The impact of tropical tropopause cooling on Sahelian 2 extreme deep convection 3

30 Previous studies have suggested that the recent increase in tropical extreme deep 31 convection, in particular over Asia and Africa during the boreal summer, has occurred in 32 association with a cooling in the tropical lower stratosphere. The present study is focused 33 on the Sahel region of West Africa, where an increased occurrence of extreme precipitation 34 events has been reported over recent decades. The results show that the changes over 35 West Africa since the 1980s involve a cooling trend in the tropical lower stratosphere and 36 tropopause layer, combined with a warming in the troposphere. This feature is similar to that 37 which might result from increased greenhouse gas levels, but is distinct from the interannual 38 variation of precipitation associated with the transport of water vapor from the Atlantic Ocean. 39 It is suggested that the decrease in the vertical temperature gradient in the tropical 40 tropopause region enhances extreme deep convection over the Sahel, where penetrating 41 convection is frequent, whereas tropospheric warming suppresses the shallower convection 42 over the Guinea Coast. The essential feature of the recent changes over West Africa is 43 therefore the depth of convection, rather than the total amount of surface precipitation.


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West Africa is particularly susceptible to the impacts of climate change, with rising 50 temperatures already threatening human health (Russo et al., 2016) and significant changes 51 in the precipitation regime likely to occur over the next few decades (Gaetani et al., 2020). 52 Assessing the role of the tropical tropopause layer (TTL; around 140-70 hPa) in driving 53 precipitation trends is a valuable step forward that will improve our understanding of the 54 present and future evolution of the rainfall regime in West Africa.     In fact, convective activity varies strongly within West Africa during the monsoon season; 129 e.g., broad stratiform clouds occur frequently over the coastal region, whereas extreme deep 130 convection is common further inland (Zuluaga and Houze, 2015). A decreasing precipitation 131 trend is particularly pronounced over the coastal regions west of the Guinea Highlands and 132 South Cameroon Plateau (Fig. 1b). Over these elevated terrains, convergence of the air 133 from the ocean (Fig. 2c) results in heavy precipitation (Fig. 1a). As the convection over the 134 coastal region is generally not deep enough to penetrate into the TTL, uplifted air diverges 135 in the upper troposphere (Fig. 2b). An increasing precipitation trend is found in regions of 136 high equivalent potential temperature near the surface (Fig. 2e), where extreme deep 137 convective clouds with overshooting tops occur (Fig. 2d). This extreme deep convection is 138 also evident in the large horizontal divergence at higher levels in the TTL (Fig. 2a).

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These results suggest that the regional differences in recent precipitation trends (   1980,1986,1988,1994,1999,2003,2010,2012)   suggests that this response is limited mainly in the troposphere (Fig. 3d). However, the 184 decadal changes indicate that upwelling generally increased in the TTL, except for a region 185 of suppressed tropospheric upwelling over the West African coast. In the following, we    should also be noted that the vertical temperature gradient in the TTL (i.e., the temperature 240 difference between 125 and 175 hPa) shows a decreasing trend, i.e. destabilization.

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In the case of the divergence at the top of the troposphere at 200 hPa, we found a good 243 correlation (r = 0.78) between MCSs with a CTT below −40°C (Fig. 7b). It was noted in T17 244 that precipitation over the Sahel is better correlated with the more common MCSs (CTT <  (10°N-15°N, 30°W-15°W) (Fig. 7c). This is consistent with the analysis in Fig. 3d that   stratosphere. An increasing trend in the upwelling is also evident near the surface around  (Figs 1 and 2). These trends support the findings of previous studies  is not influenced by cooling in the TTL, but rather suppressed by warming in the troposphere.

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Although an increasing decadal trend in summer precipitation exists over the Sahel, there 291 is also substantial year-to-year variability, which may be driven by the transport of water 292 vapor from the Atlantic Ocean (Fig. 3). Modulation of the upward velocity by this year-to-  Coast regions (Fig. 5). However, in the Sahel, upwelling produced by convection is 300 connected to the lower stratospheric circulation, whereas over the Guinea Coast, 301 tropospheric upwelling is decoupled from the stratosphere. In the present analysis, we 302 assumed that the horizontal divergence in the upper troposphere and TTL is related to 303 detrained air around the cloud top in deep convection. This relationship was verified through 304 a comparison of the horizontal divergence with the occurrence frequency of MCSs (Fig. 7).      temperature (color shading) and amplitude (%) of pressure vertical velocity relative to its 510 climatological value (contours: 100% > by black lines, and < 100% by red dashed lines).

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A 3-year running mean has been applied to the data.