Transport Of Moist Static Energy Research Articles

Convectively coupled Rossby–Gravity waves in a field campaign: How they are captured in reanalysis products

AbstractConvectively coupled equatorial waves are a significant source of atmospheric variability in the tropics. Current numerical models continue to struggle in simulating the coupled diabatic heating fields that are responsible for the development and maintenance of these waves. This study investigates how the diabatic fields associated with Mixed Rossby–Gravity waves (MRGs) are represented in four reanalysis products by using a unique observational dataset from the TRMM‐KWAJEX (Tropical Rainfall Measuring Mission‐Kwajalein Experiment) field campaign. These reanalyses include ERA5, Japanese 55‐year Reanalysis (JRA‐55), Climate Forecast System Reanalysis (CFSR), and Modern‐Era Retrospective Analysis for Research and Applications (MERRA). We found that all four reanalyses captured the MRG structures in winds and temperature, and to a lesser degree in the humidity field except in the boundary layer. However, only the ERA5 and MERRA reanalyses captured the gradual rise and succession of the diabatic heating from boundary layer turbulence, shallow convection, cumulus congestus, and deep convection within the waves. ERA5 is the only product that also captured the gradual rise of the subgrid‐scale vertical transport of moist static energy. All reanalysis products underestimated the diabatic heating from cumulus congestus. Results provide observational basis on what aspects of MRG can be trusted and what cannot in the reanalysis products.

Reply to Comment on “Moist Static Energy Transport Trends in Four Global Reanalyses: Are They Downgradient?” by Clark et al. (2022)

AbstractIn a previous study, we investigated whether reanalysis moist static energy (MSE) transport trends over the 1980 through 2018 period are consistent (a) with each other and (b) with the finding that these transport trends are downgradient, as found in climate models. Regarding point (a), our conclusion was that MSE transport trends were dependent on the reanalysis data set. However, Cox et al. (2023) correctly point out that the reanalysis dependence is reduced dramatically if a barotropic mass flux correction is applied at a monthly mean timescale prior to computing the MSE transport trends. In our reply below, we revisit point (b) after applying this correction. We find that even after the correction, reanalysis MSE transport trends are not downgradient nor poleward in the Northern Hemisphere extratropics. However, reanalysis does show a compensation between dry static and latent energy transport trends, which has been shown in climate models historically.

Comment on “Moist Static Energy Transport Trends in Four Global Reanalyses: Are They Downgradient?” by Clark et al. (2022)

AbstractGiven the key role that atmospheric heat transport plays in Earth's climate system, efforts to document its changes over the satellite era are valuable. Clark et al. (2022, https://doi.org/10.1029/2022GL098822) calculated trends in atmospheric heat transport among four reanalysis data sets and found substantial disagreements between data sets. However, after accounting for the lack of mass‐conservation in reanalysis data sets, we find much smaller magnitude trends, with much better agreement among reanalyses. This highlights the importance of mass corrections when calculating atmospheric heat transport.

A Process Model for ITCZ Narrowing under Warming Highlights Clear-Sky Water Vapor Feedbacks and Gross Moist Stability Changes in AMIP Models

