The Zonally Resolved Atmospheric Energy Budget: A Predictive Theory For Orbitally Forced Regional Rainfall Changes

The vertically integrated atmospheric energy budget has become a popular means of understanding changes in tropical rainfall. Although Neelin and Held (1987) first proposed this energy budget as a framework for understanding the general distribution of tropical rainfall, recent work has focused on using the zonal mean energy budget to understand why the zonal mean tropical precipitation maximum shifts meridionally in response to an imposed high-latitude atmospheric energy source. That recent work has even led to the development of quantitative scalings that relate anomalous meridional energy transports to meridional shifts in the tropical rainfall maximum. The zonal mean has remained a strong focus, to the degree that paleoclimate studies unsuccessfully struggle to interpret regional shifts in tropical rainfall using those zonal mean scalings.

Here we go beyond the zonal mean to show that the divergent component of vertically integrated atmospheric energy fluxes provides a means of understanding both zonal and meridional shifts in tropical rainfall. We first use an idealized model to show that the divergent component of the energy fluxes has a good correspondence with the divergent component of the time-mean mass fluxes, and thus with time-mean tropical rainfall. Then we use modern reanalysis data to show that variations in the zonal component of the divergent energy fluxes are just as important as variations in the meridional component, with the former even dominating the most prominent mode of tropical interannual variability: the El Niño-Southern Oscillation. The zonally resolved energy budget helps in understanding the northward shift of rainfall over Africa during the mid-Holocene (about 6,000 years ago) that is driven by orbitally induced insolation changes. We formalize this by developing a theory that predicts the rainfall change given only the orbitally induced insolation anomaly, the basic state top-of-atmosphere albedo, and the land-sea distribution; this quantitative theory describes the African and Eurasian rainfall changes produced in numerical simulations of mid-Holocene climate. It shows that the large-scale shift in rainfall from oceanic regions toward the center of the African-Eurasian land mass during the mid-Holocene can be understood as a consequence of the summer insolation changes being rapidly communicated to the atmosphere over land, where surface thermal inertia is small, while they are buffered by ocean heat storage.