Publications

Accurately assessing future precipitation changes presents one of the greatest challenges of climate change. In the tropics, changes in the Hadley circulation are expected to considerably affect precipitation in dry subtropical and wet equatorial regions. However, while climate models project a robust weakening of the Northern Hemisphere circulation in the coming decades, currently, there is low confidence in the magnitude of such weakening and its impact on regional precipitation patterns. Here we use emergent constraint analyses and observation-based Hadley circulation strength changes to show that the projected circulation weakening will probably be larger than in current predictions. The more pronounced weakening of the flow results in a doubling of the subtropical precipitation increase compared with current forecasts, specifically over Asia, Africa and the Pacific Ocean. Our findings provide more accurate tropical circulation and precipitation projections and have considerable societal impacts, given the scarcity of water in subtropical regions.

The winter storm track in the Southern Hemisphere has large weather and climate impacts, as it drives daily to multi-decadal variations in extratropical winds, precipitation, and temperature. In the lower mid-latitudes, where the southern parts of the continents reside, the storm track is projected to initially increase with greenhouse gas emissions but to weaken as emissions continue. Given the extensive efforts to mitigate climate change, it is critical to assess the reversibility of the storm track under a negative emissions scenario. Here, we show that while changes in the storm track are reversible under a doubling of CO₂ concentrations, beyond that point, the storm track weakening becomes irreversible for centuries. The persistent storm track weakening stems from a prolonged polar amplification, driven by ocean heat transport changes, and is associated with intensity changes of poleward heat flux and extreme warm and cold temperatures. Our results suggest that implementing mitigation pathways prior of reaching a doubling of CO₂ concentrations would allow avoiding the persistent impacts of the weakening storm track.

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.

In the Southern Hemisphere, Earth system models project an intensification of winter storm tracks by the end of the twenty-first century. Previous studies using idealized models showed that storm track intensity saturates with increasing temperatures, suggesting that the intensification of the winter storm tracks might not continue further with increasing greenhouse gases. Here, we examine the response of midlatitude winter storm tracks in the Southern Hemisphere to increasing CO2 from two to eight times preindustrial concentrations in more realistic Earth system models. We find that at high CO2 levels (beyond 4×CO2), winter storm tracks no longer exhibit an intensification across the extratropics. Instead, they shift poleward, weakening the storm tracks at lower midlatitudes and strengthening at higher midlatitudes. By analyzing the eddy kinetic energy (EKE) budget, the nonlinear storm-track response to an increase in CO2 levels in the lower midlatitudes is found to stem from a scale-dependent conversion of eddy available potential energy to EKE. Specifically, in the lower midlatitudes, this energy conversion acts to oppositely change the EKE of long and short scales at low CO2 levels, but at high CO2 levels, it mostly reduces the EKE of shorter scales, resulting in a poleward shift of the storms. Furthermore, we identify a \u201ctug of war\u201d between the upper and lower temperature changes as the primary driver of the nonlinear-scale-dependent EKE response in the lower midlatitudes. Our results suggest that in the highest emission scenarios beyond the twenty-first century, the storm tracks response may differ in magnitude and latitudinal distribution from projected changes by 2100.

The Hadley circulation plays a central role in determining the location and intensity of the hydrological cycle in tropical and subtropical latitudes. Thus, the human-induced historical and projected weakening of the Northern Hemisphere Hadley circulation has considerable societal impacts. Yet, it is currently unknown how unparalleled this weakening is relative to the response of the circulation to natural forcings in past centuries. Here, using state-of-the-art climate models, we show that in contrast to the recent and future human-induced Hadley circulation weakening, natural forcings acted to intensify the circulation by cooling the climate over the last millennium. The reversal of a naturally-forced multi-centennial trend by human emissions highlights their unprecedented impacts on the atmospheric circulation. Given the amplifying effect of natural forcings on the Hadley circulation, our analysis stresses the importance of adequately incorporating natural forcings in climate model projections to better constrain future tropical climate changes.

