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By accounting for most of the poleward atmospheric heat and moisture transport in the tropics, the Hadley circulation largely affects the latitudinal patterns of precipitation and temperature at low latitudes. To increase our preparednesses for human-induced climate change, it is thus critical to accurately assess the response of the Hadley circulation to anthropogenic emissions 1-3 . However, at present, there is a large uncertainty in recent Northern Hemisphere Hadley circulation strength changes 4 . Not only do climate models simulate a weakening of the circulation 5 , whereas atmospheric reanalyses mostly show an intensification of the circulation 4-8 , but atmospheric reanalyses were found to have artificial biases in the strength of the circulation 5 , resulting in unknown impacts of human emissions on recent Hadley circulation changes. Here we constrain the recent changes in the Hadley circulation using sea-level pressure measurements and show that, in agreement with the latest suite of climate models, the circulation has considerably weakened over recent decades. We further show that the weakening of the circulation is attributable to anthropogenic emissions, which increases our confidence in human-induced tropical climate change projections. Given the large climate impacts of the circulation at low latitudes, the recent human-induced weakening of the flow suggests wider consequences for the regional tropical-subtropical climate., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

The latitudinal position of mid-latitude storm tracks has large climate impacts affecting the distribution of precipitation, temperature, humidity, and winds over the extratropics. By the end of this century, climate models project a poleward shift of summer mid-latitude storm tracks in the Southern Hemisphere. Most previous mechanisms for the poleward shift of the storm tracks focused on the role of atmospheric temperature changes. However, the relative roles of other climate system components in the projected storm tracks' shift have not been examined to date. Here it is shown that thermodynamic ocean coupling is responsible for the future poleward shift of the storm tracks as it overcomes the effect of dynamic ocean coupling to shift the storm tracks equatorward. These results stress the importance of using full-physics ocean models to investigate the future shift of the storm tracks, and of better monitoring ocean coupling processes to improve our preparedness for future climate changes., (© 2022. The Author(s).)

The North Atlantic storm track plays a critical role in setting the regional weather and climate over Western Europe. In response to anthropogenic emissions, the summer North Atlantic storm track has weakened in recent decades and is projected to continue weakening by the end of this century. In light of growing efforts to mitigate climate change, it is crucial to assess how reversible the CO2‐induced storm track weakening is. Here, I show that under CO2 removal scenarios, the recovery timescale of the storm track is more than double its weakening period. It is found that due to a prolonged high‐latitude warming, postponing the execution of mitigation policies beyond doubling of CO2 concentrations would lead to a centennial‐scale recovery of the storms. Given the impacts of weakening summer storms on weather and extreme events, the delayed recovery of the storms might have broader regional consequences for Western Europe. Plain Language Summary: The summer North Atlantic storm track sets the regional weather and climate over Europe, by transferring heat, moisture and momentum across the North Atlantic basin. Anthropogenic emissions were found to weaken the storm track, which would greatly affect precipitation and hot‐dry extreme events over Europe. It is thus critical to assess the reversibility of the summer North Atlantic storm track weakening under a reduction in greenhouse gases. Here, using targeted numerical simulations, I show that the reversibility time‐scale of the storm track is more than double its weakening time‐scale. Thus, postponing the execution of mitigation strategies increases the reversibility time‐scales of the storm track. Specifically, avoiding centennial‐scale impacts of the weakening storm track can be achieved by executing mitigation strategies prior of reaching doubling of CO2 concentrations. Our analysis reveals that the prolonged recovery of the storm track stems from a prolonged warming of the high latitudes relative to the lower latitudes. Key Points: The reversibility time‐scale of the North Atlantic summer storm track is more than double its weakening time‐scalePostponing mitigation policies beyond a doubling of CO2 levels yields centennial‐scale recovery of the storm trackThe prolonged recovery of the storm track stems from a prolonged warming of the high latitudes relative to the lower latitudes [ABSTRACT FROM AUTHOR]

