Possible Mechanisms of a Southward Shift in Tropical Precipitation During the Little Ice Age

Abstract:

A number of tropical hydroclimate reconstructions provide evidence for substantial changes in tropical rainfall patterns over the last millennium. One of the hypothesized features of the climate during the Little Ice Age (LIA; ca. 1300-1800 CE) is a more southerly position of the Intertropical Convergence Zone (ITCZ). We evaluate the evidence for, and mechanisms of, a southward shift of tropical precipitation during the LIA, utilizing the last millennium simulations in the Coupled Model Intercomparison Project Phase 5/Paleoclimate Intercomparison Project Phase 3 archive. Six out of the seven model simulations analyzed demonstrate a southward shift in tropical precipitation during the LIA in (as determined by a decrease in tropical precipitation asymmetry between the Northern Hemisphere and Southern Hemisphere). While a southward shift of tropical precipitation during the LIA appears to be a robust feature across model simulations, the change is small and is manifested in the different model simulations in largely disparate ways. However, some common features emerge. We compare the simulated precipitation changes to proxy records and discuss to what extent the precipitation changes appear to be driven by thermodynamic scaling principles (i.e. a wet-get-drier, dry-get-wetter scenario associated with global cooling) and to what extent they appear to be tied to circulation changes in the atmosphere (e.g. a southward shift of the Intertropical Convergence Zone).