – 1057 , doi: 10.1175/2010BAMS3001.1 . Shinoda , M. , T. Okatani , and

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Zewdu T. Segele, Michael B. Richman, Lance M. Leslie, and Peter J. Lamb

JJAS rainfall, and to present a time–frequency quantification of teleconnections between the rainfall and large-scale atmospheric circulation and global sea surface temperature (SST) patterns. The results linked Ethiopian monsoon rainfall variations principally with seasonal-to-interannual time-scale atmospheric circulation patterns that involve fluctuations in the major components of the monsoon system, including the monsoon trough, tropical easterly jet (TEJ), and Somali low level jet (SLLJ

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Gang Zhang, Kerry H. Cook, and Edward K. Vizy

925-hPa wind (vectors; m s −1 ) and specific humidity (shaded; g kg −1 ) in (a) MERRA, (b) CFSR, (c) ERA-Interim, and (d) the WRF convection-permitting simulation. White denotes regions where the surface pressure is lower than 925 hPa. Figure 3 displays the 650-hPa wind and specific humidity from the model simulation and the reanalyses. The easterly flow of the African easterly jet is accurately simulated by the model, and the zonal structure of specific humidity in the model is close to the

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