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Characterization of West African Jet Streams and Their Association to ENSO Events and Rainfall in ERA-Interim 1979–2011

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Characterization of West African Jet Streams and Their Association to ENSO Events and Rainfall in ERA-Interim 1979–2011 Hindawi Publishing Corporation Advances in Meteorology Volume 2014, Article ID 405617, 12 pages http://dx.doi.org/10.1155/2014/405617 Research Article Characterization of West African Jet Streams and Their Association to ENSO Events and Rainfall in ERA-Interim 1979–2011 Churchill Okonkwo, Belay Demoz, and Sium Tesfai Beltsville Center for Climate System Observation, Atmospheric Science Program, Howard University, Washington DC, DC 20059, USA Correspondence should be addressed to Churchill Okonkwo; [email protected] Received 15 November 2013; Revised 9 January 2014; Accepted 10 January 2014; Published 5 March 2014 Academic Editor: Sven-Erik Gryning Copyright © 2014 Churchill Okonkwo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The interannual variability of West African jet streams and their association with rainfall are reexamined using European Reanalysis ERA-Interim 1979–2011. The objective of the study is to characterize their climatology and role in rainfall variability in western Sahel. Wavelet analysis was used on wind speed data and implications to ENSO were discussed subsequently. Our results show that while the low-level African Westerly Jet (AWJ) correlates well with rainfall south of the equator in boreal winter months, the Tropical Easterly Jet (TEJ) and African Easterly Jet (AEJ) correlate better with rainfall north of the equator in the boreal summer months. Results of interannual-to-decadal variability in 200 mb, 600 mb, and 850 mb of zonal wind reveal that there is enhanced variability in the 2–8 year band. Also, the TEJ, AEJ, and AWJ fluctuations are coupled with variations in southern oscillation. Further analysis suggests a statistically significant association between TEJ and the El Nino˜ events of the 1980s that led to intense drought in the Sahel region of West Africa. The 2007 moderate La Nina˜ shows a statistically significant coherence with the 500 mb, 600 mb, and 850 mb jets. These associations are also phase locked, suggesting that the association may be more than by chance. 1. Introduction overstressed [6] especially the characterization of desertifica- tion in the Sahel as irreversible. Subsequently, Eklundh and The Sahel region in West Africa is characterized by severe Olsson [7], Polgreen [8], and Herrmann et al. [9]haveall drought and a sharp decrease in rainfall from equatorial West confirmed recent greening in Sahel. Their studies show that Africa to the southern edge of the Sahara desert (Figure 1). vegetation under stress due to inadequate rain can recover Over the past three decades, studies on the possible causes quickly with improved precipitation. of drought in Sahel have either focused on forcing by sea There have also been several studies that examined surface temperature (SST) [1] or land-atmosphere interaction the teleconnection between rainfall variability in Sahel and [2]. Simulations of the hydrological impact of land surface variation in global sea surface temperature (SST) [1]. While processes studies include Charney [3] YongKang Xue and it has been accepted that SST patterns play a significant role Shukla [4], Taylor et al. [2], and Li et al. [5], which all in rainfall variability in West Africa [1, 10–12], there is still a attributed reduced rainfall to degradation of land surface at debate regarding the major drivers [13]. Several reports have least in part. As the forest and shrubs are cleared and burnt found that regional circulation patterns forced by SST of the prior to cultivation, the vegetation that absorbs incoming NorthAtlantichavemoreinfluenceonthelocalclimateinthe solar radiation is removed with a resultant increase in albedo. Sahel region [1, 14–17] than changes in land use. According to Charney [3], land cover with elevated albedo In addition to the above teleconnection, the role of has a more stable air column, which suppresses precipita- the Mediterranean [18] as having a strong influence on tion unlike surrounding areas. This mechanism is extended precipitation across the Sahel has been reported. On the other leading to self-stabilization of desert in the Sahel region [3]. hand, the influence of the Indian Ocean on both Sahel rainfall The contribution of these mechanisms has however been andIndianmonsoonhasalsobeensuggested[19]. Studies 2 Advances in Meteorology NOAA ESRL Physical Science Division on the role of El Nino˜ Southern Oscillation (ENSO) events 10N 1 1 2 1 15 3 and Pacific index in modulating Sahel precipitation include N 4 4 2 5 5 3 those of Rowell [20], Caminade and Terray [21], and Joly 12N 