The occurrence of tropical cyclones (TC) in the Horn of Africa and nearby areas is for the first time examined to document their contribution to local rainfall and their trends over the period 1990-2020. An average 1.5 TC (of any intensity) per year was observed over the Western Arabian Sea, with two asymmetrical seasons, namely May-June (30% of cyclonic days) and September-December (70%). Case studies reveal that in many instances, TC-related rainfall extends beyond 500 km from the TC center, and that substantial rains occur one to two days after the lifecycle of the TC. Despite their rarity, in the otherwise arid to semi-arid context characteristic of the region, TCs contribute in both seasons to a very high percentage of total rainfall (up to 30 to 60%) over the northwestern Arabian Sea, the Gulf of Aden and their coastlines. Over inland northern Somalia, contributions are much lower. TCs disproportionately contribute to some of the most intense daily falls, which are often higher than the mean annual rainfall. A strong increase in the number of TCs is found from 1990 to 2020, hence their enhanced contribution to local rainfall. This increase is associated with a warmer eastern / southern Arabian Sea, a decrease in vertical wind shear, and a strong increase in tropospheric moisture content.

Variations in southern African precipitation have a major impact on local communities, increasing climate-related risks and affecting water and food security, as well as natural ecosystems. However, future changes in southern African precipitation are uncertain, with climate models showing a wide range of responses from near-term projections (2020-2040) to the end of the 21st century (2080-2100). Here, we assess the uncertainty in southern African precipitation change using five Ocean-Atmosphere General Circulation single model initial-condition large ensembles (30 to 50 ensemble members) and four emissions scenarios. We show that the main source of uncertainty in 21st Century projections of southern African precipitation is the internal climate variability. In addition, we find that differences between ensemble members in simulating future changes in the location of the Angola Low explain a large proportion (~60%) of the uncertainty in precipitation change. Together, the internal variations in the large-scale circulation over the Pacific Ocean and the Angola Low explain ~64% of the uncertainty in southern African precipitation change. We suggest that a better understanding of the future evolutions of the southern African precipitation may be achieved by understanding better the model’s ability to simulate the Angola Low and its effects on precipitation.

This study provides a first analysis of future changes in mean and extreme precipitation over Northeast Africa and the Arabian Peninsula. To this aim, we exploited projections from ten selected CMIP6 models (in part I) under various SSP greenhouse gases emission scenarios for the mid to late 21st century. We found a north-south differentiation in future changes in total precipitation for the JF and MAM seasons, with decreases in the north and moderate increases elsewhere, although model uncertainties are high, particularly for MAM. In contrast, the JJAS and OND seasons show larger positive changes with less model uncertainty. These increases in mean precipitation will be accompanied by an increase in the intensity and frequency of extreme precipitation events. In addition, the JJAS (+6.7%) and OND (+4.5%) seasons will contribute more to cumulative annual precipitation, while the JF (-1.3%) and MAM (-9.9%) seasons will experience a reduction. Over Djibouti, where the selected models are shown to perform well, downscaled and bias-corrected CMIP6 data using the CDF-t method indicate in addition that the return period of intense precipitation events (≥ 80 mm/day) causing documented flooding will decrease from 5 years historically to 1.4 years by the end of the 21st century under the SSP5-8.5 scenario. This robust result indicates the need to strengthen flood adaptation measures in Djibouti. Furthermore, similar downscaling exercises are recommended for other sub-regions in Northeast Africa and Arabia, given the consistent trend towards higher intensity rainfall.

This study proposes a methodology for selecting global climate models (GCMs) from the Coupled Model Intercomparison Project phase 6 (CMIP6) on the basis of their ability to simulate characteristics of mean and extreme precipitation over North-East Africa and Arabia. The seasonal climatology, annual rainfall cycles, and spatial and temporal variability (as documented by five ETCCDI indices) of twenty-five GCMs have been assessed against rain gauge observations and seven gridded rainfall products. Most of the GCMs simulate reasonably well the climatology of mean rainfall (annual and seasonal totals and number of rainy days). Large discrepancies are found in the reference products for some indices related to rainfall intensity (SDII, P95 and R95ptot), which is a major concern for the validation of GCMs. For these indices, we evaluate whether historical CMIP6 simulations fall within the uncertainty range of the rainfall estimates. Ten CMIP6 models are finally retained based on their ability to reproduce the geography and seasonality of mean and extreme rainfall. They tend to have a higher spatial resolution, although there is no systematic relationship between resolution and skill. The selected CMIP6 models perform better, not only at the regional scale (by construction), but also, and more meaningfully, at the local scale of the Republic of Djibouti particularly for the March-to-May rainy season. In a companion paper, the projections of the selected CMIP6 models will be used to study future changes in mean and extreme precipitation over North-East Africa and Arabia.

