The Extratropical Linear Step Response to Tropical Precipitation Anomalies and Its Use in Constraining Projected Circulation Changes under Climate Warming

Abstract

Rossby wave trains triggered by tropical convection strongly affect the atmospheric circulation in the extratropics. Using daily gridded observational and reanalysis data, we demonstrate that a technique based on linear response theory effectively captures the linear response in 250-hPa geopotential height anomalies in the Northern Hemisphere using examples of steplike changes in precipitation over selected tropical areas during boreal winter. Application of this method to six models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), using the same tropical forcing, reveals a large intermodel spread in the linear response associated with intermodel differences in Rossby waveguide structure. The technique is then applied to a projected tropicswide precipitation change in the HadGEM2-ES model during 2025–45 December–February, a period corresponding to a 2°C rise in the mean global temperature under the RCP8.5 scenario. The response is found to depend on whether the mean state underlying the technique is calculated using observations, the present-day simulation, or the future projection; indeed, the bias in extratropical response to tropical precipitation because of errors in the basic state is much larger than the projected change in extratropical circulation itself. We therefore propose the linear step response method as a semiempirical method of making near-term future projections of the extratropical circulation, which should assist in quantifying uncertainty in such projections.