This study presents the meso-β/γ scale dynamical features involved in an extreme African dust outbreak, which occurred during 20-21 February 2016 over the Iberian Peninsula (IP), the southwest corner of Europe. During this episode, nearly 90% of the air quality stations in Spain exceeded the European Union’s PM10 daily limit. We used observations and performed nested-grid 2km simulations with the Weather Research and Forecasting model using coupled Chemistry (WRF-Chem) to understand the development of this dust outbreak. The surface observations and the false-color RGB dust product from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) revealed that the dust storm was initiated on the southeastern flank of the Saharan Atlas Mountains at two distinct phases of dust emissions. The first dust plume crossed the Saharan Atlas during midday on the 20th, the second one followed in the afternoon of the 21st. The first dust plume was advected towards the Western IP, while the second one towards the Eastern IP. The WRF-Chem simulation results indicated that the phase I dust emission was associated with strong barrier jet (BJ) formation on the southeastern foothills of the Saharan Atlas Mountains. The BJ strengthened just after sunrise on the 20th and emitted a massive amount of dust resulting in the first strong dust storm. In phase II, a long-lived westward propagating mesoscale gravity wave (MGW) was triggered near the northeastern edge of the Tinrhert Plateau in eastern Algeria. When this westward propagating long-lived MGW crossed the Tademaït Plateau, multiple hydraulic jumps were formed on its lee side. The strong winds accompanying these multiple hydraulic jumps emitted and mixed dust aerosols upwards which enabled the second strong dust plume to reach the IP. The lifted dust extended over 2-3 km in altitude in the growing daytime planetary boundary layer (PBL) and was advected poleward by the southerly/southeasterly wind at 700hPa. Our results underline the importance of resolving meso-scale processes to understand dust storm dynamics in detail, which are difficult to represent in coarse-resolution (aerosol-) climate models.

We investigate the synoptic precursors to the Harmattan wind and dust frontogenesis during the high impact Saharan dust outbreak over the Cape Verde Islands on 13 November 2017. We employ multi-scale observations including ship data and Weather Research and Forecasting model Coupled with Chemistry simulations. The analyses indicate that the dust storm was initiated on the leeside of the Saharan Atlas Mountains (SAM) in Algeria on the 10. This dust storm was associated with a double Rossby Wave Break (RWB) linked through non-linear wave reflection. Two successive RWB contributed to the wave amplification over the Eastern North Atlantic Ocean which transported large magnitude potential vorticity air into the North African continent. The resulting coupled pressure surge was associated with cold air advected equatorward over the SAM which organized the strong near-surface wind that ablated the dust. The simulation results indicate that the dust front was initially related to a density current which formed due to the cold airflow over the SAM. The density current then triggered undular bores on the leeside. Each bore perturbed the dust loading and then the subsequent diurnal heating generated differential planetary boundary layer (PBL) turbulence kinetic energy strengthening the dust frontogenesis. Dust became confined behind the cold surge and interacted with the daytime Saharan PBL leading to increased dust loading while the dust front propagated equatorward. Two distinct dust plumes arrived successively at low-levels at Mindelo, Cape Verde Islands; (1) from the coasts of Mauritania and Senegal and (2) from the SAM southern flank.

The pandemic in 2020 caused an abrupt change in the emission of anthropogenic aerosols and their precursors. We provide the first estimate of the associated change in the aerosol radiative forcing at the top of the atmosphere and the surface. To this end, we perform new simulations with the contemporary Earth system model EC-Earth3 participating in CMIP6, and created new data on the anthropogenic aerosol optical properties and an associated effect on clouds for the implemented aerosol parameterization, MACv2-SP. Our results highlight the small impact of the pandemic on the global aerosol radiative forcing in 2020 compared to the baseline of the order of +0.04Wm-2, which is small compared to the natural year-to-year variability in the radiation budget. Natural variability also limits the ability to detect a meaningful regional difference in the anthropogenic aerosol radiative effects. We identify the best chances to find a significant change in radiation at the surface during cloud-free conditions for regions that were strongly polluted in the past years. The new post-pandemic recovery scenarios indicate a spread in the aerosol forcing of -0.68 to -0.38Wm-2 for 2050, which translates to a difference of +0.05 to -0.25Wm-2 compared to the baseline. This spread falls within the present-day uncertainty in aerosol radiative forcing and the CMIP6 spread in aerosol forcing at the end of the 21st century. We release the new MACv2-SP data for studies on the climate response to the pandemic and the recovery scenarios.

We show how changes in the global distribution of anthropogenic aerosols favour different spatial patterns in the North Atlantic sea-surface temperature (NASST). The NASSTs largely show the expected decrease associated with the anthropogenic aerosols in the 1970s, but also a surprising warming response in the eastern sub-polar gyre, the region of the North Atlantic warming hole. The NASST response reversed for the anthropogenic aerosols in the 2000s against 1970s. The regional reduction in anthropogenic aerosols favoured (1) a strengthening of the warming hole and (2) a NASST increase at high latitudes associated with changes in the atmosphere-ocean dynamics. The gyre component of the northward Atlantic heat transport in mid- to high latitudes is an important driving mechanism. At least two-thirds of the NASST response is associated with the magnitude of aerosol-cloud interactions. Constraining the NASST response therefore depends on a better understanding of the uncertain aerosol effects on clouds.