SURFACE AND AEROSOL EFFECTS ON THE SOUTH ASIAN MONSOON HYDROCLIMATE

This work targets important couplings in the South Asian monsoon system at interannual or longer time-scales and associated processes and mechanisms: aerosol-hydroclimate, atmosphere-ocean, and land-atmosphere. Anomalous springtime absorbing aerosols loading over the Indo-Gangetic Plain (IGP) leads to large-scale variations of the monsoon: cloudiness reduction associated with increased aerosols is suggested to play an important role in triggering surface heating over India, which strengthens the monsoon. Indeed, a closer analysis with high resolution data depicts a complex interplay between aerosols, dynamics and precipitation. Interestingly, observations do not provide any evidence for the Elevated Heat Pump hypothesis, a mechanism proposed for the aerosol-monsoon link. Current coupled climate models, which have been extensively used to study aerosol-monsoon interactions, are shown to have large, systematic, and coherent biases in precipitation, evaporation, sea-surface temperature (SST) over the Indian Ocean during the monsoon. Models are also found to deficiently portray local and non-local air-sea interactions. For example, they tend to emphasize local oceanic forcing on precipitation or to poorly simulate the relationship between SST and evaporation. The Indian monsoon rainfall-SST link is also spuriously misrepresented, suggesting caution when interpreting model-based findings. Both regional and remote forcings modulate the annual cycle of the heat-low over the desert areas (including the Thar Desert) between Pakistan and northwestern India, source of most of the dust loading over India. Land-surface heating has a limited role in the development of the low. Regional orography and monsoon summertime deep-convection over the Bay of Bengal, with its upstream descent to the west and related northerlies, contribute to the strengthening of the low, indicating a monsoon modulation on desert processes, including dust emission. The Thar Desert is expanding westward and the potential impact of land-cover change (without consideration of the additional aerosol loading) on summer monsoon hydroclimate and circulation is found to be significant. Locally, the atmospheric water cycle weakens, air temperature cools and subsidence prevails. An anomalous northwesterly flow over the IGP weakens the monsoon circulation over eastern India, causing precipitation to decrease. Orographic enhanced precipitation occurs over the Eastern Himalayas and southern China

Identifying the evolving human imprint on heat wave trends over the United States and Mexico

Changes in frequency, duration and intensity of three heat wave (HW) types (compound, daytime, and nighttime) over the United States (U.S.) and Mexico during the second half of the 20th-century are investigated using the Community Earth System Model Large Ensemble (CESM-LE). The individual role of anthropogenic aerosols and greenhouse gases (GHGs), as well as the contribution from internal variability (IV), are identified and contrasted by means of the CESM-LE single forcing experiments during two periods: 1950–1975, when North American aerosol emissions peaked, and 1980–2005, when aerosol emissions declined. HW changes are strongly affected by anthropogenic forcing. During 1950–1975, aerosols, via both aerosol-radiation and aerosol-cloud interactions, dominate the decreasing trends in compound HWs over the central U.S., the daytime HWs in large parts of the domain and the nighttime HWs over Mexico. Conversely, all three HW types are considerably more frequent ( $\gt$ 2 HWs summer ^−1 decade ^−1 ), longer-lasting (with increases of up to 2 days HW ^−1 decade ^−1 in some regions) and more intense (e.g., ${\gt}3^{\circ}$ C HW ^−1 decade ^−1 in compound HWs) across large regions of the domain during the 1980–2005 period. The results show that the decline in aerosol emissions and the continuous rise in GHGs lead to widespread warming and subsequent circulation adjustments, contributing to the positive HW trends. The contribution of IV is large during 1950–1975 (over 60% in most areas), and considerably reduced later on. This study provides a comprehensive picture of the role of anthropogenic forcing and IV on the marked HW changes in the recent decades and their underpinning physical mechanisms

Hemispheric-wide climate response to regional COVID-19-related aerosol emission reductions: the prominent role of atmospheric circulation adjustments

The national and global restrictions in response to the COVID-19 pandemic led to a sudden, albeit temporary, emission reduction of many greenhouse gases (GHGs) and anthropogenic aerosols, whose near-term climate impact were previously found to be negligible when focusing on global- and/or annual-mean scales. Our study aims to investigate the monthly scale coupled climate-and-circulation response to regional, COVID-19-related aerosol emission reductions, using the output from 10 Earth system models participating in the Covid model intercomparison project (CovidMIP). We focus on January–February and March–May 2020, which represent the seasons of largest emission changes in sulfate (SO2) and black carbon (BC). During January–February (JF), a marked decrease in aerosol emissions over eastern China, the main emission region, resulted in a lower aerosol burden, leading to an increase in surface downwelling radiation and ensuing surface warming. Regional sea-level pressure and circulation adjustments drive a precipitation increase over the Maritime Continent, embedded in a negative Pacific Decadal Oscillation (PDO)- and/or El Niño–Southern Oscillation (ENSO)-like response over the Pacific, in turn associated with a northwestward displacement and zonal shrinking of the Indo-Pacific Walker cell. Remote climate anomalies across the Northern Hemisphere, including a weakening of the Siberian High and Aleutian Low, as well as anomalous temperature patterns in the northern mid-latitudes, arise primarily as a result of stationary Rossby wave trains generated over East Asia. The anomalous climate pattern and driving dynamical mechanism reverse polarity between JF and MAM (March–May) 2020, which is shown to be consistent with an underlying shift of the dominant region of SO2 emission reduction from eastern China in JF to India in MAM. Our findings highlight the prominent role of large-scale dynamical adjustments in generating a hemispheric-wide aerosol climate imprint even on short timescales, which are largely consistent with longer-term (decadal) trends. Furthermore, our analysis shows the sensitivity of the climate response to the geographical location of the aerosol emission region, even after relatively small, but abrupt, emission changes. Scientific advances in understanding the climate impact of regional aerosol perturbations, especially the rapidly evolving emissions over China and India, are critically needed to reduce current uncertainties in near-future climate projections and to develop scientifically informed hazard mitigation and adaptation policies.</p