The expansion of Antarctic sea ice since 1979 in the presence of increasing greenhouse gases remains one of the most puzzling features of current climate change. Some studies have proposed that the formation of the ozone hole, via the Southern Annular Mode, might explain that expansion, and a recent paper highlighted a robust causal link between summertime Southern Annular Mode (SAM) anomalies and sea ice anomalies in the subsequent autumn. Here we show that many models are able to capture this relationship between the SAM and sea ice, but also emphasize that the SAM only explains a small fraction of the year‐to‐year variability. Finally, examining multidecadal trends, in models and in observations, we confirm the findings of several previous studies and conclude that the SAM–and thus the ozone hole–are not the primary drivers of the sea ice expansion around Antarctica in recent decades. Plain Language Summary: Unlike its Arctic counterpart, sea ice around Antarctica has been growing since 1979, even as the levels of carbon dioxide in the atmosphere have increased. Given that the ozone hole formed over the South Pole around the same time, one is led to ask whether the ozone hole may be responsible for the growth of Antarctic sea ice (recall that there is no ozone hole over the North Pole). In this study, looking at both models and observations, we show that the ozone hole is capable of affecting the surface winds and these, in turn, can make sea ice expand. However, the magnitude of this effect is small. Also since the ozone hole started healing after the year 2000, while Antarctic sea ice kept expanding, we conclude that ozone depletion is not the main reason for the expansion of Antarctic sea ice in recent decades. Key Points: Many CMIP5 models are able to capture the observed seasonal correlation between summertime Southern Annular Mode (SAM) and Antarctic sea ice extentThe SAM, however, only explains 15 of the year‐to‐year sea ice extent variability in the fall, in both models and observationsSAM trends, and ozone depletion, are not the primary drivers of the observed Antarctic sea ice expansion in the last four decades [ABSTRACT FROM AUTHOR]

The projected widening and weakening of the Hadley circulation hold large societal impacts at low and subtropical latitudes. Previous studies suggested that ocean heat transport (OHT) might play a central role in future circulation's changes. Here, using ensembles of model integrations, we quantify the role of OHT in the evolution of the Hadley cell over the 20th and 21st centuries. We find that by the end of the current century OHT reduces the widening of the circulation by ∼35% (0.42°) and its weakening by ∼60% (1.3 × 1010 kg s−1). As a result, OHT delays the emergence from internal variability of the widening by 30 years and of the weakening by 20 years. Lastly, while oceanic heat uptake accounts for most of the reduced widening, and thus merely delays it by reducing surface warming, horizontal heat transport and net heat uptake have comparable impacts to reduce the weakening of the circulation. Key Points: By the end of the 21st century ocean heat transport is projected to reduce the widening of the Hadley cell by ∼35% and its weakening by ∼60%The reduced widening is mostly due to the increase in oceanic heat uptake, which thus delays the expansion of the tropicsHorizontal heat distribution and net heat uptake are both responsible for weakening the circulation by redistributing latent heating [ABSTRACT FROM AUTHOR]

The projected weakening of the Northern Hemisphere Hadley cell will have large climatic impacts at low latitudes. Several mechanisms have been proposed to explain this weakening. In order to isolate and assess their relative importance, we here use the abrupt 4 × CO2 experiment of the Coupled Model Intercomparison Project phase 5, as this forcing separates the different mechanisms which respond on different time scales. We find that the Hadley circulation responds relatively quickly to quadrupling CO2 concentrations, reaching its steady‐state value after less than a decade. This fast response demonstrates that the weakening could not be solely due to the much slower increase in surface temperature. In addition, we show that the Hadley cell's weakening results from a combination of an increase in tropical static stability, partially offset by an increase in the latitudinal gradient of latent heating. Plain Language Summary: The Hadley circulation has large effects on the variability of temperature and precipitation at low latitudes. By the end of the 21st century, climate models project a weakening of the circulation, which will have great societal impacts. It is thus critical to elucidate the underlying mechanisms of the projected weakening. Here, we assess different mechanisms for the projected weakening and show that it stems from an increase in the vertical temperature gradient in the tropics, which is partially offset by an increase in latent heating. Key Points: Surface warming alone is unable to explain Hadley cell weakening under 4 × CO2 forcing, as the weakening exhibits a much faster responseEach of the previously proposed mechanisms analyzed in this study cannot fully explain the weakening of the Hadley cellThe weakening stems from a combination of the opposing effects of increases in stratification and in meridional gradient of latent heating [ABSTRACT FROM AUTHOR]

