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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 StatementThe 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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Anomalous Northward Energy Transport due to Anthropogenic Aerosols during the Twentieth Century
Abstract In the tropics, the absorbed solar radiation is larger than the outgoing longwave radiation, while the opposite is true at high latitudes. This basic fact implies a poleward energy transport (PET) in both hemispheres, which is accomplished by the atmosphere and oceans. The magnitude of PET is determined by the top of atmosphere gradient in the net radiation flux, and small changes to this quantity must change the total PET in the absence of changes in heat uptake. We analyze a large ensemble of 50 historic climate simulations from the CESM LENS2 project and find a significant PET anomaly in the latter half of the twentieth century. The temporal evolution of this anomaly—with a rapid increase after 1950, a peak near 1975, and a rapid decrease in the 1990s—mirrors the historic trend of sulfur dioxide (SO2, a significant aerosol predecessor) emissions from Europe and North America. This anomaly also appears in an analysis of the PET calculated from ERA5 reanalyses and from the CESM2 Single Forcing Large Ensemble. Consistent with previous studies, we find that historic SO2emissions from Europe and North America brightened clouds, which reflected additional solar radiation back to space in the midlatitudes: this shortwave anomaly increased the meridional gradient in the net TOA radiation flux and induced an anomalous northward energy transport. Finally, our results suggest that cryosphere processes become an additional important factor in setting the PET anomaly during the first years of the twenty-first century by contributing to the difference in absorbed solar radiation between hemispheres alongside cloud radiative effects. significance statementIn this study, we analyze a large group of climate model simulations from 1850 to 2014 and find that this historical pollution changed the way that heat was transported from the tropics to Earth’s poles. We also find this change in heat transport when we analyzed an atmospheric reanalysis, which is a historical dataset that combines many meteorological observations into a best estimate of the past climate state. This extra reflection of sunlight from polluted clouds cooled the Northern Hemisphere, and we hypothesize that this cooling caused more heat transport out of the tropics. Last, we find that similar pollution emitted from China and India in more recent decades has not led to a change in Earth’s heat transport because of counteracting changes in snow and ice in the Northern Hemisphere.
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- Award ID(s):
- 2005137
- PAR ID:
- 10457430
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 36
- Issue:
- 19
- ISSN:
- 0894-8755
- Format(s):
- Medium: X Size: p. 6713-6728
- Size(s):
- p. 6713-6728
- Sponsoring Org:
- National Science Foundation
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