Anthropogenic warming can alter large-scale circulation patterns in the atmosphere, which could have serious consequences for regional climate impacts and extreme weather. Observed thermodynamic changes in boreal extratropics have been attributed to human emissions with high confidence, but most circulation changes have not. In particular, not only that in the previous suite of climate models most models do not capture the recent boreal summer storm tracks weakening, but also a quantification of the role of human emissions in the recent storm tracks weakening has not been conducted to date. Here we use the latest suite of climate models, which are found to adequately capture the recent storm tracks weakening, and show that this weakening is attributable to anthropogenic emissions. Human emissions have resulted in more-rapid warming of the high latitudes, and the associated reduction in poleward temperature gradient has weakened the storms. The physical consistency between models and reanalyses increases our confidence in the projected weakening, which presents regional risks including hot-dry extremes in summer.

Projected tropical precipitation changes by the end of the century include increased net precipitation over the Pacific Ocean and drying over the Indian Ocean, prompting ongoing debate about the underlying mechanisms. Previous studies argued for the importance of the zonal circulation in the longitudinally dependent tropical precipitation response, as the meridional circulation is often defined and analyzed as the zonal mean. Here we show that the projected changes in the meridional circulation are highly longitudinally dependent, and explain the zonally dependent changes in net precipitation. Our analysis exposes a zonal shift in the ascending branch of the meridional circulation, associated with a strengthened net precipitation over the central Pacific and weakened precipitation in the Indo Pacific. The zonal circulation has minor influence on these projected tropical precipitation changes. These results point to the importance of monitoring the longitudinal changes in the meridional circulation for improving our preparedness for climate change impacts.

Temperature anomalies considerably influence the regional climate and weather of the extratropics. By the end of this century, climate models project an intensification of synoptic temperature variability in the Southern Hemisphere mid-latitudes. This intensification, however, comprises temperature anomalies with various length scales and periods, which might respond differently to anthropogenic emissions. Here, we find a shift, in coming decades, towards spatially larger and less persistent temperature anomalies in the Southern Hemisphere mid-latitudes. A shift towards larger length scales is also found during regional extreme heat events. The shift in length scale and duration is found to stem from changes in the meridional heat flux of atmospheric perturbations. Our results emphasize the importance of investigating the length scale and period-dependent changes in the mid-latitude climate, to prevent masking the different impacts of various length scales and periods, and thus provide more accurate climate projections for the mid-latitudes.

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.

The strength of mid-latitude storm tracks shapes weather and climate phenomena in the extra-tropics, as these storm tracks control the daily to multi-decadal variability of precipitation, temperature and winds. By the end of this century, winter mid-latitude storms are projected to intensify in the Southern Hemisphere, with large consequences over the entire extra-tropics. Therefore, it is critical to be able to accurately assess the impacts of anthropogenic emissions on these storms to improve societal preparedness for future changes. Here we show that current climate models severely underestimate the intensification in mid-latitude storm tracks in recent decades. Specifically, the intensification obtained from reanalyses has already reached the model-projected end-of-the-century intensification. The biased intensification is found to be linked to biases in the zonal flow. These results question the ability of climate models to accurately predict the future impacts of anthropogenic emissions in the Southern Hemisphere mid-latitudes.

Climate models project an intensification of the wintertime North Atlantic Ocean storm track, over its downstream region, by the end of this century. Previous studies have suggested that ocean-atmosphere coupling plays a key role in this intensification, but the precise role of the different components of the coupling has not been explored and quantified. In this paper, using a hierarchy of ocean coupling experiments, we isolate and quantify the respective roles of thermodynamic (changes in surface heat fluxes) and dynamic (changes in ocean heat flux convergence) ocean coupling in the projected intensification of North Atlantic transient eddy kinetic energy (TEKE). We show that dynamic coupling accounts for nearly all of the future TEKE strengthening as it overcomes the much smaller effect of surface heat flux changes to weaken the TEKE. We further show that by reducing the Arctic amplification in the North Atlantic, ocean heat flux convergence increases the meridional temperature gradient aloft, causing a larger eddy growth rate and resulting in the strengthening of North Atlantic TEKE. Our results stress the importance of better monitoring and investigating the changes in ocean heat transport, for improving climate change adaptation strategies. SIGNIFICANCE STATEMENT: By the end of this century, the North Atlantic Ocean storm track is projected to intensify on its eastward flank. Such intensification will have large societal impacts, mostly over western Europe. Thus, it is critical to better understand the mechanism underlying the intensification of the storm track. Here we investigate the role of ocean coupling in the future intensification of the North Atlantic storm track and find that ocean heat transport processes are responsible for the strengthening of the storm track. Our results suggest that better monitoring the changes in ocean heat transport will hopefully improve climate change adaption strategies.

