Future changes in the influence of the NAO on Mediterranean winter precipitation extremes in the EC-Earth3 large Ensemble: The prominent role of internal variability

One of the largest uncertainties in future climate projections is the interplay between internally generated and externally forced changes. This study investigates the changes in the link between the North Atlantic Oscillation (NAO) and Mediterranean winter extreme rainfall and dry days by the end of the 21st century compared to present day. We compare two different future pathways and estimate the extent to which the NAO imprint is affected by the global warming level using the latest EC-Earth3 large ensemble historical and future experiments. It is shown that the expected range of winter extremes changes due to internal and unpredictable fluctuations of the NAO largely overcomes the signal associated with externally-forced NAO variations. The NAO is found to exert a similar control on European climate variability, regardless of the amount of warming. For most of the Mediterranean region, magnitude and even sign of projected changes in the NAO-congruent precipitation indices vary substantially across the individual ensemble members according to the corresponding evolution of the NAO. Internal variability provides an average basin-wide contribution of up to 90% or more to the total NAO-driven variability in SSP1–1.9, and of about 80% in SSP5–8.5. Sub-regionally, the anthropogenic component of the NAO link is more evident over the Iberian Peninsula and parts of the central Mediterranean. This emphasises the role of internal variability and related uncertainty in determining the future impact of the NAO via the large spread in the circulation responses. However, the NAO is found to exert a weaker influence on the extreme precipitation total variability in both future scenarios given their future marked increase in total intensity and variance as opposed to the negligible NAO-related trends. Opposite conclusions are drawn for dry days, which are projected to decrease in the future, especially in the northern Mediterranean. Thus, this study also highlights how the variability of future extreme precipitation intensity in the Mediterranean basin will be less dependent on the principal mode of internal climate variability, posing further challenges for prediction and adaptation to weather-related hazards

Decreased monsoon precipitation in the Northern Hemisphere due to anthropogenic aerosols

The Northern Hemisphere monsoons are an integral component of Earth's hydrological cycle and affect the lives of billions of people. Observed precipitation in the monsoon regions underwent substantial changes during the second half of the 20th century, with drying from the 1950s to mid-1980s and increasing precipitation in recent decades. Modeling studies suggest anthropogenic aerosols has been a key factor driving changes in tropical and monsoon precipitation. Here we apply detection and attribution methods to determine whether observed changes are driven by human influences using fingerprints of individual forcings (i.e. greenhouse gas, anthropogenic aerosol and natural) derived from climate models. The results show that the observed changes can only be explained when including the influence of anthropogenic aerosols, even after accounting for internal climate variability. Anthropogenic aerosol, not greenhouse gas or natural forcing, has been the dominant influence on Northern Hemisphere monsoon precipitation over the second half of the 20th century

Emerging Asian aerosol patterns

Anthropogenic aerosol emissions over Asia are changing rapidly, both in composition and spatial distribution1. The Shared Socioeconomic Pathways (SSPs), potential narratives of development used by the Intergovernmental Panel for Climate Change in future projections, span a range of influences of aerosols on climate over the next decades. Several of these narratives project the continuation of a trend manifested in observations since 2010, with a clear dipole between South and East Asia. The patterns of radiative forcing that result from these distributions of aerosols will differ from those of the late 20th century. They may instigate large-scale atmospheric responses that could have wide ranging impacts on climate and society well beyond the aerosol source regions. South and East Asia are particularly vulnerable to climate change because of strong seasonal variations in precipitation, high average temperature, and very high population density. Therefore, any aerosol impacts on the strength or seasonal variations in monsoon rainfall, freshwater availability, or climate extremes, will incur large societal costs. We urge the scientific community to make definite progress towards understanding and quantifying the impacts of Asian aerosols and to tackle the potentially large regional and hemispheric implications of these emerging trends

Anthropogenic Aerosols and the Weakening of the South Asian Summer Monsoon

Observations show that South Asia underwent a widespread summertime drying during the second half of the 20th century, but it is unclear whether this trend was due to natural variations or human activities. We used a series of climate model experiments to investigate the South Asian monsoon response to natural and anthropogenic forcings. We find that the observed precipitation decrease can be attributed mainly to human-influenced aerosol emissions. The drying is a robust outcome of a slowdown of the tropical meridional overturning circulation, which compensates for the aerosol-induced energy imbalance between the Northern and Southern Hemispheres. These results provide compelling evidence of the prominent role of aerosols in shaping regional climate change over South Asia. TheSouthAsian summermonsoon providesup to 80 % of the annual mean precipi-tation for most regions of India and has tremendous impacts on agriculture, health, wa-ter resources, economies, and ecosystems through-out South Asia (1). It is also an important part of the global-scale atmospheric circulation, because its vigorous ascent dominates the boreal summe

Mechanisms for a remote response to Asian aerosol emissions in boreal winter

Asian emissions of anthropogenic aerosols have increased rapidly since 1980, with half of the increase since the pre-industrial era occurring in this period. Transient experiments with the HadGEM3-GC2 coupled model were designed to isolate the impact of Asian aerosols on global climate. In boreal winter, it is found that this increase has resulted in local circulation changes, which in turn have driven increases in temperature and decreases in precipitation over China, alongside an intensification of the offshore monsoon flow. Over India, the opposite response is found, with decreasing temperatures and increasing precipitation. The dominant feature of the local circulation changes is an increase in low-level convergence, ascent, and precipitation over the Maritime continent, which forms part of a tropical-Pacific-wide La-Nina-like response. HadGEM3-GC2 also simulates pronounced far-field responses. A decreased meridional temperature gradient in the North Pacific leads to a positive-Pacific-North-American circulation pattern, with associated temperature anomalies over the North Pacific and North America. An anomalous anticyclonic circulation over the North Atlantic, and an anomalous cyclonic circulation over the Mediterranean drive advection of cold air into Europe, causing cooling in this region. Using a steady-state primitive equation model, LUMA, we demonstrate that these far-field midlatitude response arise primarily as a result of Rossby waves generated over China, rather than in the Equatorial Pacific