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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]

In spite of the unabated emissions of greenhouse gases into the atmosphere, sea ice around Antarctica has increased over most of the satellite era. Such an increase is not captured by climate models, which simulate a melting over the same period. Over the last few years, moreover, the observed sea ice trends have drastically changed, and this might act to cancel the models‐observations discrepancy. Here we show that in spite of the very recent Antarctic sea ice trend changes, such discrepancy still exists. Analyzing multiple large ensembles of model simulations, we elucidate the origin of the models‐observations discrepancy. We show that internal variability cannot account for the discrepancy, which therefore is likely to stem from biases in the models' forced response to the external forcing. These biases, we show, reside in thermodynamic ocean‐atmosphere coupling, as models fail to simulate the trends in surface heat fluxes from reanalyses over the period 1979–2019. Key Points: In spite of the Antarctic sea ice loss reported in recent years, climate models still fail to capture the observed sea ice trendThe source of discrepancy resides in the models' forced response, not in their ability to capture the internal variabilityThe models' inability to capture the observed sea ice trends is associated with a bias in simulating recent surface heat flux trends [ABSTRACT FROM AUTHOR]

One of the most robust responses of the climate system to future greenhouse gas emissions is the melting of Arctic sea ice. It is thus essential to elucidate its impacts on other components of the climate system. Here we focus on the response of the annual mean Hadley cell (HC) to Arctic sea ice loss using a hierarchy of model configurations: atmosphere only, atmosphere coupled to a slab ocean, and atmosphere coupled to a full‐physics ocean. In response to Arctic sea ice loss, as projected by the end of the 21st century, the HC shows negligible changes in the absence of ocean‐atmosphere coupling. In contrast, by warming the Northern Hemisphere thermodynamic coupling weakens the HC and expands it northward. However, dynamic coupling acts to cool the Northern Hemisphere which cancels most of this weakening and narrows the HC, thus opposing its projected expansion in response to increasing greenhouse gases. Plain Language Summary: The climate's response to anthropogenic emissions comprises different feedbacks of the different components in the climate system. One of the robust responses to increased greenhouse gases is the melting of Arctic sea ice, which is found to have large effects on the hydrological circulation in the atmosphere. Here we examine the effect of Arctic sea ice loss on the tropical circulation. We find that under Arctic sea ice loss ocean heat transport acts to transfer the Arctic signal to the tropics and to contract the tropical circulation. This contraction opposes the projected widening of the tropical circulation and thus shows that Arctic sea ice loss acts as a negative internal feedback in the response of the tropical circulation to increased greenhouse gases. Key Points: Arctic sea ice loss affects the Hadley cell only through ocean‐atmosphere couplingArctic sea ice loss acts as a negative internal feedback in the response of the Hadley cell width to anthropogenic emissionsOcean heat transport opposes the thermodynamic effect of Arctic sea ice loss on the Hadley circulation [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]

Poleward migration of eddy-driven jets is found to occur in the extratropics when the subtropical and eddy-driven jets are clearly separated, as achieved by simulations at high-rotation rates. The poleward migration of these eddy-driven baroclinic jets over time is consistent with variation of eddy momentum flux convergence and baroclinicity across the width of the jet. We demonstrate this using a high-resolution idealized GCM where we systematically examine the eddy-driven jets over a wide range of rotation rates (up to 16 times the rotation rate of Earth). At the flanks of the jets, the poleward migration is caused by a poleward bias in baroclinicity across the width of the jet, estimated through measures such as Eady growth rate and supercriticality. The poleward biased baroclinicity is due to the meridional variation of the Coriolis parameter, which causes a poleward bias of the eddy momentum flux convergence. At the core of the jets, the poleward biased eddy momentum flux convergence relative to the mean jet deflects over time the baroclinicity and the jets poleward. As the rotation rate is increased, and more (narrower) jets emerge the migration rate becomes smaller due to less eddy momentum flux convergence over the narrower baroclinic zones. We find a linear relation between the migration rate of the jets and the net eddy momentum flux convergence across the jets. This poleward migration might be related to the slow poleward propagation of temporal anomalies of zonal winds observed in the upper troposphere. [ABSTRACT FROM AUTHOR]