Increasing global precipitation whiplash due to anthropogenic greenhouse gas emissions

Abrupt shifts in precipitation patterns, termed "precipitation whiplash," severely impact ecosystems and human societies. Whiplash events, defined as rapid transitions between wet and dry extremes with minimal intermediate periods, cause cascading effects. Wet extremes followed by dry extremes can exacerbate wildfires, while dry-to-wet transitions, while beneficial for water resources, can lead to flooding, landslides, and erosion due to soil crusting. These rapid shifts leave little time for adaptation, worsening the consequences of droughts and floods. Climate change projections indicate increased precipitation variability, raising concerns about heightened vulnerability. Previous research has analyzed regional wet-dry transitions at seasonal or annual scales; however, sub-seasonal global-scale analysis of precipitation whiplash remains limited. This study addresses this gap by developing a metric for quantifying sub-seasonal precipitation whiplash globally, using multiple datasets to investigate spatiotemporal changes and disentangle the effects of different anthropogenic forcings.

Existing literature explores regional precipitation variability and transitions, primarily at seasonal or annual timescales. Studies have employed standardized precipitation indices to characterize transitions and analyzed annual anomalous precipitation dipole events (e.g., drought followed by flood). While some research focuses on sub-seasonal shifts in atmospheric circulation for regional climate extremes, a comprehensive, global-scale examination of sub-seasonal precipitation whiplash characteristics and changes is lacking. This research builds upon these studies by providing a global perspective and focusing on the rapid, sub-seasonal timescale of these events.

This study uses a combination of observed and simulated data to analyze precipitation whiplash. Observed data includes daily precipitation from ERA5, MERRA2, JRA-55, CHIRPS, GPCC, and REGEN. Simulated data comes from the CESM Large Ensemble Community Project (CESM-LENS) – 40 ensemble members forced with historical and RCP8.5 scenarios – and the Coupled Model Intercomparison Project Phase 6 (CMIP6) – 55 realizations from 22 climate models. The CESM 'all-but-one' experiments (CESM-XLENS) are used to isolate the impact of individual forcings (GHGs, aerosols, biomass burning). A new metric is developed to quantify sub-seasonal precipitation whiplash, considering occurrence frequency, transition duration, and intensity. Data is detrended to remove the long-term effects of climate change and the annual cycle is removed to account for seasonality. Wet and dry extremes are identified based on exceedances or deficits of 30-day cumulative precipitation relative to the 90th and 10th percentiles of the 1979-2019 baseline. Whiplash is defined as a rapid transition between these extremes. Signal-to-noise ratios are calculated to determine the emergence of anthropogenic signals. The risk ratio is calculated to estimate the contribution of various forcings to the changes observed in precipitation whiplash. Finally, large-scale atmospheric circulation patterns are analyzed to understand the mechanisms behind the whiplash events using composite maps of 500 hPa geopotential height anomalies and vertically integrated water vapor transport.

Both observed and simulated data consistently demonstrate an increasing global frequency of precipitation whiplash since the late 1990s. By the end of the 21st century under the RCP8.5 scenario, the global frequency of precipitation whiplash events is projected to increase by 156% (2.56 times) compared to the 1979-2019 baseline, with land areas experiencing an even greater increase of 243% (3.43 times). This increase is accompanied by shorter transition durations (-10%) and increased intensity (13%), indicating more rapid and intense shifts between wet and dry extremes. The polar and monsoon regions show the most dramatic increases. Changes in precipitation whiplash frequency show a far greater percentage change compared to changes in total precipitation. Analysis of individual forcings using CESM-XLENS reveals that anthropogenic GHGs are the dominant driver of these changes, contributing to a 55 ± 4% increase in global whiplash risk by 2079. Aerosols exhibit an opposing effect, but their impact diminishes in the latter half of the 21st century as GHG-driven warming increasingly dominates. Large-scale atmospheric circulation analyses using the northeastern China region (NEC) as a case study reveal that shifts in high and low-pressure systems, linked to variations in the western Pacific subtropical high and East Asian summer monsoon, are instrumental in the occurrence and intensification of precipitation whiplash events. These circulation patterns appear to amplify in the future scenario, potentially explaining the increasing intensity and wider impacts.

The findings strongly support the hypothesis that anthropogenic GHG emissions are the primary driver of the observed and projected increases in global precipitation whiplash. The substantial increase in whiplash frequency, coupled with shorter transition durations and greater intensity, highlights the significant and potentially devastating consequences of climate change. The disproportionate impact on polar and monsoon regions underscores the vulnerability of these regions to rapid changes in precipitation regimes. The strong correlation between changes in whiplash and total precipitation, with whiplash changes exceeding those of total precipitation in many regions, suggests a more volatile hydrological cycle under a warmer climate. The detailed analysis of large-scale atmospheric circulation provides a mechanistic explanation for these changes, linking them to shifts in major circulation patterns. This work underscores the critical need for adaptation strategies to mitigate the impacts of more frequent and severe precipitation whiplash events.

This study demonstrates a substantial increase in global precipitation whiplash frequency driven predominantly by anthropogenic GHG emissions. The projected changes in whiplash characteristics – higher frequency, shorter durations, and increased intensity – pose significant challenges for water resource management, disaster preparedness, and ecosystem health. Future research should focus on improved regional-scale mechanistic studies of the interactions between atmospheric circulation and precipitation patterns, further refining projections of precipitation whiplash under different climate scenarios and developing more effective adaptation strategies. Investigating the impacts of short-lived, intense precipitation events within the whiplash framework is also warranted.

While this study utilizes multiple datasets and advanced modeling techniques, some limitations exist. The method for identifying whiplash events, using a 30-day sliding window, may smooth out short-duration, high-intensity events, such as intense rainstorms immediately following drought. Future research could explore the use of alternative methodologies to capture such events more effectively. The reliance on specific climate models and emission scenarios (RCP8.5) introduces some uncertainty, and the results should be interpreted in light of the inherent uncertainties in climate projections. While the study provides a global-scale analysis, the mechanistic investigations of atmospheric circulation are limited to a case study in northeastern China, necessitating further regional-scale analyses.