Abstract Tropical areas with mean upward motion—and as such the zonal-mean intertropical convergence zone (ITCZ)—are projected to contract under global warming. To understand this process, a simple model based on dry static energy and moisture equations is introduced for zonally symmetric overturning driven by sea surface temperature (SST). Processes governing ascent area fraction and zonal mean precipitation are examined for insight into Atmospheric Model Intercomparison Project (AMIP) simulations. Bulk parameters governing radiative feedbacks and moist static energy transport in the simple model are estimated from the AMIP ensemble. Uniform warming in the simple model produces ascent area contraction and precipitation intensification—similar to observations and climate models. Contributing effects include stronger water vapor radiative feedbacks, weaker cloud-radiative feedbacks, stronger convection-circulation feedbacks, and greater poleward moisture export. The simple model identifies parameters consequential for the inter-AMIP-model spread; an ensemble generated by perturbing parameters governing shortwave water vapor feedbacks and gross moist stability changes under warming tracks inter-AMIP-model variations with a correlation coefficient ∼0.46. The simple model also predicts the multimodel mean changes in tropical ascent area and precipitation with reasonable accuracy. Furthermore, the simple model reproduces relationships among ascent area precipitation, ascent strength, and ascent area fraction observed in AMIP models. A substantial portion of the inter-AMIP-model spread is traced to the spread in how moist static energy and vertical velocity profiles change under warming, which in turn impact the gross moist stability in deep convective regions—highlighting the need for observational constraints on these quantities. Significance Statement A large rainband straddles Earth’s tropics. Most, but not all, climate models predict that this rainband will shrink under global warming; a few models predict an expansion of the rainband. To mitigate some of this uncertainty among climate models, we build a simpler model that only contains the essential physics of rainband narrowing. We find several interconnected processes that are important. For climate models, the most important process is the efficiency with which clouds move heat and humidity out of rainy regions. This efficiency varies among climate models and appears to be a primary reason for why climate models do not agree on the rate of rainband narrowing.

Identifying the Deep‐Inflow Mixing Features in Orographically‐Locked Diurnal Convection

AbstractOrographically‐locked diurnal convection involves interactions between local circulation and the thermodynamic environment of convection. Here, the relationships of convective updraft structures over orographic precipitation hotspots and their upstream environment in the TaiwanVVM large‐eddy simulations are analyzed for the occurrence of the orographic locking features. Strong convective updraft columns within heavily precipitating, organized systems exhibit a mass flux profile gradually increasing with height through a deep lower‐tropospheric inflow layer. Enhanced convective development is associated with higher upstream moist static energy (MSE) transport through this deep‐inflow layer via local circulation, augmenting the rain rate by 36% in precipitation hotspots. The simulations provide practical guidance for targeted observations within the most common deep‐inflow path. Preliminary field measurements support the presence of high MSE transport within the deep‐inflow layer when organized convection occurs at the hotspot. Orographically‐locked convection facilitate both modeling and field campaign design to examine the general properties of active deep convection.

Surface impacts and associated mechanisms of a moisture intrusion into the Arctic observed in mid-April 2020 during MOSAiC

Distinct events of warm and moist air intrusions (WAIs) from mid-latitudes have pronounced impacts on the Arctic climate system. We present a detailed analysis of a record-breaking WAI observed during the MOSAiC expedition in mid-April 2020. By combining Eulerian with Lagrangian frameworks and using simulations across different scales, we investigate aspects of air mass transformations via cloud processes and quantify related surface impacts. The WAI is characterized by two distinct pathways, Siberian and Atlantic. A moist static energy transport across the Arctic Circle above the climatological 90th percentile is found. Observations at research vessel Polarstern show a transition from radiatively clear to cloudy state with significant precipitation and a positive surface energy balance (SEB), i.e., surface warming. WAI air parcels reach Polarstern first near the tropopause, and only 1–2 days later at lower altitudes. In the 5 days prior to the event, latent heat release during cloud formation triggers maximum diabatic heating rates in excess of 20 K d-1. For some poleward drifting air parcels, this facilitates strong ascent by up to 9 km. Based on model experiments, we explore the role of two key cloud-determining factors. First, we test the role moisture availability by reducing lateral moisture inflow during the WAI by 30%. This does not significantly affect the liquid water path, and therefore the SEB, in the central Arctic. The cause are counteracting mechanisms of cloud formation and precipitation along the trajectory. Second, we test the impact of increasing Cloud Condensation Nuclei concentrations from 10 to 1,000 cm-3 (pristine Arctic to highly polluted), which enhances cloud water content. Resulting stronger longwave cooling at cloud top makes entrainment more efficient and deepens the atmospheric boundary layer. Finally, we show the strongly positive effect of the WAI on the SEB. This is mainly driven by turbulent heat fluxes over the ocean, but radiation over sea ice. The WAI also contributes a large fraction to precipitation in the Arctic, reaching 30% of total precipitation in a 9-day period at the MOSAiC site. However, measured precipitation varies substantially between different platforms. Therefore, estimates of total precipitation are subject to considerable observational uncertainty.