Future changes in Sahel precipitation are uncertain because of large differences among projections from various models. In order to explore this uncertainty, we use a storyline approach which seeks to identify alternative plausible evolutions of Sahel precipitation and their driving factors. By analysing projections from the CMIP6 climate models, we show that changes in North Atlantic and in Euro-Mediterranean temperatures explain up to 50% of the central Sahel precipitation change uncertainty. We then construct several storylines of Sahel precipitation change based on future plausible changes in North Atlantic and Euro-Mediterranean temperatures. In one storyline, an amplified warming of both the North Atlantic and the Euro-Mediterranean areas promotes a northward shift of the West African monsoon, increasing precipitation over the central Sahel, while, in another storyline, a moderate warming in both regions is associated with a small change in precipitation over the central Sahel and a decrease in precipitation over the western Sahel, at the end of the 21st century. These results indicate that reducing uncertainty in future changes in Sahel precipitation could be achieved by reducing uncertainty in the future warming of the North Atlantic and the Euro-Mediterranean areas.

During atmospheric river (AR) landfalls on the Antarctic ice sheet, the high waviness of the circumpolar polar jet stream allows for sub-tropical air masses to be advected towards the Antarctic coastline. These rare but high-impact AR events are highly consequential for the Antarctic mass balance; yet little is known about the various atmospheric dynamical components determining their life cycle. By using an AR detection algorithm to retrieve AR landfalls at Dumont d’Urville and non-AR analogues based on 700 hPa geopotential height, we examined what makes AR landfalls unique and studied the complete life cycle of ARs to affect Dumont d’Urville. ARs form in the mid-latitudes/sub-tropics in areas of high surface evaporation, likely in response to tropical deep convection anomalies. These convection anomalies likely lead to Rossby wave trains that help amplify the upper-tropospheric flow pattern. As the AR approaches Antarctica, condensation of isentropically lifted moisture causes latent heat release that – in conjunction with poleward warm air advection – induces geopotential height rises and anticyclonic upper-level potential vorticity tendencies downstream. As evidenced by a blocking index, these tendencies lead to enhanced ridging/blocking that persist beyond the AR landfall time, sustaining warm air advection onto the ice sheet. Finally, we demonstrate a connection between tropopause polar vortices and mid-latitude cyclogenesis in an AR case study. Overall, the non-AR analogues reveal that the amplified jet pattern observed during AR landfalls is a result of enhanced poleward moisture transport and associated diabatic heating which is likely impossible to replicate without strong moisture transport.

El Niño-Southern Oscillation (ENSO) shows a large diversity of events that is modulated by climate variability and change. The representation of this diversity in climate models limits our ability to predict their impact on ecosystems and human livelihood. Here, we use multiple observational datasets to provide a probabilistic description of historical variations in event location and intensity, and to benchmark models, before examining future system trajectories. We find robust decadal variations in event intensities and locations in century-long observational datasets, which are associated with perturbations in equatorial wind-stress and thermocline depth, as well as extra-tropical anomalies in the North and South Pacific. Some climate models are capable of simulating such decadal variability in ENSO diversity, and the associated large-scale patterns. Projections of ENSO diversity in future climate change scenarios strongly depend on the magnitude of decadal variations, and the ability of climate models to reproduce them realistically over the 21st century.

Here, we analyze the inter-relationships between weather types (WTs) and atmospheric rivers (ARs) around Aotearoa New Zealand (ANZ), their respective properties, as well as their combined and separate influence on daily precipitation amounts and extremes. Results show that ARs are often associated with 3-4 WTs, but these WTs change depending on the regions where ARs landfall. The WTs most frequently associated with ARs generally correspond to those favoring anomalously strong westerly wind in the mid-latitudes, especially for southern regions of ANZ, or northwesterly anomalies favoring moisture export from the lower latitudes, especially for the northern regions. WTs and ARs show strong within-type and inter-event diversity. The synoptic patterns of the WTs significantly differ when they are associated with AR occurrences, with atmospheric centers of actions being shifted so that moisture fluxes towards ANZ are enhanced. Symmetrically, the location, angle, and persistence of ARs appear strongly driven by the synoptic configurations of the WTs. Although total moisture transport shows weaker WT-dependency, it appears strongly related to zonal wind speed to the south of ANZ, or the moisture content of the air mass to the north. Finally, WT influence on daily precipitation may completely change depending on their association, or lack thereof, with AR events. WTs traditionally considered as favorable to wet conditions may conceal daily precipitation extremes occurring during AR days, and anomalously dry days or near-climatological conditions during non-AR days.