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  1. Abstract

    We investigate the linear trends in meridional atmospheric heat transport (AHT) since 1980 in atmospheric reanalysis datasets, coupled climate models, and atmosphere-only climate models forced with historical sea surface temperatures. Trends in AHT are decomposed into contributions from three components of circulation: (i) transient eddies, (ii) stationary eddies, and (iii) the mean meridional circulation. All reanalyses and models agree on the pattern of AHT trends in the Southern Ocean, providing confidence in the trends in this region. There are robust increases in transient-eddy AHT magnitude in the Southern Ocean in the reanalyses, which are well replicated by the atmosphere-only models, while coupled models show smaller magnitude trends. This suggests that the pattern of sea surface temperature trends contributes to the transient-eddy AHT trends in this region. In the tropics, we find large differences between mean-meridional circulation AHT trends in models and the reanalyses, which we connect to discrepancies in tropical precipitation trends. In the Northern Hemisphere, we find less evidence of large-scale trends and more uncertainty, but note several regions with mismatches between models and the reanalyses that have dynamical explanations. Throughout this work we find strong compensation between the different components of AHT, most notably in the Southern Ocean where transient-eddy AHT trends are well compensated by trends in the mean-meridional circulation AHT, resulting in relatively small total AHT trends. This highlights the importance of considering AHT changes holistically, rather than each AHT component individually.

     
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  2. ABSTRACT

    Cross-correlating 21cm and Lyα intensity maps of the Epoch of Reionization promises to be a powerful tool for exploring the properties of the first galaxies. Next-generation intensity mapping experiments such as the Hydrogen Epoch of Reionization Array (HERA) and SPHEREx will individually probe reionization through the power spectra of the 21cm and Lyα lines respectively, but will be limited by bright foregrounds and instrumental systematics. Cross-correlating these measurements could reduce systematics, potentially tightening constraints on the inferred astrophysical parameters. In this study, we present forecasts of cross-correlation taking into account the effects of exact uv-sampling and foreground filtering to estimate the feasibility of HERAxSPHEREx making a detection of the 21cm-Lyα cross-power spectrum. We also project the sensitivity of a cross-power spectrum between HERA and the proposed next-generation Cosmic Dawn Intensity Mapper. By isolating the sources of uncertainty, we explore the impacts of experimental limitations such as foreground filtering and Lyα thermal noise uncertainty have on making a detection of the cross-power spectrum. We then implement this strategy in a simulation of the cross-power spectrum and observational error to identify redshifts where fiducial 21cmFAST models predict the highest signal-to-noise detection (z ∼ 8). We conclude that detection of the SPHEREx-HERA cross-correlation will require an optimistic level of 21cm foreground filtering, as well as deeper thermal noise integrations due to a lack of overlapping sensitive modes but for CDIM with its larger range of scales and lower noise forecast detection levels, may be possible even with stricter 21cm foreground filtering.

     
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  3. Abstract

    Total poleward atmospheric heat transport (AHT) is similar in both magnitude and latitudinal structure between the Northern and Southern Hemispheres. These similarities occur despite more major mountain ranges in the Northern Hemisphere, which help create substantial stationary eddy AHT that is largely absent in the Southern Hemisphere. However, this hemispheric difference in stationary eddy AHT is compensated by hemispheric differences in other dynamic components of AHT so that total AHT is similar between hemispheres. To understand how AHT compensation occurs, we add midlatitude mountain ranges in two different general circulation models that are otherwise configured as aquaplanets. Even when midlatitude mountains are introduced, total AHT is nearly invariant. We explore the near invariance of total AHT in response to orography through dynamic, energetic, and diffusive perspectives. Dynamically, orographically induced changes to stationary eddy AHT are compensated by changes in both transient eddy and mean meridional circulation AHT. This creates an AHT system with three interconnected components that resist large changes to total AHT. Energetically, the total AHT can only change if the top-of-the-atmosphere net radiation changes at the equator-to-pole scale. Midlatitude orography does not create large-enough changes in the equator-to-pole temperature gradient to alter outgoing longwave radiation enough to substantially change total AHT. In the zonal mean, changes to absorbed shortwave radiation also often compensate for changes in outgoing longwave radiation. Diffusively, the atmosphere smooths anomalies in temperature and humidity created by the addition of midlatitude orography, such that total AHT is relatively invariant.

    Significance Statement

    The purpose of this study is to better understand how orography influences heat transport in the atmosphere. Enhancing our understanding of how atmospheric heat transport works is important, as heat transport helps moderate Earth’s surface temperatures and influences precipitation patterns. We find that the total amount of atmospheric heat transport does not change in the presence of mountains in the midlatitudes. Different pieces of the heat transport change, but they change in compensatory ways, such that the total heat transport remains roughly constant.

     
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