A Vertically Resolved Analysis of Radiative Feedbacks on Moist Static Energy Variance in Tropical Cyclones

Abstract A vertically resolved moist static energy (MSE) variance budget framework is used to diagnose processes associated with the development of tropical cyclones (TCs) in a general circulation model (GCM) under realistic boundary conditions. Previous studies have shown that interactions between radiation and MSE promote TC development. Here, we examine the vertical contributions of radiation and its interactions with MSE by performing several mechanism-denial experiments in which synoptic-scale radiative interactions are suppressed either in the boundary layer or in the free troposphere. Partly suppressing radiative interactions results in a reduction in global TC frequency. However, the magnitude of reduction and structure of the feedback depend on the intensity and structure of the TCs in these mechanism-denial experiments, indicating that both the magnitude and the vertical location of radiative interactions can impact global TC frequency. Using instantaneous 6-hourly outputs, an explicit computation reveals distinct spatial patterns of the advection term: the vertical component is positive in the mid- to upper troposphere, which reflects an upward transport of MSE by deep convection, whereas the horizontal component is positive in the boundary layer. These results illustrate the impact of the vertical distribution of radiative interactions and vertically varied contribution of the advection term in the development of TCs.

Sensitivity of Convective Self‐Aggregation to Subsidence

AbstractSimulations using the convection‐permitting MesoNH model with imposed vertical motion document the influence of large‐scale circulation on convective self‐aggregation. Results show that subsidence favors self‐aggregation in this model and that this sensitivity includes a hysteresis: for weak subsidence, only one stationary state is found, without aggregation; for moderate subsidence, two stationary states are found, one with aggregation and one without; for large subsidence, only the aggregated stationary state is found. In that state, the area covered by deep convection decreases with subsidence. Using the variance of column moist static energy (MSE) as a metric for aggregation, we detail the role of physical processes in the development or maintenance of aggregation. Solar radiation always contributes positively to aggregation. The effect of terrestrial radiation results from a competition between the effects of the top‐of‐atmosphere and surface fluxes, and can therefore vary in amplitude and change sign both with time in transient simulations and with subsidence intensity in stationary state. The effect of turbulent surface fluxes also result from a competition between the effect of air‐sea thermodynamic contrast and the wind‐induced contribution, and it can also vary very significantly. The contribution of MSE transport within the model domain also changes from negative to positive as the intensity of subsidence is increased because the shallow circulation accelerates and causes a change of sign in the gross moist stability associated with the internal circulation.

Moist Static Energy Transport Trends in Four Global Reanalyses: Are They Downgradient?

AbstractTrends in moist static energy (MSE) transport are investigated for the years 1980 through 2018 using four different reanalysis data sets. The reanalysis data sets show agreement in the eddy MSE transport trends and the latitudinal structure of the MSE trends, but vary widely in the trend of the flux of the climatological zonal mean MSE by the anomalous zonal mean meridional wind. The latter dominates the total MSE transport trends in all four data sets. Therefore, none of the four total MSE flux trends is downgradient of the corresponding MSE trend. Further analysis of the MSE trends reveals that dry static energy increases strongly dominate MSE trends at all latitudes, including in the tropics where climate models and theory predict latent energy increases to dominate. As changes in MSE transport are routinely assumed to be downgradient when interpreting changes in climate, including Arctic amplification, further investigation of reanalysis MSE transport is warranted.

