1JUNE 2014 P O K A M E T A L . 4245 Identification of Processes Driving Low-Level Westerlies in West Equatorial Africa WILFRIED M. POKAM Department of Physics, Higher Teacher Training College, University of Yaounde 1, and Center for International Forestry Research, Central Africa Regional Office, Yaounde, Cameroon CAROLINE L. BAIN,ROBIN S. CHADWICK, AND RICHARD GRAHAM Met Office Hadley Centre, Exeter, United Kingdom DENIS JEAN SONWA Center for International Forestry Research, Central Africa Regional Office, Yaounde, Cameroon FRANCOIS MKANKAM KAMGA University of Mountain, Bangangte, Cameroon (Manuscript received 8 August 2013, in final form 16 January 2014) ABSTRACT This paper investigates and characterizes the control mechanisms of the low-level circulation over west equatorial Africa (WEA) using four reanalysis datasets. Emphasis is placed on the contribution of the di- vergent and rotational circulation to the total flow. Additional focus is made on analyzing the zonal wind component, in order to gain insight into the processes that control the variability of the low-level westerlies (LLW) in the region. The results suggest that the control mechanisms differ north and south of 68N. In the north, the LLW are primarily a rotational flow forming part of the cyclonic circulation driven primarily by the heat low of the West African monsoon system. This northern branch of the LLW is well developed from June to August and disappears in December–February. South of 68N, the seasonal variability of the LLW is controlled by the heating contrast between cooling associated with subsidence over the ocean and heating over land regions largely south of the equator, where ascent prevails. The heating contrasts lead to a Walker- type circulation with development of LLW as its lower branch. Thus, evidence is presented that the LLW are driven by differential heating. This contrasts with the traditional conceptual view that the Saint Helena high is the primary driver of low-level circulation off the Atlantic Ocean to WEA. Forest cover in WEA may modulate the latent heating that helps to drive the differential heating and maintain the LLW, and this interaction should be the focus of further study. 1. Introduction moisture advection throughout the year (McCollum et al. 2000; Matsuyama et al. 1994). This low-level moisture The economy of west equatorial Africa (WEA) (108S– advection is dominated by moist air from the Atlantic 108N, 98–308E) is dominated by natural resources and ag- Ocean. The strongest moisture inflow is registered during riculture and is highly dependent on climate. Over WEA, the second and main rainy season from September to the climate is strongly influenced by changes in low-level November, when the highest amount of rainfall is re- corded (Pokam et al. 2012). Pokam et al. (2012) found that at interannual time scales, for both year-to-year Denotes Open Access content. comparisons and wet minus dry composites, low-level moisture flux from the Atlantic Ocean controls the Corresponding author address: Wilfried M. Pokam, Department of Physics, Higher Teacher Training College, University of Publisher’s Note: This article was revised on 7 August 2017 to Yaounde 1, P.O. Box 47, Yaounde, Cameroon. correct the name of the first author, whose given name and E-mail: [email protected] surname was inadvertently reversed when originally published. DOI: 10.1175/JCLI-D-13-00490.1 Ó 2014 American Meteorological Society Unauthenticated | Downloaded 10/08/21 02:34 AM UTC 4246 JOURNAL OF CLIMATE VOLUME 27 moisture content of the entire atmospheric column. southern site (Zhang et al. 2006). At the northern site, this Changes in associated low-level westerlies control the limit moves upward from 925hPa in April to 750hPa interannual variability of rainfall in the coastal region in August (Fontaine and Janicot 1992). In the vicinity (Nicholson and Dezfuli 2013). Because of the important of the equator, LLW are defined from August to January. role of the water cycle in climate variability and change The rest of the year, westerlies disappear and east- (Burde et al. 1996) and the heavy dependence of the erlies dominate throughout the entire atmospheric col- economy and livelihood of the region on water cycle umn (Zhang et al. 2006). (Molua and Lambi 2007), it is important to explore the There are well-documented examples of regions of low-level circulation driving this moisture advection from concentrated low-level westerly flow in West Africa the Atlantic Ocean to WEA. This is essential for ad- (Grodsky et al. 2003; PuandCook2010). However, vancing the physical understanding and modeling of there are few documented examples in WEA. This study climate in the region. It is also important for exploring focuses on the investigation of mechanisms governing the reasons behind the disagreement between climate LLW over WEA. Very little is known about the pro- model responses to expected future climate change cesses that control the height of these LLW and the (James et al. 2013), as Washington