Some aspects of the West African monsoon circulation As deduced from a geostationary satellite

This paper focusses on some aspects or the West African monsoonal circulation observed during the period 15 July-l0 August 1979 of the PGGE, as derived from the satellite cloud windvectors. Temporal averages of the computed winsfields reveal that the flow at the low level is southerly (monsoonal), and Its line of discontinuity with the continental northeasterly was found at approximately 16°-18°N, lying about 300 km south of the accepted mean position. At both the middle and upper tropospheres the flow is easterly with axis about 12o-14,N and, latitude 8 No respectively, such that it is a circulation south of the axis and northwards, it is anticyclonic. The satellite-observed tropospheric circulation IS then discussed in relation to the, weather manifestations over the sub-region typical of the July / August period. The mass fields obtained from our gridded satellite-winds indicate that inflow into the land area occur mainly at the lowest layer (1000:850 hPa), whereas at the upper, levels (that is, above 850 hPa) it is predominantly an outflow, The tropospheric average gives a net mass for divergence from within the area, The significance of this result in relation to the observed weather phenomenology of a temporary cessation of the monsoon precipitations occurring about the peak of the season IS also discussed.

The effect of a vegetated Sahara on the West African monsoon rainbelt in mid-Holocene storm-resolving simulations

<p>Im mittleren Holozän dehnten sich die westafrikanischen Monsunniederschläge deutlich weiter nach Norden aus als es heute der Fall ist. Modellsimulation stellen, im Vergleich zu Rekonstruktionen, eine zu schwache Verschiebung des Niederschlags nach Norden dar mit einem zu starken meridionalen Niederschlagsgradienten. Studien zeigen, dass die Repräsentation von Wechselwirkungen zwischen Land und Atmosphäre dafür von entscheidender Bedeutung sind. Wechselwirkungen zwischen Land und Atmosphäre können jedoch stark variieren, abhängig davon, ob konvektive Prozesse in Klimamodellen explizit aufgelöst oder parametrisiert werden. Daher untersuchen wir, ob und wie Wechselwirkungen zwischen Land und Atmosphäre die westafrikanischen Monsunniederschläge in Simulationen mit explizit aufgelöster und parametrisierter Konvektion beeinflussen.<br /><br />Unabhängig von der Darstellung der Konvektion weisen Simulationen mit einer höheren Vegetationsdichte während des mittleren Holozäns - im Vergleich zu Simulation mit heutiger Vegetation - eine positive Wechselwirkungen zwischen Land und Atmosphäre über Nordafrika auf. Sowohl in unseren Simulationen mit explizit aufgelöster als auch mit parametrisierter Konvektion dehnt sich das Niederschlagsband über Nordafrika um 4-5° nach Norden aus, wenn wir eine höhere Vegetation vorschreiben. Diese nördliche Ausdehnung der Monsunniederschläge ist eine Folge von höheren latenten Wärmeflüssen in der Sahel-Sahara Region und einer Abschwächung und Nordwärts-Verschiebung des afrikanischen Ostjets.<br /><br />Während sich die Art der Wechselwirkungen zwischen Land und Atmosphäre in Simulationen mit explizit aufgelöster und parametrisierter Konvektion nicht unterscheidet, ist die Stärke der Wechselwirkungen zwischen Land und Atmosphäre deutlich verschieden. In Simulationen mit expliziter Konvektion sind die positiven Wechselwirkungen zwischen Land und Atmosphäre schwächer ausgeprägt als in Simulationen mit parametrisierter Konvektion. Der Grund für diese schwächeren Wechselwirkungen - im Gegensatz zu bisherigen Studien - ist nicht die abgeschwächte Reaktion des Niederschlags auf eine Änderung des latenten Wärmeflusses, sondern die abgeschwächte Reaktion der Bodenfeuchte auf eine Änderung des Niederschlags. Die Darstellung der Konvektion beeinflusst maßgeblich die Niederschlagseigenschaften, wie beispielsweise deren Intensität, räumliche Verteilung oder Häufigkeit. Diese verschiedenen Niederschlagseigenschaften beeinflussen den hydrologischen Kreislauf in unseren Simulationen maßgeblich. Lokale Starkniederschläge in Simulationen mit expliziter Konvektion führen zu einem hohen Wasserabfluss, weshalb sich die Bodenfeuchte weniger gut regenerieren kann als in Simulationen mit parametrisierter Konvektion. Wir zeigen, dass diese Limitierung der Bodenfeuchte in Simulationen mit expliziter Konvektion im Vergleich zu Simulationen mit parametrisierter Konvektion, die potenzielle Stärke der Wechselwirkungen zwischen Land und Atmosphäre einschränkt und die nördliche Ausdehnung der Monsunniederschläge begrenzt.</p>

The effect of explicit convection on climate change in the West African monsoon and central West African Sahel rainfall

The West African monsoon (WAM) is the dominant feature of West African climate providing the majority of annual rainfall. Projections of future rainfall over the West African Sahel are deeply uncertain with a key reason likely to be moist convection, which is typically parameterized in global climate models. Here, we use a pan-Africa convection permitting simulation (CP4), alongside a parameterized convection simulation (P25), to determine the key processes that underpin the effect of explicit convection on the climate change of the central West African Sahel (8°W-2°E, 12-17°N). In current climate, CP4 affects WAM processes on multiple scales compared to P25. There are differences in the diurnal cycles of rainfall, moisture convergence, and atmospheric humidity. There are upscale impacts: the WAM penetrates farther north, there is greater humidity over the north Sahel and the Saharan heat low regions, the sub-tropical subsidence rate over the Sahara is weaker, and ascent within the tropical rain belt is deeper. Under climate change, the WAM shifts northwards and Hadley circulation weakens in P25 and CP4. The differences between P25 and CP4 persist, however, underpinned by process differences at the diurnal and large-scales. Mean rainfall increases 17.1% in CP4 compared to 6.7% in P25 and there is greater weakening in tropical ascent and sub-tropical subsidence in CP4. These findings show the limitations of parameterized convection and demonstrate the value that explicit convection simulations can provide to climate modellers and climate policy decision makers.

