Congo Basin Hydrology, Climate, and Biogeochemistry. Группа авторов

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СКАЧАТЬ 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). This is in contradiction with the two main and well‐studied convective hotspots, namely the Amazon Basin and Maritime Continent, where ascent in rainiest months begins at the surface (Nie et al., 2010). This classical picture of rainfall‐producing mechanisms conjectured by previous works over central Africa contributes to the misleading picture and disregards the contribution of features like the Saharan heat low (Lavaysse et al., 2009) and Angola low (Howard & Washington, 2018) located over northern and southern Africa, respectively. While these features are prominent in the regulation of climate over northern and southern Africa, their role in modulating the central African climate is largely unexplored. More recent studies underline the contribution of these lows to the central African climate mainly through the components of the African easterly jets (Creese & Washington, 2016; Kuete et al., 2019; Tamoffo et al., 2019), while other related features such as the shallow meridional circulation are understudied (Shekar & Boos, 2017) and merit more attention. To this effect, Longandjo and Rouault (submitted) explored the contribution of this circulation to central Africa’s rainfall. Results show that the dynamics of shallow meridional circulation are not identical during the two rainy seasons, March–April–May and September–October–November, and appear important to understand rainfall seasonality over central Africa. These thermal lows modulated African easterly jets dynamics, which are associated with secondary transverse circulations driven by ageostrophic wind (Uccellini & Johnson, 1979) and which may contribute to lower level subsidence over central Africa. This mechanism is unexplored and merits more attention to understand the reasons behind lower‐level subsidence underneath regions of deep convection. Another candidate that merits more attention to provide insights into seasonality over central Africa is the Congo Air Boundary, a largely forgotten element of regional circulation, which controls the southward migration of rainfall during the second half of the year (Howard & Washington, 2019). At interannual timescale, thermal lows over northern and southern Africa also modulate rainfall trends over central Africa (Cook & Vizy, 2019). Further investigations of these key processes are needed to promote the understanding of seasonality and trend of rainfall over central Africa. Other investigations of rainfall mechanisms led to increased interest on Walker‐like circulation (Cook & Vizy, 2015; Hua et al., 2016; Neupane, 2016; Pokam et al., 2014) and identification of a closed shallow overturning cell, named the Congo Basin Cell, that determines the rainfall maximum location over eastern central Africa (Longandjo & Rouault, 2020) and the upper level Zonal Asymmetric Pattern (Dezfuli et al., 2015). Despite this progress in the understanding of the regional dynamic, drivers of regional precipitation deserve more attention. What drives the genesis of mesoscale convective systems, which provide more than 70% of regional precipitation (Nesbitt et al., 2006), is not fully resolved.

      Although the shallow meridional circulation and African easterly jet have been intensively studied over West Africa (e.g., Hagos & Zhang, 2010; Thorncroft et al., 2011), they could contribute in a different way to regional rainfall processes over central Africa. Over West Africa the well‐known “monsoon jump” and the associated seasonal meridional migration of rainfall is modulated by the northern component of the African Easterly Jet through inertial instability, whereas over Central Africa such modulation is not observed (Cook, 2015). In addition, differences in the shape of the northern and southern hemisphere of Africa lead to differences in the driving mechanisms of each local mid‐level tropospheric jet (Kuete et al., 2019) and the associated features (Adebiyi & Zuidema, 2017). Moreover, in a geographic sense, central Africa is unique as a region of deep convection bordered north and south by semi‐arid regions. This suggests that although understanding drivers of climate variability over central Africa may benefit from advances in knowledge of features as shallow meridional circulation and African easterly jet over other regions in Africa, specific investigation of their role over central Africa is needed to avoid misleading information.

Definition of seasons over central Africa rely on the ITCZ concept. Henceforth, challenging mechanisms linking the ITCZ structure and rainbelt (Nicholson, 2018; Suzuki, 2011) also suggest that reconsideration of the definition of seasons is necessary as they are also three‐month traditional seasons mostly based on mid‐ to high‐latitude seasonality and not suitable for tropical regions (Bombardi et al., 2019). Figure 2.1a shows that this traditional view of seasonality could apply only along a latitudinal band of about 5° width centred at the equator. However, several studies used the three‐month approach over a wider region (e.g., Dezfuli & Nicholson, 2013; Dyer et al., 2017; Hua et al., 2016; Pokam et al., 2011; Sandjon et al., 2012), which may inappropriately combine wet and dry precipitation regimes. Over the latitudinal band of bimodal rainfall regime (Figure 2.1a), we note an inhomogeneity of the length of rainy seasons (Figures 2.1b, 2.1c). An east–west contrast in rainfall seasonality is also observed across this bimodal region (Figure 2.1c). This picture questions the common three‐month seasons usually defined over central Africa and suggests complex interactions that shape regional seasonality.

2.3. CONVECTION

Weather over central Africa involves mechanisms on wide spatiotemporal scales. These processes span from individual convective cells to organized mesoscale convective systems interacting with global systems such as tropical waves (Hartman, 2020; Nguyen et al., 2008). Precipitation occurring from moist convection makes it a crucial component of the water budget in the atmosphere. The Congo Basin appears as one of the major areas of convective activities in the world (Jackson et al., 2009; Nesbitt & Zipser, 2003; Taylor et al., 2018; Webster, 1983) and hotspot of high‐frequency lightning (Zipser et al., 2006). Predisposed large‐scale conditions for strong thunderstorms and lighting are not yet fully explained over the basin due to СКАЧАТЬ