Atmospheric waves control the weather and climate variability, by affecting winds, temperature, and precipitation. It is thus critical to assess their future response to anthropogenic emissions. Most previous studies investigated the projected regional changes in the intensity of atmospheric waves, by pooling across waves with different scales. However, the waves' projected changes might vary with their scale, and thus, their future climate impacts might also be scale dependent. Here we show that both in the tropics and midlatitudes while large waves will get stronger, small waves will get weaker by the end of this century. Thus, investigating the response of atmospheric waves to human activity by pooling across all wave scales masks the future climate impacts of large waves. We further reveal that the opposite response of large and small waves stems from the opposite effect of static stability and zonal wind on the growth rate of the different waves. Key Points: By the end of the 21st century large tropical and midlatitudes waves will get stronger, while small waves will get weakerThe opposite response of large and small waves to anthropogenic emissions stems from the waves' opposite growth rate responseStudying future wave changes by pooling all scales together masks the different climate impacts of large and small waves [ABSTRACT FROM AUTHOR]

Abstract: The Hadley cell (HC) plays an important role in setting the strength and position of the hydrological cycle. Climate projections show a weakening of the HC, together with widening of its vertical and meridional extents. These changes are projected to have profound global climatic impacts. Current theories for the HC response to increased greenhouse gases account only for atmospheric and oceanic thermodynamic changes and not for oceanic circulation changes. Here the effects of ocean circulation changes on the HC response to increased greenhouse gases are examined by comparing fully coupled and slab ocean model configurations. By reducing the warming of both the sea surface and the atmosphere, changes in ocean circulation reduce the HC response to increased CO2 concentrations. This reduced warming suppresses convective heating, which reduces the weakening of the HC and the stabilization at low latitudes, and thus also reduces the meridional (in the Southern Hemisphere) and vertical HC expansion. [ABSTRACT FROM AUTHOR]

The projected changes in temperature due to global warming will have a profound effect on the behaviour of midlatitude eddies. Using an idealized moist global circulation model, the atmospheric barotropic energy balance is studied over a wide range of climates. The barotropic energy cycle is found to shift poleward as the long-wave optical thickness increases in concert with the poleward shift of the static stability, driven by the poleward shift of the upper-level baroclinicity. The baroclinic-barotropic conversion (barotropization) shows a non-monotonic behaviour at midlatitudes as the climate becomes warmer, and reaches a maximum value around present-day climate with lower values for colder and warmer climates. This is found to be associated with the non-monotonic behaviour of the stratification. Similar to the barotropization, the strength of the inverse energy cascade also shows a non-monotonic behaviour as the climate becomes warmer. However, the inverse energy cascade does not shift poleward, but rather corresponds to the uniform latitudinal distribution of the quasi-geostrophic supercriticality through all climates. The eddy-mean flow interactions increase and transfer kinetic energy from the eddies to the mean flow at low and high latitudes, and from the mean flow to the eddies at midlatitudes, as the climate becomes warmer. This occurs mostly due to the decrease of the latitudinal extent of the mean flow at high latitudes, which increases and shifts equatorward the meridional shear of the barotropic mean zonal wind. The findings of this study imply that under global warming the eddy flow is dominated by eddy-mean flow interactions and has a more baroclinic nature. [ABSTRACT FROM AUTHOR]