Dependence of Convective Cloud Properties and Their Transport on Cloud Fraction and GCM Resolution Diagnosed from a Cloud-Resolving Model Simulation

The scale-aware convective parameterization for high resolution global climate models must satisfy the requirement that the parameterized subgrid convective transport diminishes as the model resolution increases to convection-resolving resolutions. A major assumption in current scale-aware convection schemes is that the differences between convective cloud properties and their environmental counterparts are independent of cloud fraction. This study examines convective cloud vertical velocity, moist static energy (MSE), moisture, and the vertical eddy transport of MSE and moisture for different averaging subdomain sizes and fractional convective cloudiness using a cloud resolving model simulation of a midlatitude mesoscale convective system. Results show that convective cloud fraction, mass flux, and vertical transport of MSE and moisture increase with decreasing subdomain size. The differences between convective cloud properties in both updrafts and downdrafts and their environment depend on both cloud fraction and the averaging subdomain size. For a given subdomain size, the differences increase with cloud fraction, in contrast to the assumption used in current scale-aware convection parameterization schemes. A consequence of this is that the parameterized convective eddy transport reaches maximum at a higher cloud fraction than believed in previous studies. This has implications on how fast the subgrid convective transport should diminish as GCM resolution increases.

Eddy-Mediated Hadley Cell Expansion due to Axisymmetric Angular Momentum Adjustment to Greenhouse Gas Forcings

Abstract The poleward expansion of the Hadley cells is one of the most robust modeled responses to increasing greenhouse gas concentrations. There are many proposed mechanisms for expansion, and most are consistent with modeled changes in thermodynamics, dynamics, and clouds. The adjustment of the eddies and the mean flow to greenhouse gas forcings, and to one another, complicates any effort toward a deeper understanding. Here we modify the Gray Radiation and Moist Aquaplanet (GRANDMA) model to uncouple the eddy and mean flow responses to forcings. When eddy forcings are held constant, the purely axisymmetric response of the Hadley cell to a greenhouse gas–like forcing is an intensification and poleward tilting of the cell with height in response to an axisymmetric increase in angular momentum in the subtropics. The angular momentum increase drastically alters the circulation response compared to axisymmetric theories, which by nature neglect this adjustment. Model simulations and an eddy diffusivity framework demonstrate that the axisymmetric increase in subtropical angular momentum—the direct manifestation of the radiative–convective equilibrium temperature response—drives a poleward shift of the eddy stresses which leads to Hadley cell expansion. Prescribing the eddy response to the greenhouse gas–like forcing shows that eddies damp, rather than drive, changes in angular momentum, moist static energy transport, and momentum transport. Expansion is not driven by changes in baroclinic instability, as would otherwise be diagnosed from the fully coupled simulation. These modeling results caution any assessment of mechanisms for circulation change within the fully coupled wave–mean flow system.

Hemisphere-asymmetric tropical cyclones response to anthropogenic aerosol forcing

How anthropogenic forcing could change tropical cyclones (TCs) is a keen societal concern owing to its significant socio-economic impacts. However, a global picture of the anthropogenic aerosol effect on TCs has not yet emerged. Here we show that anthropogenic aerosol emission can reduce northern hemisphere (NH) TCs but increase southern hemisphere (SH) TCs primarily through altering vertical wind shear and mid-tropospheric upward motion in the TC formation zones. These circulation changes are driven by anthropogenic aerosol-induced NH-cooler-than-SH and NH-increased versus SH-decreased meridional (equator to mid-latitudes) temperature gradients. The cooler NH produces a low-level southward cross-equatorial transport of moist static energy, weakening the NH ascent in the TC formation zones; meanwhile, the increased meridional temperature gradients strengthen vertical wind shear, reducing NH TC genesis. The opposite is true for the SH. The results may help to constrain the models’ uncertainty in the future TC projection. Reduction of anthropogenic aerosol emission may increase the NH TCs threat.