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.

By modulating the distribution of heat, precipitation and moisture, the Hadley cell holds large climate impacts at low and subtropical latitudes. Here we show that the interannual variability of the annual mean Hadley cell strength is~30% less in the Northern Hemisphere than in the Southern Hemisphere. Using a hierarchy of ocean coupling experiments, we find that the smaller variability in the Northern Hemisphere stems from dynamic ocean coupling, which has opposite effects on the variability of the Hadley cell in the Southern and Northern Hemispheres; it acts to increase the variability in the Southern Hemisphere, which is inversely linked to equatorial upwelling, and reduce the variability in the Northern Hemisphere, which shows a direct relation with the subtropical wind-driven overturning circulation. The important role of ocean coupling in modulating the tropical circulation suggests that further investigation should be carried out to better understand the climate impacts of ocean-atmosphere coupling at low latitudes.

The enhanced warming of the Arctic, relative to other parts of the Earth, a phenomenon known as Arctic amplification, is one of the most striking features of climate change, and has important climatic impacts for the entire Northern Hemisphere. Several mechanisms are believed to be responsible for Arctic amplification; however, a quantitative understanding of their relative importance is still missing. Here, using ensembles of model integrations, we quantify the contribution of ocean coupling, both its thermodynamic and dynamic components, to Arctic amplification over the 20th and 21st centuries. We show that ocean coupling accounts for ~80% of the amplification by 2100. In particular, we show that thermodynamic coupling is responsible for future amplification and sea-ice loss as it overcomes the effect of dynamic coupling which reduces the amplification and sea-ice loss by ~35%. Our results demonstrate the utility of targeted numerical experiments to quantify the role of specific mechanisms in Arctic amplification, for better constraining climate projections.

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 SAMand thus the ozone holeare not the primary drivers of the sea ice expansion around Antarctica in recent decades.

We explore the climate system response to abrupt CO₂ forcing, spanning the range 1× to 8×CO₂, with two state-of-the-art coupled atmosphere-ocean-sea-ice-land models: the NASA Goddard Institute for Space Studies Model E2.1-G (GISS-E2.1-G) and the Community Earth System Model (CESM-LE). We find that the effective climate sensitivity is a non-monotonic function of CO₂ in both models, reaching a minimum at 3×CO₂ for GISS-E2.1-G, and 4×CO₂ for CESM-LE. A similar non-monotonic response is found in Northern Hemisphere surface temperature, sea-ice, precipitation, the latitude of zero precipitation-minus-evaporation, and the strength of the Hadley cell. Interestingly, the Atlantic meridional overturning circulation collapses when non-monotonicity appears and does not recover for larger CO₂ forcings. Analyzing the climate response over the same CO₂ range with slab-ocean versions of the same models, we demonstrate that the climate systems non-monotonic response is linked to ocean dynamics.

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.

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.

The Antarctic surface mass balance (SMB) has global climatic impacts through its effects on global sea-level rise. The forced increase in Antarctic SMB over the second half of the 20th century was argued to stem from multiple forcing agents, including ozone and ozone-depleting substances (ODSs). Here we use ensembles of fixed-forcing model simulations to quantify and contrast the contributions of stratospheric ozone, tropospheric ozone and ODSs to increases in the Antarctic SMB. We show that ODSs and stratospheric ozone make comparable contributions and together account for 44% of the increase in the annual mean Antarctic SMB over the second half of the 20th century. In contrast, tropospheric ozone has an insignificant impact on the SMB increase. A large portion of the annual mean SMB increase occurs during austral summer, when stratospheric ozone is found to account for 63% of the increase. Furthermore, we demonstrate that stratospheric ozone increases the SMB by enhancing the meridional mean and eddy flows towards the continent, thus converging more water vapor over the Antarctic.

