The relationship between the energy flux equator and tropical rainfall on interannual timescales

Theoretical and idealized modeling studies suggest that monsoon precipitation and the marine Intertropical Convergence Zone (ITCZ) are associated with interhemispheric atmospheric energy transport, and these relationships have been used to understand future climate projections. However, the observational connection of tropical rainfall to interhemispheric energy transport has remained uncertain, due to the dominant role played by sea surface temperatures in fixing the marine ITCZ, and difficulties in constraining atmospheric energy fluxes. In this study, we attempt to characterize the relationship between tropical rainfall and interhemispheric energy transport using zonal mean GPCP precipitation data and ERA-Interim circulation and energy transport indices from 1979-2010. As a transport index, we define the energy flux equator as the latitude of zero zonal mean meridional energy transport. We show that the energy flux equator is strongly correlated with cross-equatorial meridional energy flux, and we also find that it is related to meridional energy transport at all latitudes, particularly in the northern hemisphere. Regressing monthly zonal mean precipitation anomalies onto the latitude of the energy flux equator shows a significant relationship in the tropics. Comparing the regression slopes with the precipitation climatology, we find that when the energy flux equator is farther north (south), there is more precipitation along the northern (southern) margins of the tropical rain belts. This relationship holds for both land and ocean precipitation considered separately. Ocean precipitation varies over a narrow latitude range resembling marine ITCZ shifts, whereas the land precipitation signal is spread over a much wider tropical range. We examine the correlation of the energy flux equator with the difference of tropical northern and southern rainfall (an indicator of the ITCZ latitude), and the precipitation annual range (an indicator of monsoon intensity) in both hemispheres. Additionally, we compare similar regressions for sea surface temperature; preliminary analysis points to the Pacific Meridional Mode as an important pattern linking tropical rainfall to the energy flux equator.