Dry and moist atmospheric circulation with uniform sea‐surface temperature

AbstractThe steady and transient response of “dynamically” dry and moist atmospheres to uniform sea‐surface temperature (SST) is studied using an aquaplanet general circulation model (GCM). Specifically, the latent heat () of water vapour is varied, so that for small , water substance is essentially a passive tracer from a dynamical point of view. Despite the lack of SST gradients, Hadley and Ferrel cells in the same direction as that on present‐day Earth are observed for relatively stronger moist coupling. Organized convergence by equatorial waves drives tropical overturning, and along with the equatorial deformation radius, the Hadley cell width increases with coupling strength. An abrupt switch to a much shallower tropical cell is noted when the system becomes completely passive. Moist static energy is transported equatorward in the tropics and a larger amount is directed poleward in the midlatitudes. As a whole, there is an almost invariant poleward transport of moist static energy for relatively strong coupling of water substance. Transient extratropical activity is seen in the form of intense warm‐core vortices for strong coupling, and these systems become weaker and smaller as decreases. The drift of these moist vortices results in the observed poleward energy transport in the midlatitudes. In the tropics, intraseasonal variability is dominant and systematically shifts to longer time periods with stronger coupling. In fact, large‐scale, low‐frequency Kelvin waves and MJO‐like modes disappear as water vapour becomes passive in nature. Further, the direction of tropical overturning reverses as decreases due to the off‐equatorial convergence imposed by tropical Rossby waves. Finally, extreme rainfall events associated with cyclonic storms vanish as water vapour becomes dynamically inactive, however, moderate precipitation events increase leading to higher total precipitation for weaker coupling of water substance. Tropospheric heating due to a saturation of the outgoing longwave radiation results in an increase in the stability of the atmosphere for strong coupling, and provides a plausible physical mechanism for interpreting the behaviour of precipitation.

The role of background SST changes in the ENSO-driven rainfall variability revealed from the atmospheric model experiments in CMIP5/6

The tropical Pacific rainfall variability driven by El Niño–Southern Oscillation (ENSO rainfall) is the crucial bridge across which ENSO exerts its influence on global climate. Previous modeling studies have documented a robust intensification and eastward shift of ENSO-driven rainfall under global warming despite large inter-model biases in ENSO-related sea surface temperature variability (ENSO SST), but the formation mechanisms of the robust ENSO rainfall changes are not well clarified. Here, we utilized a set of atmospheric model experiments—namely, “amip”, forced by observed SST; “amip-p4K”, forced by observed SST plus uniform 4-K warming, and “amip-future4K”, forced by observed SST plus spatially patterned SST warming, in CMIP5/CMIP6—to disentangle the role of the changes in mean-state SST (uniform and spatially patterned SST warming) with an unchanged ENSO SST. Relative to the weak ENSO rainfall changes in amip-p4K, those in amip-future4K forced by an El Niño-like SST warming pattern show an increase and eastward shift over the equatorial central–eastern Pacific. The results in amip-future4K without ENSO SST changes coincide well with previous results of the coupled models in CMIP5/3, suggesting a dominant role played by the mean-state SST warming pattern in ENSO rainfall changes. By diagnosing the moisture budget decomposition and moist static energy transport, we revealed that the structural and amplitude changes in ENSO rainfall are mainly controlled by the dynamic and thermodynamic components, respectively. In addition, the El Niño-like SST warming pattern in amip-future4K can increase low-level specific humidity with a remarkable spatial gradient, particularly in the equatorial central–eastern Pacific. As a result, the low-level specific humidity dominates the pattern of changes in the vertical transport of moist static energy, and subsequently the ENSO rainfall changes, in amip-future4K. This study further verified the importance of background SST changes to ENSO rainfall under global warming.

Modes of tropical convection and their roles in transporting moisture and moist static energy: contrast between deep and shallow convection

