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Trends 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.

Applying composite analysis to ERA-Interim data, the surface air temperature (SAT) anomaly pattern of the Pacific–North American (PNA) teleconnection is shown to include both symmetric and asymmetric SAT anomalies with respect to the PNA phase. The symmetric SAT anomalies, overlying the Russian Far East and western and eastern North America, grow through advection of the climatological temperature by the anomalous meridional wind and vertical mixing. The asymmetric SAT anomalies, overlying Siberia during the positive PNA and the subtropical North Pacific during the negative PNA, grow through vertical mixing only. For all SAT anomalies, vertical mixing relocates the temperature anomalies of the PNA teleconnection pattern from higher in the boundary layer downward to the level of the SAT. Above the level of the SAT, temperature anomaly growth is caused by horizontal temperature advection in all locations except for the subtropical North Pacific, where adiabatic cooling dominates. SAT anomaly decay is caused by longwave radiative heating/cooling, except over Siberia, where SAT anomaly decay is caused by vertical mixing. Additionally, temperature anomaly decay higher in the boundary layer due to nonlocal mixing contributes indirectly to SAT anomaly decay by weakening downgradient diffusion. These results highlight a diverse array of mechanisms by whichmore »individual anomalies within the PNA pattern grow and decay. Furthermore, with the exception of Siberia, throughout the growth and decay stages, horizontal temperature advection and/or vertical mixing is nearly balanced by longwave radiative heating/cooling, with the former being slightly stronger during the growth stage and the latter during the decay stage.

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Radiation changes at the Earth's surface alter climate, however, the causes of observed surface radiation changes are not precisely quantified globally. With complete global coverage by ERA‐Interim, the drivers of the clear sky surface downwelling longwave irradiance (SDLI) trends from 1984 to 2017 are quantifiable everywhere. Trends in atmospheric temperature and water vapor contributed significantly (∼90%) to clear sky SDLI trends, including trends consistent with Arctic warming and Southern Ocean cooling. CO₂contributed ∼10% and other greenhouse gases (CH₄, N₂O, CFC‐11, and CFC‐12) ∼1% to the SDLI trends. These observation‐based results are consistent with early CO₂‐doubling climate model calculations wherein temperature and water vapor changes drove ∼90% of the SDLI change. The well‐mixed greenhouse gases drive location‐dependent SDLI trends that are strongest over regions with climatologically high temperatures and low water vapor amounts.

According to baroclinic adjustment theory, the isentropic slope maintains its marginal state for baroclinic instability. However, the recent trend of Arctic warming raises the possibility that there could have been a systematic change in the extratropical isentropic slope. In this study, global reanalysis data are used to investigate this possibility. The result shows that tropospheric isentropes north of 50°N have been flattening significantly during winter for the recent 25 years. This trend pattern fluctuates at intraseasonal time scales. An examination of the temporal evolution indicates that it is the planetary-scale (zonal wavenumbers-1–3) eddy heat fluxes, not the synoptic-scale eddy heat fluxes, that flatten the isentropes; synoptic-scale eddy heat fluxes instead respond to the subsequent changes in isentropic slope. This extratropical planetary-scale wave growth is preceded by an enhanced zonal asymmetry of tropical heating and poleward wave activity vectors. A numerical model is used to test if the observed latent heating can generate the observed isentropic slope anomalies. The result shows that the tropical heating indeed contributes to the isentropic slope trend. The agreement between the model solution and the observation improves substantially if extratropical latent heating is also included in the forcing. The model temperature response shows a patternmore »resembling the warm-Arctic–cold-continent pattern. From these results, it is concluded that the recent flattening trend of isentropic slope north of 50°N is mostly caused by planetary-scale eddy activities generated from latent heating, and that this change is accompanied by a warm-Arctic–cold-continent pattern that permeates the entire troposphere.

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Atmospheric stationary waves play an important role in regional climate. In phase 5 of the Coupled Model Intercomparison Project (CMIP5), a prior study found that there are systematic biases in Arctic moisture intrusions caused by stationary eddy meridional wind biases. In this study, using initial‐value model calculations, it is shown that CMIP5 latent heating biases in the tropics and midlatitudes play a substantial role in generating the systematic meridional wind bias poleward of 50°N. It is further shown that the midlatitude heating biases are in part driven by the circulation caused by the tropical and subtropical heating biases. These results indicate that the systematic stationary meridional wind biases poleward of 50°N can be traced to systematic model biases in tropical and extratropical latent heating. Therefore, reliable regional climate projections likely hinge on accurate representations of moist processes upstream of the region of interest and in the tropics.

