Congo Basin Hydrology, Climate, and Biogeochemistry. Группа авторов

Part I Influences on Rainfall

2 Central African Climate: Advances and Gaps

       Wilfried Pokam Mba1,2, Derbetini Appolinaire Vondou2, and Pierre Honore Kamsu-Tamo3

       1 Department of Physics, Higher Teacher Training College, University of Yaoundé I, Yaoundé, Cameroon

       2 Laboratory for Environmental Modeling and Atmospheric Physics, Department of Physics, University of Yaoundé 1, Yaoundé, Cameroon

       3 Climate Prediction Center, NOAA Center for Weather and Climate Prediction, College Park, Maryland, USA

      ABSTRACT

      Less attention is given to the study of the central African climate, such that many concepts were imported from other regions of the globe. This study underlines misleading information due to the simplistic view of mechanisms driving annual rainfall regime based on the intertropical convergence zone concept, and presents recent advances in regional climate dynamics and modeling. Up‐to‐date findings highlight links between major African climatic features such as Saharan heat low and Angola heat low and central African climate, which was unexplored due to the fragmented approach used to investigate African climate. In addition, the role of lows’ related features as shallow meridional circulations and mid‐tropospheric easterly jets were underlined, but remain understudied. This underlines the need of an integrated approach to provide an improvement in the understanding of central African climate, thereby breaking the rigid regional view of the continent’s climate system. Investigations demonstrate that intense observation campaigns are required to explain singularities of central African climate and improve understanding of mechanisms prevailing in this region.

      

2.1. INTRODUCTION

      The prominence of central Africa in the tropical climate system is well established. Central Africa is one of the three major global hotspots of convective activity, which are a key part of the large‐scale air circulation that transfers warm air from the tropics toward the poles, thereby contributing to the regulation of global climate. The regional climate system is associated with the world’s greatest frequency of thunderstorms and lightning (Cecil et al., 2015; Clulow et al., 2018). Central Africa has an extensive wetland and the regional hydrography is dominated by the Congo Basin, which is the biggest water catchment in Africa, holding 30% of the continent’s water resources (Brummett et al., 2008), and the second‐largest catchment in the world. The Congo Basin, with over thousands of kilometers of navigable waterways, presents an opportunity to adapt and reduce the effect of climate change on the Lake Chad Basin through the possibility of interbasin water transfer (Salman & Momha, 2009). The wealth of biodiversity over central Africa is abundant. It has the second‐largest tropical forest on Earth, with evergreen trees and thousands of species of plants, and hundreds of species of mammals and birds. Carbon sequestration by the Congo forest contributes to climate regulation and thereby mitigates climate change (Eba’a et al., 2015).

It follows that gaps in the understanding of the climate of central Africa, and how it may change in the future, are substantial limitations. Such gaps hinder the confidence in the risk to which biodiversity is exposed and, in turn, the Earth’s future. Over the African continent, central Africa is the region where climate models generally suffer from poor performance. The direction of the future climate of central Africa is uncertain as there is strong disagreement between climate model projections (Dosio et al., 2019). These deficiencies are because the models fail to represent observed climate well, and the contemporary climate system is poorly observed (Washington et al., 2013). Deficiency in the investigation of the central African climate leads to development of theories on its functioning built on concepts imported from other regions that are sometimes not suitable to the regional dynamics (Nicholson, 2018). These conjectures likely induce misleading information in the current understanding on the regional climate, and suggest the investigation of gaps. To this effect, our goal is to explore key gaps in the understanding of the climate of central Africa, while presenting the advances made recently.

      This study reviews recent advances in drivers of the climate regime over central Africa and highlights aspects that merit further attention. The focus is on annual rainfall regime and associated mechanisms. Current knowledge in regional atmospheric dynamic and associated potential for understanding rainfall regime is presented. This paper is organized as follows: Section 2.2 presents mechanisms linked to the annual rainfall regime and related insufficiency. Section 2.3 deals with convection and associated gaps. The consequence of the knowledge gap in the functioning of regional climate on climate modeling is presented in Section 2.4, and conclusions are presented in Section 2.5.

2.2. RAINBELT MECHANISMS

      This section starts with a brief description of the main climate features and their drivers over central Africa. Drivers of rainfall producing mechanisms found over the region encompass the northern and the southern components of the African Easterly Jet, the shallow meridional circulation, the Congo Basin Cell, the Congo Air Boundary, and the Intertropical Convergence Zone (ITCZ). The dynamics of the jets are well documented and owe their existence to surface temperature contrast between the warm and dry Sahara and the cold and wet Congo Basin for the northern component, and the warm and dry Kalahari and the Congo for the southern component (Cook, 2015; Kuete et al., 2020). These jets modulate mid‐tropospheric moisture convergence and impact rainfall at annual and interannual time scales. Therefore, the jets are influenced by the dynamic of heat lows over the northern and southern African continent. Also associated with these heat lows are shallow meridional circulation cells (Shekar & Boos, 2017) associated with poleward flow at the surface, ascent over dry lands in northern and southern Africa, and a return flow at mid‐troposphere. The Congo Basin Cell is a shallow zonal overturning circulation with ascent over central Africa, an upper eastward flow at mid‐level, and descent branch over the coastal region, and is associated at lower level with the low‐level westerlies from the southeast Altlantic Ocean, which are a main source of moisture for central Africa (Longandjo & Rouault, 2019). The Congo Air Boundary, defined as the location where low‐level westerlies meet with the easterly Indian Ocean trade winds at the surface (Howard and Washington 2019), lies over southern central Africa in August–September and controls the southward shift of the African rainbelt afterward.

      The ITCZ is the climate feature with which the dynamic has long been associated, and the climate regime over Central Africa (Collier & Hughes, 2011; Sandjon et al., 2012). This assumes that, over central Africa, surface wind convergence is collocated with maximum temperature, high cloudiness, rainfall, low pressure (McGregor & Nieuwolt, 1998), and minimum longwave radiation (Sandjon et al., 2012). Suzuki (2011) couldn’t find a strong relation between these surface meteorological variables and the mechanisms underlying the formation and maintenance of the ITCZ. Over Central Africa, surface wind convergence is discernible in the northern part and associated with a local heat low, shallow convection (Suzuki, 2011), and little rainfall (Nicholson, 2009). This surface convergence is effectively independent of the system that produces deep convection and most of the rainfall. Over much of the area of maximum rainfall, Nicholson (2018) found lower tropospheric subsidence and concludes that over central Africa, the ITCZ paradigm, which entails surface convergence leading directly to ascent and hence rainfall, is incorrect.

Over Central Africa, ascent associated with rainfall begins in the middle atmosphere (Nicholson, 2018).