Abhik Santra: Global rainfall impact of projected tropical Pacific warming pattern uncertainty
Future changes in tropical rainfall remain one of the largest sources of uncertainty in climate projections. A major contributor to this uncertainty is the spatial pattern of tropical Pacific sea surface temperature (SST) warming, yet its influence on global rainfall responses remains poorly quantified. We isolate the impact of Pacific warming-pattern uncertainty using the atmosphere-only ACCESS ESM1.5 simulations forced by present-day SSTs and a suite of plausible future warming patterns. These patterns are derived from leading modes of uncertainty in CMIP6 projections of tropical Pacific warming and span a range from the commonly projected El Niño-like warming state to the rarely projected La Niña-like state.
We find that rainfall responses are highly sensitive to changes in the tropical Pacific zonal SST gradient, with rainfall changes across many regions scaling about linearly with the imposed zonal gradient. The rainfall response is dominated by circulation-driven moisture convergence changes, whereas thermodynamic contributions are comparatively weak. Changes in rainfall are closely linked to the modulation of the Walker circulation, with El Niño-like warming producing an eastward shift of tropical Pacific convection and enhanced drying over parts of the Indo-Pacific warm pool. In contrast, the less commonly projected La Niña-like warming strengthens rainfall over the Indo-Pacific warm pool and surrounding regions.
Our results demonstrate that future rainfall projections depend not only on the magnitude of global warming but also on the spatial pattern of tropical Pacific warming. The tropical Pacific, therefore, represents a key source of uncertainty in future hydroclimate change and a critical target for improving regional rainfall projection.