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.

In spite of the unabated emissions of greenhouse gases into the atmosphere, seaice 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 seaice trends have drastically changed, and this might act to cancel the modelsobservations discrepancy. Here we show that in spite of the very recent Antarctic seaice trend changes, such discrepancy still exists. Analyzing multiple large ensembles of model simulations, we elucidate the origin of the modelsobservations 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 oceanatmosphere coupling, as models fail to simulate the trends in surface heat fluxes from reanalyses over the period 19792019.

North Atlantic sea surface temperatures have large climate impacts affecting the weather of the Northern Hemisphere. In addition to a substantial warming over much of the North Atlantic, caused by increasing greenhouse gases over the 21st century, climate projections show a surprising region of considerable future cooling at midlatitudes, referred to as the North Atlantic warming hole. A similar pattern of surface temperature trends has been observed in recent decades, but it remains unclear whether this pattern is of anthropogenic origin or a simple manifestation of internal climate variability. Here, analyzing state-of-the-art climate models and observations, we show that the recent North Atlantic warming hole is of anthropogenic origin. Our analysis reveals that the anthropogenic signal has only recently emerged from the internal climate variability, and can be attributed to greenhouse gas emissions. We further show that a declining northward oceanic heat flux in recent decades, which is linked to this surface temperature pattern, is also of anthropogenic origin.

Eddy heat fluxes play the important role of transferring heat from low to high latitudes, thus affecting midlatitude climate. The recent and projected polar warming, and its effects on the meridional temperature gradients, suggests a possible weakening of eddy heat fluxes. We here examine this question in reanalyses and state-of-the-art global climate models. In the Northern Hemisphere we find that the eddy heat flux has robustly weakened over the last four decades. We further show that this weakening emerged from the internal variability around the year 2000, and we attribute it to increasing greenhouse gases. In contrast, in the Southern Hemisphere we find that the eddy heat flux has robustly strengthened, and we link this strengthening to the recent multi-decadal cooling of Southern-Ocean surface temperatures. The inability of state-of-the-art climate models to simulate such cooling prevents them from capturing the observed Southern Hemisphere strengthening of the eddy heat flux. This discrepancy between models and reanalyses provides a clear example of how model biases in polar regions can affect the midlatitude climate.

The Hadley circulation has large climate impacts at low latitudes by transferring heat and moisture between the tropics and subtropics. Climate projections show a robust weakening of the Northern Hemisphere Hadley circulation by the end of the twenty-first century. Over the past several decades, however, atmospheric reanalyses indicate a strengthening of the Hadley circulation. Here we show that the strengthening of the circulation in the Northern Hemisphere is not seen in climate models; instead, these models simulate a weakening of the circulation in the past 40 years. Using observations and a large ensemble of model simulations we elucidate this discrepancy between climate models and reanalyses, and show that it does not stem from internal climate variability or biases in climate models, but appears related to artefacts in the representation of latent heating in the reanalyses. Our results highlight the role of anthropogenic emissions in the recent slowdown of the atmospheric circulation, which is projected to continue in coming decades, and question the reliability of reanalyses for estimating trends in the Hadley circulation.

Future emissions of greenhouse gases into the atmosphere are projected to result in significant circulation changes. One of the most important changes is the widening of the tropical belt, which has great societal impacts. Several mechanisms (changes in surface temperature, eddy phase speed, tropopause height, and static stability) have been proposed to explain this widening. However, the coupling between these mechanisms has precluded elucidating their relative importance. Here, the abrupt quadrupled-CO2 simulations of phase 5 of the Coupled Model Intercomparison Project (CMIP5) are used to examine the proposed mechanisms. The different time responses of the different mechanisms allow us to disentangle and evaluate them. As suggested by earlier studies, the Hadley cell edge is found to be linked to changes in subtropical baroclinicity. In particular, its poleward shift is accompanied by an increase in subtropical static stability (i.e., a decrease in temperature lapse rate) with increased CO2 concentrations. These subtropical changes also affect the eddy momentum flux, which shifts poleward together with the Hadley cell edge. Transient changes in tropopause height, eddy phase speed, and surface temperature, however, were found not to accompany the poleward shift of the Hadley cell edge. The widening of the Hadley cell, together with the increase in moisture content, accounts for most of the expansion of the dry zone. Eddy moisture fluxes, on the other hand, are found to play a minor role in the expansion of the dry zone.

