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.
The projected changes in temperature due to global warming will have a profound effect on the behavior of midlatitude eddies. Using an idealized moist global circulation model the barotropic energy cycle is found to shift poleward along with a non‐monotonic behavior at midlatitudes as the climate becomes warmer. 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.
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 trend
The source of discrepancy resides in the models' forced response, not in their ability to capture the internal variability
The models' inability to capture the observed sea ice trends is associated with a bias in simulating recent surface heat flux trends
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 weaker
The opposite response of large and small waves to anthropogenic emissions stems from the waves' opposite growth rate response
Studying future wave changes by pooling all scales together masks the different climate impacts of large and small waves
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.
Key Points
Ocean circulation acts to reduce the Hadley cell response to increased greenhouse gases
Convective heating is suppressed due to the reduced warming by ocean circulation
The suppressed convective heating reduces the weakening and the expansion of the Hadley cell
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 coupling
Arctic sea ice loss acts as a negative internal feedback in the response of the Hadley cell width to anthropogenic emissions
Ocean heat transport opposes the thermodynamic effect of Arctic sea ice loss on the Hadley circulation
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.
Key Points
A new possible aerosol effect on the tropical circulation and rain patterns is proposed
Aerosol loading increases water vapor content in the environment of trade wind clouds due to a suppression of precipitation
A decrease in subtropical precipitation in a GCM results in intensification of the equatorial precipitation and Hadley circulation
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.
Key Points
Oceanic barotropization and inverse energy cascade are found at high latitudes
The energy‐containing and conversion scales follow the Rhines and most unstable scales, respectively
Supercriticality and stratification affect the barotropization and inverse energy cascade
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.
Key Points
Near‐surface temperature increases with atmospheric mass, and the meridional temperature gradient decreases
Higher atmospheric heat capacity leads to decreased net radiative cooling
Suppressed vertical eddy fluxes further warm the lower atmosphere
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 tropics
Horizontal heat distribution and net heat uptake are both responsible for weakening the circulation by redistributing latent heating
Poleward migration of eddy‐driven jets Chemke, R.; Kaspi, Y.
Journal of advances in modeling earth systems,
September 2015, Volume:
7, Issue:
3
Journal Article
Peer reviewed
Open access
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.
Key Points:
Eddy driven jets migrate poleward with time
The planet's sphericity causes a poleward biased baroclinicity and eddy momentum flux convergence
At high rotation rates as narrower jets form the migration rate decreases
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