6 7 6 6 9 5 9 N 13 7 et al. [22]. One of the conclusions that came out of these 4 8 11 10 4 6N 8 3 multiple studies is that the interannual to decadal variability 7 6 3 2 8 in precipitation over West Africa is controlled by competing N 1 EQ 2 physical mechanisms [23]. These physical mechanisms have 5 2 3S 4 been summarized by Caminade and Terray [21]intotwo 6 3 (1) S broad conditions; those that enhance precipitation— 9S 1 positive (negative) SST anomalies north (south) of equator, as 15W 10W 5W 0 5E 10E 15E 20E 25E 30E 35E 40E 45E 50E increased water vapor holding capacity, and La Nina event— and (2) conditions that are less favorable to precipitation such as negative (positive) SST anomalies north (south) of equator 1 2345 678 9 10 11 12 13 14 15 16 and El Nino events. Figure 1: July, August, and September (JAS) precipitation Recently, the strong link between Sahel region and West (mm/month) averaged from 1979 to 2010 (data source: GPCP; this African monsoon—the precursor to most of its rainfall— figure was created using the NOAA/ESRL Physical Science Division has been reexamined [24–26]. Of particular importance website (http://www.esrl.noaa.gov/psd/cgi-bin/data/composites/print- are the shortcomings of the classical picture of the West page.pl). African monsoon and the association of the rainfall to the Intertropical Convergence Zone (ITCZ). Some of these shortcomings include the association of rainfall with the convergence of southwest monsoonal flow and the north- typicallyassociatedwithENSOevents.Theorganization easterly Harmattan wind with the increasing movement of of the paper is as follows. Section 2 describes the data deep moist layer equator-ward from the ITCZ [27, 28]. This and correlation methodology. Section 3 describes the results concept is, however, less likely because it originates from the under annual cycle, interannual to multidecadal cycle, and Hadley circulation cell which is valid over the ocean [25]and jet stream association with ENSO. Summary and conclusions describes the global mean state rather than regional [13]. Also, will be presented in Section 4. the association of ITCZ to several variables (surface pressure, wind convergence, and rainfall) makes its use in charac- 2. Data and Methodology terizing the West African monsoon ambiguous. Following the discussion, the classical picture of the West African The primary dataset used for this study was the ERA- ∘ ∘ monsoon has been revised and ITCZ is interchangeably being Interim reanalysis (1.5 ×1.5 resolution) [30]. This global used with “tropical rainbelt” [24] and “rain band” [25]. The reanalysis provides meteorological variables needed for a revised picture of West African monsoon is pointing to a better understanding of West African monsoon circulation diminished importance of ITCZ in controlling precipitation andprecipitation.TheERA-Interimoutputdatawereused whileemphasizingtheroleofAfricaneasterlyjet(AEJ), inthisanalysisbecauseoftheabsenceofdetailedlong- tropical easterly jet (TEJ), and low level westerlies as some of term observational wind data. According to Dee et al. [31] the main tropical circulation features in the region [24–26]. (2011), the ERA-Interim data assimilation system consists Despite these advances on the possible causes and trends ofa12hourly4D-Varoftheupper-airatmosphericstate. of persistent drought in the Sahel region, there are still In regions where observations are sparse, like the Sahel projection uncertainties and models cannot reliably predict region in the study domain, the ability of ERA-Interim’s data future climates in the region [10, 12]. This led to a large spread assimilation system to exploit physical information implicit in projections [27, 28]. This is partly due to the complexity in the model equations can be advantageous [31]. The 4D- in dynamics from the AEJ to TEJ, which makes prediction Var in ERA-Interim has been shown to outperform 3D-Var of precipitation difficult [29]. To better simulate the West of ERA-40 in such situations [32, 33]. African monsoon and improve in precipitation projection, a AncillarydatausedinthisworkalsoincludestheGlobal good understanding of the complexity of the West African Precipitation Climatology Project (GPCP; World Climate monsoondynamicsisvital. Research Program 1990), a one-degree daily estimate of In this paper, we used the improved ECMWF-Interim precipitation. The GPCP data (merged from satellite and a reanalysis data 1979–2011 (hereafter referred to as ERA- gauge observation) is an excellent validation for precipitation Interim) to provide further details on the annual to interan- produced by models [34]. nual variability of the West African jet streams. The objective This study is focused primarily on the West African of this first of two parts study is to represent more accurately monsoon circulation
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