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.

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.Plain Language Summary Given the importance of tropical circulation in affecting low-latitude climate, it is crucial to understand the projected response of tropical circulation to anthropogenic emissions. To fully understand the tropical circulation response, one must account for the internal feedbacks between the different components of the climate system. Here we study the effect of changes in ocean circulation in response to increased greenhouse gases on the tropical circulation. We find that ocean circulation acts to reduce the projected response of tropical circulation to anthropogenic emissions. This emphasizes the importance in using models with ocean circulation, when studying the long-term atmospheric response to increased greenhouse gases.

The large uncertainty in estimating the global aerosol radiative forcing (ARF) is one of the major challenges the climate community faces for climate projection. While the global-mean ARF may affect global quantities such as surface temperature, its spatial distribution may result in local thermodynamical and, thus, dynamical changes. Future changes in aerosol emissions distribution could further modulate the atmospheric circulation. Here, the effects of the spatial distribution of the direct anthropogenic ARF are studied using an idealized global circulation model, forced by a range of estimated-ARF amplitudes, based on the Copernicus Atmosphere Monitoring Service data. The spatial distribution of the estimated-ARF is globally decomposed, and the effects of the different modes on the circulation are studied. The most dominant spatial distribution feature is the cooling of the Northern Hemisphere in comparison to the Southern Hemisphere. This induces a negative meridional temperature gradient around the equator, which modulates the mean fields in the tropics. The ITCZ weakens and shifts southward, and the Northern (Southern) Hemisphere Hadley cell strengthens (weakens). The localization of the ARF in the Northern Hemisphere midlatitudes shifts the subtropical jet poleward and strengthens both the eddy-driven jet and Ferrel cell, because of the weakening of high-latitude eddy fluxes. Finally, the larger aerosol concentration in Asia compared to North America results in an equatorial superrotating jet. Understanding the effects of the different modes on the general circulation may help elucidate the circulation's future response to the projected changes in ARF distribution.

The many recently discovered terrestrial exoplanets are expected to hold a wide range of atmospheric masses. Here the dynamic-thermodynamic effects of atmospheric mass on atmospheric circulation are studied using an idealized global circulation model by systematically varying the atmospheric surface pressure. On an Earth analog planet, an increase in atmospheric mass weakens the Hadley circulation and decreases its latitudinal extent. These changes are found to be related to the reduction of the convective fluxes and net radiative cooling (due to the higher atmospheric heat capacity), which, respectively, cool the upper troposphere at mid-low latitudes and warm the troposphere at high latitudes. These together decrease the meridional temperature gradient, tropopause height and static stability. The reduction of these parameters, which play a key role in affecting the flow properties of the tropical circulation, weakens and contracts the Hadley circulation. The reduction of the meridional temperature gradient also decreases the extraction of mean potential energy to the eddy fields and the mean kinetic energy, which weakens the extratropical circulation. The decrease of the eddy kinetic energy decreases the Rhines wavelength, which is found to follow the meridional jet scale. The contraction of the jet scale in the extratropics results in multiple jets and meridional circulation cells as the atmospheric mass increases.

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.

Observations suggest that Earth's early atmospheric mass differed from the present day. The effects of a different atmospheric mass on radiative forcing have been investigated in climate models of variable sophistication, but a mechanistic understanding of the thermodynamic component of the effect of atmospheric mass on early climate is missing. Using a 3-D idealized global circulation model (GCM), we systematically examine the thermodynamic effect of atmospheric mass on near-surface temperature. We find that higher atmospheric mass tends to increase the near-surface temperature mostly due to an increase in the heat capacity of the atmosphere, which decreases the net radiative cooling effect in the lower layers of the atmosphere. Additionally, the vertical advection of heat by eddies decreases with increasing atmospheric mass, resulting in further near-surface warming. As both net radiative cooling and vertical eddy heat fluxes are extratropical phenomena, higher atmospheric mass tends to flatten the meridional temperature gradient.