The dominant modes of convection (vertical motion) in the tropical convective regions were explored using the EOF analysis method and the ERA-interim reanalysis data, with an emphasis on comparing their roles in transporting moisture and moist static energy (MSE). Two major types of convection orthogonal to each other were identified in the deep tropics, including deep and shallow convection. The deep convection, derived from the 1st EOF mode of vertical motion, shows different vertical structures (e.g., top-heavy versus bottom-heavy) under different thermodynamic environments (e.g., west versus east Pacific); while the shallow convection, derived from the 2nd EOF mode of vertical motion, exhibits opposite signs of vertical structure (e.g., positive and negative) between upper and lower troposphere. The two subtypes of deep convection exhibit distinct tendencies in moisture and MSE transport, i.e., while both import moisture into the atmosphere, the top-heavy one tends to export MSE, leading to a stabilized atmosphere; whereas the bottom-heavy counterpart inclines to import MSE, resulting in a destabilized atmosphere. On the contrary, the two subtypes of shallow convection display a consistent tendency in moisture and MSE transport, i.e., the positive shallow convection imports moisture and MSE; whereas the negative counterpart exports moisture and MSE. More importantly, the MSE transport by shallow convection (including positive and negative ones) can be comparable in size with the MSE transport by deep convection (including top-heavy and bottom-heavy ones), implying a non-negligible role played by shallow convection in charging and discharging the atmosphere in addition to the commonly recognized deep convection.

Precipitation and the Associated Moist Static Energy Budget off Western Australia in Conjunction with Ningaloo Niño

Ningaloo Nino is a well-known ocean-atmosphere coupled climate event in the southeastern Indian Ocean that interacts with large-scale atmospheric circulations at the interannual time scale. When a Ningaloo Nino develops, remarkably enhanced precipitation anomalies occur primarily off the coast of northwestern Australia (NWA) rather than over the offshore area of western Australia (WA), where the most significant sea surface temperature warming prevails. This enhanced NWA precipitation manifests as jointly intensified stratiform and convective precipitation accompanied by an increase in high cloud cover. Further analyses of the column-integrated moist static energy (MSE) budget reveal that during Ningaloo Nino events, positive vertical MSE transport moistens and heats the atmosphere in the WA region. However, these moistening and heating effects are mostly offset by the inhibitory effect of the negative horizontal MSE advection, leading to the relatively weak and nonsignificant variation in local precipitation anomalies off the WA coastal region. In contrast, off the coast of the NWA region, the recharge of column-integrated MSE, which is induced by the significant positive radiative and surface heating, horizontal MSE advection, and vertical MSE advection, contributes to the heaviest austral summer precipitation associated with Ningaloo Nino. Therefore, the distinct MSE transport processes result in the particular rainfall pattern in which the most enhanced rainfall is associated with Ningaloo Nino events over the NWA region.

Trends, abrupt shifts and interannual variability of the Arctic Wintertime Seasonal Sea Ice from 1979 to 2019

AbstractThe Arctic winter seasonal sea ice (WSSI) concentration from 1979 to 2019 is derived from passive microwave data. Based on Empirical Orthogonal Function (EOF) analysis, the WSSI time series includes regionally different trends, abrupt shifts and interannual variations. The time series of the first EOF mode (PC1) mainly represents the WSSI trend, which is characterized by an increase, particularly in the Pacific sector. PC1 confirms two abrupt shifts in WSSI in 1989 and 2007, with a variance of 31%. After 2007, the large-scale atmospheric circulation anomaly shows a strengthened wavenumber 3 structure at high latitudes associated with a mid-tropospheric low-pressure anomaly in central and western Siberia and a high-pressure anomaly in eastern Siberia in summer and autumn. These patterns have promoted the increased transport of moist static energy to the central Arctic and contributed to increased near-surface air temperatures that may enhance ice melting in summer and reduce ice growth in autumn and winter. The changes in ice melt and growth have had opposite effects in the Pacific and Atlantic sectors: WSSI has increased in the Pacific sector due to the replacement of multi-year ice by WSSI, and decreased in the Atlantic sector due to the replacement of WSSI by open water.

