The Energetic Constraints on the Zonal Mean Atmospheric Circulations in the Tropics, Midlatitudes, and High Latitudes
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In this doctoral thesis, I have studied the processes that affect the atmospheric energy budget and their coupling relationships with atmospheric circulations. The equator-to-pole radiation gradient at the top of the atmosphere is the fundamental driver of atmospheric and oceanic circulations. Any anomaly in the energy budget due to variations in different climate components (such as clouds, aerosols, atmospheric properties, and land surfaces) will have an effect on the atmospheric and oceanic circulations and energy transport. Variations in the energy budget of extratropical regions have a non-local effect on tropical climate and vice versa. We first investigated climate components that affect the atmospheric energy budget and their coupled relationships with the atmospheric energy transport, using CMIP multi-model ensembles. We studied how individual components affect energy transport in three latitude bands: (1) at 70 degrees, where increasing poleward energy transport may cause polar amplification, (2) at 40 degrees, where eddies are the strongest, and (3) in the deep tropics, where global climate models (GCMs) do not agree on the changes in transport in global warming scenarios. In high latitudes, positive radiative effects from melting sea ice decrease the equator-to-pole temperature gradient and prevent poleward fluxes from increasing. Models that have more melting ice tend to predict a smaller increase in the energy transport, which is counterintuitive based on the argument that increasing poleward transport can lead to melting sea ice. The cooling effect of increasing low clouds over newly open ocean along the ice edge sharpens the temperature gradient and increases the energy transport in midlatitudes. Clouds and sea ice in the extratropics can also influence energy transport at the equator. We then shifted our focus to the tropical rain belt, built on the first part that demonstrated a directly linkage from hemispheric asymmetry of the atmospheric energy budget to cross-equatorial atmospheric energy transport. The cross-equatorial atmospheric energy transport is anti-correlated with the displacement of the tropical rain belt, since the Hadley circulation governs both energy and moisture transport within the deep tropical atmosphere. Using this energetic framework and some idealized experiments, we investigated the atmospheric energy budget in satellite observations, reanalysis, and CMIP3 and CMIP5 models, and report the following results: (1) In the latter half of the 20th century, anthropogenic sulfate emissions cause a southward tropical precipitation shift in CMIP3 and CMIP5 models. This southward shift is also observed in rain gauge data, marine cloud observations and reanalysis products. (2) In models' climatologies, biases in shortwave cloud radiative forcing over the Southern Ocean explain much of the excessive precipitation in the southern tropics and are responsible for part of the double intertropical convergence zone (ITCZ) problem in most GCMs.
- Atmospheric sciences