Cloud-aerosol interactions are considered as one of the largest sources of uncertainties in the study of climate change. Here another possible cloud-aerosol effect on climate is proposed. A series of large eddy simulations (LES) with bin microphysics reveal a sensitivity of the total atmospheric water vapor amount to aerosol concentration. Under polluted conditions the rain is suppressed and the total amount of water vapor in the atmosphere increases with time compared to clean precipitating conditions. Theoretical examination of this aerosol effect on water vapor transport from the subtropics to the tropics, and hence on the equatorial rain and Hadley circulation, is conducted using an idealized general circulation model (GCM). It is shown that a reduction in the subtropical rain amount results in increased water vapor advection to the tropics and enhanced equatorial rain and Hadley circulation. This joins previously proposed mechanisms on the radiative aerosol effect on the general circulation.

The midlatitude atmosphere is characterized by turbulent eddies that act to produce a depth-independent (barotropic) mean flow. Using the NCEP (National Centers for Environmental Prediction) Reanalysis 2 data, the latitudinal dependence of barotropic kinetic energy and enstrophy are investigated. Most of the barotropization takes place in the extratropics with a maximum value at midlatitudes, due to the latitudinal variations of the static stability, tropopause height, and sphericity of the planet. Barotropic advection transfers the eddy kinetic energy to the zonal mean flow and thus maintains the barotropic component of the eddy-driven jet. The classic description of geostrophic turbulence exists only at high latitudes, where the quasi-geostrophic flow is supercritical to baroclinic instability; the eddy-eddy interactions carry both the barotropization of eddy kinetic energy upscale to the Rhines scale and the barotropization of eddy potential enstrophy downscale.

The effect of eddy-eddy interactions on zonal and meridional macroturbulent scales is investigated over a wide range of eddy scales, using high-resolution idealized GCM simulations with and without eddy-eddy interactions. The wide range of eddy scales is achieved through systematic variation of the planetary rotation rate and thus multiple-jet planets. It is found that not only are eddy-eddy interactions not essential for the formation of jets, but the existence of eddy-eddy interactions decreases the number of eddy-driven jets in the atmosphere. The eddy-eddy interactions have little effect on the jet scale, which in both types of simulations coincides with the Rhines scale through all latitudes. The decrease in the number of jets in the presence of eddy-eddy interactions occurs because of the narrowing of the latitudinal region where zonal jets appear. This narrowing occurs because eddy-eddy interactions are mostly important at latitudes poleward of where the Rhines scale is equal to the Rossby deformation radius. Thus, once eddy-eddy interactions are removed, the conversion from baroclinic to barotropic eddy kinetic energy increases, and eddy-mean flow interactions intrude into these latitudes and maintain additional jets there. The eddy-eddy interactions are found to increase the energy-containing zonal scale so it coincides with the jets' scale and thus make the flow more isotropic. While the conversion scale coincides with the most unstable scale, the Rossby deformation radius does not provide a good indication to these scales in both types of simulations.

Geostrophic turbulence theory predicted already a few decades ago an inverse energy cascade in the barotropic mode, yet there has been limited evidence for it in the ocean. In this study, the latitudinal behavior of the oceanic barotropic energy balance and macroturbulent scales is studied using the ECCO2 (Estimating the Circulation and Climate of the Ocean) state estimate, which synthesizes satellite data and in situ measurements with a high-resolution general circulation model containing realistic bathymetry and wind forcing. It is found that inverse energy cascade occurs at high latitudes, as eddy-eddy interactions spread the conversion of eddy kinetic energy from the baroclinic to the barotropic mode, both upscale and downscale. At these latitudes, the conversion scale of baroclinic eddy kinetic energy and the energy-containing scale follow the most unstable and Rhines scales, respectively. Even though an inverse energy cascade occurs at high latitudes, the energy spectrum follows a steeper slope than the -5/3 slope. Different than classic arguments, the Rossby deformation radius does not follow the baroclinic conversion and most unstable scales.

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.

