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  1. Free, publicly-accessible full text available May 16, 2023
  2. Abstract The Atlantic multidecadal variability (AMV), a dominant mode of multidecadal variations in North Atlantic sea surface temperatures (NASST), has major impacts on global climate. Given that both internal variability and external forcing have contributed to the historical AMV, how future anthropogenic forcing may regulate the AMV is of concern but remains unclear. By analyzing observations and a large ensemble of model simulations [i.e., the Max Planck Institute Grand Ensemble (MPI-GE)], the internally generated (AMV IV ) and externally forced (AMV EX ) components of the AMV and their climatic impacts during the twenty-first century are examined. Consistent with previous findings, the AMV IV would weaken with future warming by 11%–17% in its amplitude by the end of the twenty-first century, along with reduced warming anomaly over the midlatitude North Atlantic under future warming during the positive AMV IV phases. In contrast, the AMV EX is projected to strengthen with reduced frequency under future warming. Furthermore, future AMV IV -related temperature variations would weaken over Eurasia and North Africa but strengthen over the United States, whereas AMV IV -related precipitation over parts of North America and Eurasia would weaken in a warmer climate. The AMV EX ’s impact on globalmore »precipitation would also weaken. The results provide new evidence that future anthropogenic forcing (i.e., nonlinear changes in GHGs and aerosols) under different scenarios can generate distinct multidecadal variations and influence the internally generated AMV, and that multidecadal changes in anthropogenic forcing are important for future AMV.« less
    Free, publicly-accessible full text available May 1, 2023
  3. Abstract

    Winter surface air temperature (Tas) over the Barents–Kara Seas (BKS) and other Arctic regions has experienced rapid warming since the late 1990s that has been linked to the concurring cooling over Eurasia, and these multidecadal trends are attributed partly to internal variability. However, how such variability is generated is unclear. Through analyses of observations and model simulations, we show that sea ice–air two-way interactions amplify multidecadal variability in sea-ice cover, sea surface temperatures (SST) and Tas from the North Atlantic to BKS, and the Atlantic Meridional Overturning Circulation (AMOC) mainly through variations in surface fluxes. When sea ice is fixed in flux calculations, multidecadal variations are reduced substantially (by 20–50%) not only in Arctic Tas, but also in North Atlantic SST and AMOC. The results suggest that sea ice–air interactions are crucial for multidecadal climate variability in both the Arctic and North Atlantic, similar to air-sea interactions for tropical climate.

  4. Abstract Drought is projected to become more severe and widespread as global warming continues in the 21 st century, but hydroclimatic changes and their drivers are not well examined in the latest projections from the Phase Six of the Coupled Model Inetercomparison Project (CMIP6). Here, precipitation (P), evapotranspiration (E), soil moisture (SM), and runoff (R) from 25 CMIP6 models, together with self-calibrated Palmer Drought Severity Index with Penman-Monteith potential evapotranspiration (scPDSIpm), are analyzed to quantify hydroclimatic and drought changes in the 21 st century and the underlying causes. Results confirm consistent drying in these hydroclimatic metrics across most of the Americas (including the Amazon), Europe and the Mediterranean region, southern Africa, and Australia; although the drying magnitude differs, with the drying being more severe and widespread in surface SM than in total SM. Global drought frequency based on surface SM and scPDSIpm increases by ~25%–100% (50%–200%) under the SSP2-4.5 (SSP5-8.5) scenario in the 21 st century together with large increases in drought duration and areas, which result from a decrease in the mean and flattening of the probability distribution functions of SM and scPDSIpm; while the R-based drought changes are relatively small. Changes in both P and E contribute tomore »the SM change, whereas scPDSIpm decreases result from ubiquitous PET increases and P decreases over subtropical areas. The R changes are determined primarily by P changes, while the PET change explains most of the E increase. Inter-model spreads in surface SM and R changes are large, leading to large uncertainties in the drought projections.« less
  5. Free, publicly-accessible full text available August 14, 2023
  6. Abstract South American climate is influenced by both Atlantic multidecadal variability (AMV) and Pacific multidecadal variability (PMV). But how they jointly affect South American precipitation and surface air temperature is not well understood. Here we analyze composite anomalies to quantify their combined impacts using observations and reanalysis data. During an AMV warm (cold) phase, PMV-induced JJA precipitation anomalies are more positive (negative) over 0°-10°S and southeastern South America, but more negative (positive) over the northern Amazon and central Brazil. PMV-induced precipitation anomalies in DJF are more positive (negative) over Northeast Brazil and southeastern South America during the warm (cold) AMV phase, but more negative (positive) over the central Amazon Basin and central-eastern Brazil. PMV’s impact on AMV-induced precipitation anomalies shows similar dipole patterns. The precipitation changes result from perturbations of the local Hadley and Walker Circulations. In JJA, PMV- and AMV-induced temperature anomalies are more positive (negative) over entire South America when the other basin is in a warm (cold) phase, but in DJF temperature anomalies are more positive (negative) only over the central Andes and central-eastern Brazil and more negative (positive) over southeastern South America and Patagonia. Over central Brazil in JJA and southern Bolivia and northern Argentina inmore »DJF, the temperature and precipitation anomalies are negatively correlated. Our results show that the influence of Pacific and Atlantic multidecadal variability need to be considered jointly, as significant departures from the mean AMV or PMV fingerprint can occur during a cold or warm phase of the other basin’s mode.« less