Stable water isotope signals in tropical ice clouds in the West African monsoon simulated with a regional convection-permitting model

Tropical ice clouds have an important influence on the Earth’s radiative balance. They often form as a result of tropical deep convection, which strongly affects the water budget of the tropical tropopause layer. Ice cloud formation involves complex interactions on various scales, which are not fully understood yet and lead to large uncertainties in climate predictions. In this study, we investigate the formation of tropical ice clouds related to deep convection in the West African monsoon, using stable water isotopes as tracers of moist atmospheric processes. We perform simulations using the regional isotope-enabled model COSMOiso with different resolutions and treatments of convection for the period of June–July 2016. First, we evaluate the ability of our simulations to represent the isotopic composition of monthly precipitation through comparison with GNIP observations, and the precipitation characteristics related to the monsoon evolution and convective storms based on insights from the DACCIWA field campaign in 2016. Next, a case study of a mesoscale convective system (MCS) explores the isotope signatures of tropical deep convection in atmospheric water vapour and ice. Convective updrafts within the MCS inject enriched ice into the upper troposphere leading to depletion of vapour within these updrafts due to the preferential condensation and deposition of heavy isotopes. Water vapour in downdrafts within the same MCS are enriched by non-fractionating sublimation of ice. In contrast to ice within the MCS core regions, ice in widespread cirrus shields is isotopically in approximate equilibrium with the ambient vapour, which is consistent with in situ formation of ice. These findings from the case study are supported by a statistical evaluation of isotope signals in the West African monsoon ice clouds. The following five key processes related to tropical ice clouds can be distinguished based on their characteristic isotope signatures: (1) convective lofting of enriched ice into the upper troposphere, (2) cirrus clouds that form in situ from ambient vapour under equilibrium fractionation, (3) sedimentation and sublimation of ice in the mixed-phase cloud layer in the vicinity of convective systems and underneath cirrus shields, (4) sublimation of ice in convective downdrafts that enriches the environmental vapour, and (5) the freezing of liquid water in the mixed-phase cloud layer at the base of convective updrafts. Importantly, the results show that convective systems strongly modulate the humidity budget and the isotopic composition of the lower tropical tropopause layer. They contribute to about 40 % of the total water and 60 % of HDO in the 175–125 hPa layer in the African monsoon region according to estimates based on our model simulations. Overall, this study demonstrates that isotopes can serve as useful tracers to disentangle the role of different processes in the Earth’s water cycle, including convective transport, the formation of ice clouds, and their impact on the tropical tropopause layer.

West African monsoon precipitation impacted by the South Eastern Atlantic biomass burning aerosol outflow

The West African Monsoon (WAM) is a complex system depending on global climate influences and multiple regional environmental factors. Central and Southern African biomass-burning (SABB) aerosols have been shown to perturb WAM during episodic northward inter-hemispheric transport events, but a possible dynamical connection between the core of the SABB aerosol outflow and the WAM system remains unexplored. Through regional climate modeling experiments, we show that SABB aerosols can indeed impact WAM dynamics via two competitive regional scale and inter-hemispheric dynamical feedbacks originating from (i) enhanced diabatic heating occurring in the Southeastern Atlantic low-cloud deck region, and (ii) aerosol and cloud-induced sea surface temperature cooling. These mechanisms, related to aerosol direct, semi-direct, and indirect effects, are shown to have different seasonal timings, resulting in a reduction of June to September WAM precipitation, while possibly enhancing late-season rainfall in WAM coastal areas.

A review of West African monsoon penetration during Green Sahara periods; implications for human evolution and dispersals over the last three million years

Green Sahara periods (GSPs) represent episodes during which the present-day Sahara was transformed into a savannah in response to intensification of the West African monsoon (WAM). Although GSPs might have dramatically altered the size, structure, and connectivity of human populations in Africa and nearby regions of Asia, their significance for human evolution remains unknown due to the problems involved in gauging the penetration of the WAM over the Sahara at evolutionary timescales. Here I reanalyse monsoon run-off and dust records back to 3 million years ago from Eastern Mediterranean ODP Site 967, and assimilate them with North African palaeoenvironmental data to substantiate penetration of the WAM front during GSPs to latitudes beyond 28°N. These results, coupled with demographic and ecological data for modern hunter-gatherers, point to a significant expansion of human populations during GSPs compared with background desert conditions. Given the clustering of GSPs around long-term maxima in the eccentricity of the Earth´s orbit, I propose that recurrent periods of human population expansion driven by GSPs led to an increased number of favourable mutations. Along with environmental factors favourable for triggering epigenetic changes, this might have led to the rise in enhanced phenotypic plasticity that underpins the speciation of hominin lineages at times of high climate variability envisaged by the variability selection hypothesis. Clustering of GSPs around the Pliocene/Pleistocene boundary, simultaneously with a protracted period of wetter conditions in East Africa and the Sinai Peninsula, further suggests that the initial colonization of Eurasia by hominins occurred circa 2.6 Ma, much earlier than typically considered.