The latitudinal width of atmospheric eddy-driven jets and scales of macroturbulence are examined latitude by latitude over a wide range of rotation rates using a high-resolution idealized GCM. It is found that for each latitude, through all rotation rates, the jet spacing scales with the Rhines scale. These simulations show the presence of a "supercriticality latitude" within the baroclinic zone, where poleward (equatorward) of this latitude, the Rhines scale is larger (smaller) than the Rossby deformation radius. Poleward of this latitude, a classic geostrophic turbulence picture appears with a - spectral slope of inverse cascade from the deformation radius up to the Rhines scale. A shallower slope than the -3 slope of enstrophy cascade is found from the deformation radius down to the viscosity scale as a result of the broad input of baroclinic eddy kinetic energy. At these latitudes, eddy-eddy interactions transfer barotropic eddy kinetic energy from the input scales of baroclinic eddy kinetic energy up to the jet scale and down to smaller scales. For the Earth case, this latitude is outside the baroclinic zone and therefore an inverse cascade does not appear. Equatorward of the supercriticality latitude, the - slope of inverse cascade vanishes, eddy-mean flow interactions play an important role in the balance, and the spectrum follows a -3 slope from the Rhines scale down to smaller scales, similar to what is observed on Earth. Moreover, the length scale of the energy-containing zonal wavenumber is equal to (larger than) the jet scale poleward (equatorward) of the supercriticality latitude.

In-situ aircraft measurements of aerosol chemical and cloud microphysical properties were conducted during the CalWater campaign in February and March 2011 over the Sierra Nevada Mountains and the coastal waters of central California. The main objective was to elucidate the impacts of aerosol properties on clouds and precipitation forming processes. In order to accomplish this, we compared contrasting cases of clouds that ingested aerosols from different sources. The results showed that clouds containing pristine oceanic air had low cloud drop concentrations and started to develop rain 500 m above their base. This occurred both over the ocean and over the Sierra Nevada, mainly in the early morning when the radiatively cooled stable continental boundary layer was decoupled from the cloud base. Supercooled rain dominated the precipitation that formed in growing convective clouds in the pristine air, up to the -21 degrees C isotherm level.A contrasting situation was documented in the afternoon over the foothills of the Sierra Nevada, when the clouds ingested high pollution aerosol concentrations produced in the Central Valley. This led to slow growth of the cloud drop effective radius with height and suppressed and even prevented the initiation of warm rain while contributing to the development of ice hydrometeors in the form of graupel. Our results show that cloud condensation and ice nuclei were the limiting factors that controlled warm rain and ice processes, respectively, while the unpolluted clouds in the same air mass produced precipitation quite efficiently. These findings provide the motivation for deeper investigations into the nature of the aerosols seeding clouds. (C) 2013 Elsevier B.V. All rights reserved.

Highly supercooled rain and drizzle from cloud tops at -12 to -21 degrees C down to the 0 degrees isotherm was documented by aircraft observations in clouds over a wide range of meteorological situations under relatively pristine marine aerosol conditions. The Gulfstream-1 aircraft during the CalWater campaign in February and early March 2011 measured clouds over the coastal waters of northern California, orographically triggered convective clouds over the foothills of the Sierra Nevada, and orographic layer clouds over Yosemite National Park. Supercooled drizzle in layer clouds near Juneau, Alaska, was measured by the Wyoming King Air as part of a FAA project to study aircraft icing in this region. Low concentrations of cloud condensation nuclei (CCN) were commonly observed in all of these clouds, allowing for the formation of clouds with small concentrations of mostly large drops that coalesced into supercooled drizzle and raindrops. Another common observation was the absence of ice nuclei (IN) and/or ice crystals in measurable concentrations, associated with persistent supercooled drizzle and rain. Average ice crystal concentrations were 0.007l(-1) at the top of convective clouds at -12 degrees C and 0.03l(-1) in the case of layer clouds at -21 degrees C. In combination, these two conditions of low concentrations of CCN and very few IN provide ideal conditions for the formation of highly supercooled drizzle and rain. These results help explain the anomalously high incidences of aircraft icing at cold temperatures in U.S. west coast clouds and highlight the need to include aerosol effects when simulating aircraft icing with cloud models.