Energy Budget Theory for Interannual Shifts of the South Pacific Convergence Zone

Numerous studies have used the atmospheric energy budget to constrain the latitude of tropical rainfall maxima, but quantitative theories for rainfall shifts have focused on the zonal mean with limited relevance to regional climate. Here we present a quantitative theory for regional tropical rainfall shifts produced by an arbitrary energy source; this theory was originally used to predict the response of precipitation to the mid-Holocene orbital forcing, but here is applied to shifts in the South Pacific Convergence Zone (SPCZ) that occur during the El Niño-Southern Oscillation (ENSO). Using either resolved atmospheric energy fluxes or estimates of column-integrated moist energy sources, this framework predicts well the observed SPCZ shifts during ENSO, at least when anomalous ENSO-region SSTs are relatively small. In large-amplitude ENSO events such as the 1997-1998 El Niño, the framework breaks down because the SPCZ undergoes large changes in intensity as well as position. The atmospheric energy flux framework permits decomposition of the ENSO forcing into various components, e.g., column radiative heating vs. surface turbulent fluxes, and local vs. remote contributions. Column energy source anomalies in the equatorial central and eastern Pacific dominate the SPCZ shift. Furthermore, although the radiative flux anomaly is larger than the surface turbulent flux anomaly in the SPCZ region, the radiative flux anomaly, which can be viewed as a feedback on the ENSO forcing, accounts for slightly less than half of SPCZ precipitation anomalies during ENSO. We also introduce an idealized analytical model to illustrate atmospheric energy flux anomalies during ENSO and to obtain a scaling for the SPCZ response to an anomalous equatorial energy source.