The midwinter minimum in North Pacific storm‐track intensity is a perplexing phenomenon because the associatedlocalbaroclinity in the North Pacific is maximum during midwinter. Here, a new mechanism is proposed wherein the midwinter minimum occurs in part because global planetary‐scale waves consume the zonal available potential energy, limiting its availability for storm‐track eddy growth. During strong midwinter suppression years, the midwinter minimum is preceded by anomalously large planetary‐scale eddy kinetic energy and subsequent reduction in zonal available potential energy andglobalbaroclinity. Consistent with previous studies, this large planetary‐scale eddy kinetic energy takes place after enhanced Pacific warm pool convection, which peaks during winter. These results indicate that the midwinter minimum is in part caused by heightened warm pool convection, which, through excitation of planetary‐scale waves, leads to a weaker storm‐track. This finding also helps explain the existence of the midwinter North Atlantic storm‐track minimum.

The summer of 2010 was characterized by weather and climate extremes such as the western Russia heatwave and the Pakistan floods. A recent study found that summer was dominated by a particular 200 hPa geopotential height pattern featuring an anomalous Rossby wave train with ridges centred over Greenland, Europe and Russia. The daily frequency of this pattern has dramatically increased recently and closely resembles the mean‐state difference in 200 hPa geopotential height fields between 1998–2014 (P2) and 1979–1997 (P1). Because anomalous wave trains are often driven by localized diabatic heating, it is tested in this study whether the P2 minus P1 pattern is caused by diabatic heating anomalies near Greenland. While it is found that sea ice concentrations declined and sea‐surface temperatures rose over Baffin Bay to the west of Greenland during P2, surface latent heat fluxes actually increased downward, indicating that surface processes were likely not the source of diabatic heating. Rather, an increase in vertically integrated horizontal latent‐heat flux convergence over Baffin Bay was observed in P2, which led to the condensation of water vapour and latent heating. Thus, the mid‐tropospheric circulation established the diabatic heating. A set of initial‐value calculations with idealized heating over Baffin Baymore »show solutions that remarkably resemble the P2 minus P1 pattern and provide a plausible explanation as to why the pattern has been occurring more frequently. This study demonstrates that changes in the Arctic can arise from moisture transport from the midlatitudes, and, in turn, these changes can induce weather and climate extremes in distant midlatitude regions.

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Arctic moisture intrusions have played an important role in warming the Arctic over the past few decades. A prior study found that Coupled Model Intercomparison Project Phase 5 (CMIP5) models exhibit large regional biases in the moisture flux across 70°N. It is shown here that the systematic misrepresentation of the moisture flux is related to the models' overprediction of zonal wavenumberk = 2 contribution and underprediction ofk = 1 contribution to the flux. Models with a warmer tropical upper troposphere and El‐Niño‐like tropical surface temperature tend to simulate strongerk = 2 flux, whilek = 1 flux is uncorrelated with tropical upper tropospheric temperature and is associated with La‐Niña‐like surface temperature. The models also overpredict the transient eddy moisture flux while underpredicting the stationary eddy flux. Moreover, future projections in Representative Concentration Pathway 8.5 (RCP8.5) simulations show trends in moisture flux that is consistent with biases in historical simulations, suggesting that these CMIP5 projections reflect the same error(s) that cause the model biases.

Abstract Future projections of the poleward eddy heat flux by the atmosphere are often regarded as being uncertain because of the competing effect between surface and upper-tropospheric meridional temperature gradients. Previous idealized modeling studies showed that eddy heat flux response is more sensitive to the variability of lower-tropospheric temperature gradient. However, observational evidence is lacking. In this study, observational data analyses are performed to examine the relationships between eddy heat fluxes and temperature gradients during boreal winter by constructing daily indices. On the intraseasonal time scale, the surface temperature gradient is found to be more effective at regulating the synoptic-scale eddy heat flux (SF) than is the upper-tropospheric temperature gradient. Enhancements in surface temperature gradient, however, are subject to an inactive planetary-scale eddy heat flux (PF). The PF in turn is dependent on the zonal gradient in tropical convective heating. Consistent with these interactions, over the past 40 winters, the zonal gradient in tropical heating and PF have been trending upward, while the surface temperature gradient and SF have been trending downward. These results indicate that for a better understanding of eddy heat fluxes, attention should be given to zonal convective heating gradients in the tropics as much as tomore »meridional temperature gradients.« less