et al. (2013) have shown seasonal and interannual variability of their strength. that moisture flux is a useful quantity to understand model The purpose of this study is to investigate the processes rainfall biases over WEA. that control the depth, the intensity, and the seasonal The atmospheric circulation over WEA has been de- and interannual variability of LLW using reanalysis scribed in several previous studies. Broadly, the atmo- data. We will use the National Centers for Environ- spheric circulation is dominated by a large seasonal shift in mental Prediction–National Center for Atmospheric the position of the intertropical convergence zone (ITCZ), Research (NCEP–NCAR) reanalysis (NCEP-1; Kalnay which is determined by the northeast and southwest trade et al. 1996), the NCEP–U.S. Department of Energy winds, and the monsoon circulation from the Atlantic (DOE) Atmospheric Model Intercomparison Project (Fontan et al. 1992). The regional upper-level dynamics phase 2 (AMIP-II) reanalysis (NCEP-2; Kanamitsu are influenced by the high pressure cells over the Sahara et al. 2002), the European Centre for Medium-Range and the south of Africa that drive high-altitude easterlies Weather Forecasts (ECMWF) Interim Re-Analysis (Fontan et al. 1992). Nicholson and Grist (2003) found (ERA-Interim; Dee et al. 2011), and the Modern-Era that at midlevel (around 600–700 hPa), the annual cycle Retrospective Analysis for Research and Applications of easterlies are dominated by the north component of (MERRA; Rienecker et al. 2011). Over WEA, the den- the African easterly jet (AEJ-N) and the south compo- sity of the observation/meteorological station network is nent of the African easterly jet (AEJ-S). low and observations are sparse and sometimes unavail- In the lower troposphere, moist air from the Atlantic able (Aguilar et al. 2009). In the region, the reanalysis Ocean (Fontan et al. 1992), known as low-level equa- data rely strongly on the physical parameterizations in torial westerlies (Nicholson and Grist 2003), are asso- the global models used to create the reanalyses. There- ciated with the southeasterly trades on the northeastern fore, the reanalyses may differ because of different flank of the Saint Helena (South Atlantic) high. Because analysis systems and different model physics. Other dis- of Coriolis forces, the southeasterlies become westerlies crepancies may arise from the difference in spatial resolu- when crossing the equator. The low-level westerlies tion between the reanalyses or the number of observations (LLW) are defined throughout the year and are well used. Some aspects of the atmospheric dynamics may be developed from July to September (Nicholson and more visible in ERA-Interim and MERRA because of Grist 2003). Using soundings, the atmospheric circu- their finescale resolution (see section 2a)comparedtothe lation was described over the west coast of WEA, in- coarser resolution of NCEP-1 and NCEP-2. Use of more cluding one site in the Northern Hemisphere (Douala: than one reanalysis dataset reduces the susceptibility of 4.38N, 9.428E) (Fontaine and Janicot 1992), one near the results to errors in the underlying model used, and con- equator (Libreville: 0.238N, 9.278E), and one in the sistency between the reanalyses is an indicator (though Southern Hemisphere (Luanda: 8.488N, 13.148E) (Zhang not a guarantee) of robustness. et al. 2006). It appears that the annual cycle of LLW Theobjectiveofthisstudyistoidentifycommon varies from the southern to the Northern Hemisphere. At features between the reanalyses in the representation of the northern (southern) site, LLW are well developed the mean characteristics of the low-level circulation over from July to September (October–February) and are WEA, provide detailed information about the drivers of the strongest and the deepest in August (December– the low-level westerlies, and dissect the contributions of January). The upper limit of westerlies migrates from rotational and divergent flow. The related driving pro- around 950 hPa in July to 700 hPa in January in the cesses are investigated at both seasonal and interannual Unauthenticated | Downloaded 10/08/21 02:34 AM UTC 1JUNE 2014 P O K A M E T A L . 4247 time scales. Focus is made on the drivers of the low-level b. Wind decomposition and heating calculations zonal flow from the Atlantic Ocean to WEA. This study In section 3 we explore the processes that control the also revises the conventional view that LLW are driven variability of LLW. Focus is made on the contribution of solely by the anticyclone in the southeastern Atlantic the divergent and the rotational (nondivergent) circulation to Ocean. The paper is organized as follows: After presenting the total flow. To achieve this, the horizontal wind fields were the data and methods used in section 2, section 3 presents partitioned into the rotational and divergent circulations. the structure of the low-level circulation over WEA as Through such an approach, predominance of a circulation seen from the four reanalyses.