Regional Tropical Rainfall Shifts Under Global Warming: An Energetic Perspective
In order to better understand the mechanisms of future tropical rainfall shifts and their association with remote forcings, we utilize a recently developed framework first presented in Boos and Korty (2016, hereafter BK) that generalizes the atmospheric energy flux approach to two dimensions (see also Adam et al 2016). One of the main benefits of this method is the ability to associate changes in atmospheric energy transport with rainfall shifts, which then allows us to attribute the rainfall changes to remote regional energy sources. Additionally, the linearity in the BK method allows us to decompose the influence of anomalous atmospheric energy transports into regional contributions, and contributions of individual flux components (latent, sensible, and radiative). Moreover, the two-dimensional nature of the energetic perspective motivates the examination of the next-century change in the two-dimensional distribution of atmospheric energy fluxes. To this end, we identify two main centers of action in climate change scenarios: the North Atlantic (AMOC) and the eastern equatorial Pacific (permanent El Niño). We apply this method to diagnose annual mean future tropical rainfall shifts in the CESM2-LE. Since precipitation changes in the tropical Pacific feature both amplitude changes as well as spatial shifts, we focus on and apply the theory to the greater tropical Atlantic sector, where simple spatial shifts in rainfall dominate and thus, the theory is more relevant. As this is the first application of the BK method that we know of to future rainfall, we also discuss its strengths and weaknesses.
Two main sources of future atmospheric energy flux changes, and hence rainfall shifts, are identified by the analysis: the high-latitude North Atlantic due to a weakened Atlantic Meridional Overturning Circulation that shifts tropical rainfall southwards over the greater Tropical Atlantic sector and eastern Pacific; and the eastern tropical Pacific due to a permanent El-Niño-like response that produces zonal shifts over the Maritime Continent and South America. To first order, the shifts in the EFE and EFPM mirror gross distributional changes in tropical precipitation, with a southward shift in rainfall over the tropical Atlantic, West Africa, and eastern tropical Pacific and an eastward shift over the Maritime Continent and western Pacific. When used to reconstruct future rainfall shifts in the tropical Atlantic and Sahel, the method reasonably represents the simulated meridional structure of rainfall shifts but does not do so for the zonal structures.
We apply the method of Boos and Korty (2016) to the CESM2 large ensemble SSP370 simulations to evaluate the role of atmospheric energy transports in future tropical rainfall shifts. For the late 21st century, we find significant southward EFE shifts over the tropical Atlantic sector and the eastern equatorial Pacific. In general, we find that shifts in the EFE and EFPM align with changes to the tropical precipitation, specifically with the change to the precipitation centroid. However, how the detailed precipitation change expresses itself as a result of the EFE/EPFM shift differs from region to region. Over the tropical Atlantic and Sahel region, the peak rainfall shifts southwards, indicating a horizontal shift of those rainbands. Over the eastern equatorial Pacific, the rainfall change is expressed in the formation of a 'double-ITCZ' structure where rainfall increases in the southern ITCZ and decreases in the northern ITCZ. Rainfall changes in the western Pacific and Maritime continent are expressed as a distributional change with an increase over the western Pacific and a decrease over the Maritime Continent, but the location of the peak rainfall remains the same.