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1. Rainfall trends in the African Sahel: Characteristics, processes, and causes

   https://doi.org/10.1002/wcc.591  

   Biasutti, Michela ( June 2019 , WIREs Climate Change)

   Sahel rainfall is dynamically linked to the global Hadley cell and to the regional monsoon circulation. It is therefore susceptible to forcings from remote oceans and regional land alike. Warming of the oceans enhances the stability of the tropical atmosphere and weakens deep ascent in the Hadley circulation. Warming of the Sahara and of the nearby oceans changes the structure and position of the regional shallow circulation and allows more of the intense convective systems that determine seasonal rain accumulation. These processes can explain the observed interannual to multidecadal variability. Sea surface temperature anomalies were the dominant forcing of the drought of the 1970s and 1980s. In most recent decades, seasonal rainfall amounts have partially recovered, but rainy season characteristics have changed: rainfall is more intense and intermittent and wetting is concentrated in the late rainy season and away from the west coast. Similar subseasonal and subregional differences in rainfall trends characterize the simulated response to increased greenhouse gases, suggesting an anthropogenic influence. While uncertainty in future projections remains, confidence in them is encouraged by the recognition that seasonal mean rainfall depends on large‐scale drivers of atmospheric circulations that are well resolved by current climate models. Nevertheless, observational and modeling efforts are needed to provide more refined projections of rainfall changes, expanding beyond total accumulation to metrics of intraseasonal characteristics and risk of extreme events, and coordination between climate scientists and stakeholders is needed to generate relevant information that is useful even under deep uncertainty.

   This article is categorized under:

   Paleoclimates and Current Trends > Modern Climate Change

    

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2. Modulation of Mid‐Holocene African Rainfall by Dust Aerosol Direct and Indirect Effects

   https://doi.org/10.1029/2018GL081225  

   Thompson, Alexander J. ; Skinner, Christopher B. ; Poulsen, Christopher J. ; Zhu, Jiang ( April 2019 , Geophysical Research Letters)

   Climate model simulations of the mid‐Holocene (MH) consistently underestimate northern African rainfall for reasons not fully understood. While most models incorporate orbital forcing and vegetation feedbacks, they neglect dust reductions associated with greater vegetation cover. Here we simulate the MH climate response to reduced Saharan dust using CESM CAM5‐chem, which resolves direct and indirect dust aerosol effects. Direct aerosol effects increase Saharan and Sahel convective rainfall by ~16% and 8%. In contrast, indirect aerosol effects decrease stratiform rainfall, damping the dust‐induced total rainfall increase by ~13% in the Sahara and ~59% in the Sahel. Sensitivity experiments indicate the dust‐induced precipitation anomaly in the Sahara and Sahel (0.27 and 0.18 mm/day) is smaller than the anomaly from MH vegetation cover (1.19 and 1.08 mm/day). Although sensitive to dust radiative properties, sea surface temperatures, and indirect aerosol effect parameterization, our results suggest that dust reductions had competing effects on MH African rainfall.

    

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3. The characteristics of African easterly waves coupled to Saharan mineral dust aerosols

   https://doi.org/10.1002/qj.3483  

   Grogan, Dustin F. P. ; Thorncroft, Christopher D. ( March 2019 , Quarterly Journal of the Royal Meteorological Society)

   The characteristics of African easterly waves (AEWs) and their relationship with synoptic‐scale plumes of Saharan mineral dust (SMD) are examined using 37 summers (July–September, 1980–2016) of 3‐D fields from the Modern‐Era Retrospective analysis for Research and Applications, version 2 (MERRA‐2). The AEWs and SMD plumes are obtained by projecting the kinematic, thermodynamic and dust fields onto the 2–6‐day time‐filtered aerosol optical depth (AOD) near the west coast of North Africa (15°N, 15°W). Results show that the kinematic fields of the AEWs have significant structure north of the African easterly jet (AEJ). For the SMD, signals occur in two regions relative to the AEW over Africa: in the northerlies ahead of the AEW trough in the Sahel (10–20°N), and in the southerlies behind the AEW trough in the Sahara (20–27°N). The SMD signals are emitted on the flanks of a low surface‐pressure system associated with the northern circulation, mixed vertically in ascent regions below 600 hPa and removed by dry and wet deposition processes. Over the eastern Atlantic Ocean, the kinematic fields of the AEWs weaken, which modifies the propagation of the SMD signals. Consequently, the SMD signals from the two sources merge in the Saharan air layer (SAL) over the Atlantic.

   The direct radiative effects of SMD on the AEWs are also investigated over Africa.

   The perturbation diabatic heating rate is combined with the perturbation temperature field to infer the structure of the diabatic generation term in the energetics of the AEWs. The implied energetics show that an SMD‐induced generation region of eddy potential energy occurs at mid‐levels (850–650 hPa) north of the AEJ, which replenishes energy that is converted by baroclinic processes.

    

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4. Atlantic tropical cyclones downscaled from climate reanalyses show increasing activity over past 150 years

   https://doi.org/10.1038/s41467-021-27364-8  

   Emanuel, Kerry ( December 2021 , Nature Communications)

   Historical records of Atlantic hurricane activity, extending back to 1851, show increasing activity over time, but much or all of this trend has been attributed to lack of observations in the early portion of the record. Here we use a tropical cyclone downscaling model driven by three global climate analyses that are based mostly on sea surface temperature and surface pressure data. The results support earlier statistically-based inferences that storms were undercounted in the 19^thcentury, but in contrast to earlier work, show increasing tropical cyclone activity through the period, interrupted by a prominent hurricane drought in the 1970s and 80 s that we attribute to anthropogenic aerosols. In agreement with earlier work, we show that most of the variability of North Atlantic tropical cyclone activity over the last century was directly related to regional rather than global climate change. Most metrics of tropical cyclones downscaled over all the tropics show weak and/or insignificant trends over the last century, illustrating the special nature of North Atlantic tropical cyclone climatology.

    

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5. Changes in Atlantic major hurricane frequency since the late-19th century

   https://doi.org/10.1038/s41467-021-24268-5  

   Vecchi, Gabriel A. ; Landsea, Christopher ; Zhang, Wei ; Villarini, Gabriele ; Knutson, Thomas ( July 2021 , Nature Communications)

   Atlantic hurricanes are a major hazard to life and property, and a topic of intense scientific interest. Historical changes in observing practices limit the utility of century-scale records of Atlantic major hurricane frequency. To evaluate past changes in frequency, we have here developed a homogenization method for Atlantic hurricane and major hurricane frequency over 1851–2019. We find thatrecordedcentury-scale increases in Atlantic hurricane and major hurricane frequency, and associated decrease in USA hurricanes strike fraction, are consistent with changes in observing practices and not likely a true climate trend. After homogenization, increases in basin-wide hurricane and major hurricane activity since the 1970s are not part of a century-scale increase, but a recovery from a deep minimum in the 1960s–1980s. We suggest internal (e.g., Atlantic multidecadal) climate variability and aerosol-induced mid-to-late-20th century major hurricane frequency reductions have probably masked century-scale greenhouse-gas warming contributions to North Atlantic major hurricane frequency.

    

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