Amplified tropical Pacific rainfall variability related to background SST warming

Previous studies have found that under global warming, El Niño/Southern Oscillation (ENSO)-related rainfall variability will become enhanced over the tropical central-eastern Pacific and weakened over the western Pacific. The climatological sea surface temperature (SST) warming pattern exhibits a warming center in the equatorial eastern Pacific in projections. How this pattern contributes to projected changes in ENSO-driven rainfall variability has not been fully addressed. Here, we use “time-slice” experiments to investigate the response of interannual variability in tropical Pacific rainfall in boreal winter to a warming background SST and associated physical mechanisms. A high-resolution Atmosphere General Circulation Model is driven by the detrended observational SST (1979–2003) plus a warming pattern from the coupled model under the A1B emission scenario (2075–2099). The results show that precipitation interannual variability over the tropical central-eastern Pacific will be enhanced more than the surrounding regions under warming, which is mostly contributed by a faster increase in rainfall amount during the El Niño year relative to non-El Niño years. Based on a moisture budget analysis, both the dynamic and thermodynamic components in the vertical advection of climatological specific humidity contribute to the enhancement of El Niño-induced precipitation anomalies in the tropical central-eastern Pacific where the dynamic effect is dominant. Moist static energy budget analysis further illustrates that vertical velocity is enhanced due to the increased transport of moist static energy from the lower troposphere into the middle-upper troposphere and the intensified warming effect of cloud longwave radiation caused by the increase of high cloud amount and altitude.

Atmospheric Diffusivity: A New Energetic Framework for Understanding the Midlatitude Circulation Response to Climate Change

AbstractThe midlatitude circulation response to climate change is not well understood. Within an energetic framework the circulation is connected to the atmospheric diffusivity. Here we define a novel diagnostic diffusivity framework as a tool for interpreting changes in the midlatitude atmospheric circulation. The diffusivity is computed by dividing the zonally and vertically integrated transient eddy moist static energy (MSE) transport by the zonal‐mean 925 hPa MSE gradient, and we investigate the diffusivity response to global‐scale sea surface temperature (SST) perturbations in aquaplanet and Atmospheric Model Intercomparison Project (AMIP) general circulation model simulations. The midlatitude diffusivity maximum shifts poleward with increasing SST in both the aquaplanet simulations and Southern Hemisphere winter of the AMIP simulations. Our framework indicates that this diffusivity shift is associated with Ferrel circulation and 925 hPa MSE gradient changes, rather than with total MSE transport changes. The MSE gradient‐related shift cannot be understood purely in terms of Clausius‐Clapeyron changes in specific humidity following the global‐mean temperature response, because atmospheric temperature gradients change significantly even when SST gradients are held fixed. The intensity of the diffusivity maximum also increases with SST for the aquaplanet configuration but is relatively insensitive to SST for AMIP Southern Hemisphere winter. Additional energy balance model experiments suggest that while diffusivity changes caused by SST perturbations can be large, their effect on zonal‐mean temperature is minor. Finally, the latitudes and intensities of the diffusivity maxima are strongly correlated with the same properties of the zonal‐mean eddy kinetic energy maxima. However, existing theories cannot adequately explain these linear relationships.

Arctic sea-ice variability is primarily driven by atmospheric temperature fluctuations

The anthropogenically forced decline of Arctic sea ice is superimposed on strong internal variability. Possible drivers for this variability include fluctuations in surface albedo, clouds and water vapour, surface winds and poleward atmospheric and oceanic energy transport, but their relative contributions have not been quantified. By isolating the impact of the individual drivers in an Earth system model, we here demonstrate that internal variability of sea ice is primarily caused directly by atmospheric temperature fluctuations. The other drivers together explain only 25% of sea-ice variability. The dominating impact of atmospheric temperature fluctuations on sea ice is consistent across observations, reanalyses and simulations from global climate models. Such atmospheric temperature fluctuations occur due to variations in moist-static energy transport or local ocean heat release to the atmosphere. The fact that atmospheric temperature fluctuations are the key driver for sea-ice variability limits prospects of interannual predictions of sea ice, and suggests that observed record lows in Arctic sea-ice area are a direct response to an unusually warm atmosphere. Atmospheric temperature fluctuations are the main influence on Arctic sea-ice variability, whereas other factors explain only 25% of variability, according to an analysis of Earth system model simulations.