Climate warming contributes to the record-shattering 2022 Pakistan rainfall

Pakistan, situated at the South Asian summer monsoon's western edge, experiences varying rainfall patterns. While the northern mountains receive substantial rainfall due to orographic lift, southern coastal areas and the Indus River plains remain arid. The summer of 2022, however, witnessed an unprecedented sequence of record-breaking deluges, surpassing the 2010 flood and all prior instrumental records by massive margins. This catastrophic event resulted in significant socioeconomic consequences, including nearly 2000 fatalities, over 2.1 million displaced residents, over 75,000 km² of inundated land, and at least $30 billion in economic losses. The sheer scale of this disaster necessitates a comprehensive investigation into its underlying physical mechanisms and causative factors. Previous research has explored the causes of historical Pakistan floods, generally attributing them to South Asian monsoon low-pressure systems (LPSs) triggering deep convection and heavy rainfall. The upper-level divergence from extratropical troughs aloft northern Pakistan has also been implicated. Planetary-scale influences, including sea surface temperature (SST) anomalies linked to El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), have also been identified as contributing factors. However, the exceptional intensity of the 2022 event, far exceeding previously studied scenarios, necessitates a more detailed investigation into its multiscale triggers and the potential role of anthropogenic climate change.

Existing literature extensively examines the causes of historical floods in Pakistan. Synoptically, South Asian monsoon low-pressure systems (LPSs) are frequently identified as triggers for intense convection and rainfall. Many LPSs originate near the Bay of Bengal and move inland, significantly impacting South Asia's rainfall distribution. Studies also suggest that upper-level divergence from extratropical troughs over northern Pakistan contributed to extreme rainfall events in 1988, 2010, and 2013. On a planetary scale, Pakistan's rainfall and its synoptic triggers are influenced by large-scale South Asian monsoon circulation, regulated by SST anomalies in the tropical Pacific and Indian Oceans (ENSO and IOD). While prior research highlights these factors, the unprecedented scale of the 2022 event challenges these existing models. Although tropical SST anomalies (La Niña and IOD) and upstream blocking over northeastern Europe have been suggested as contributors, these weren't unique to 2022. Studies have indicated climate warming's role in enhancing Pakistan rainfall intensity, but the mechanisms and longer-term impacts remain largely unexplored. This study aims to address these gaps by investigating the multiscale drivers of the 2022 event and determining the relative contributions of natural variability and anthropogenic climate change.

This study utilizes multiple datasets to analyze the 2022 Pakistan floods. Gridded daily rainfall data from the Climate Prediction Center (CPC) Unified Gauge-Based Analysis, Climate Prediction Center Merged Analysis of Precipitation (CMAP), Global Precipitation Climatology Centre (GPCC), and Climate Hazards Center InfraRed Precipitation with Station data (CHIRPS) are employed. Reanalysis data from the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) provides six-hourly and monthly data at 0.5° x 0.625° resolution. MERRA-2 was selected due to its high consistency with gauge-based observations compared to other reanalysis products. Monthly sea surface temperature (SST) data comes from the NOAA Extended Reconstruction of Historical Sea Surface Temperature version 5 (ERSST v5). For model data, the study uses data from the Coupled Model Intercomparison Project phase 6 (CMIP6), including historical all-forcing and single-forcing experiments, as well as future projections under shared socioeconomic pathways (SSPs). The objective tracking algorithm, built upon TempestExtreme, utilizes MERRA-2 reanalysis data to track South Asian monsoon low-pressure systems (LPSs). The algorithm mimics manual tracking procedures, using 850 hPa relative vorticity and sea level pressure fields. To account for intra-seasonal and interannual variability, a 21-day high-pass filter is applied. The algorithm identifies LPS candidates with local vorticity maxima exceeding 4 × 10⁻⁵ s⁻¹ and applies criteria ensuring closed circulation centers. Nearby candidates are stitched together to form trajectories, and those inconsistent with transiting features or lasting less than 2 days are eliminated. LPS activities (genesis, track density, and translation velocity) are calculated. Daily rainfall is considered LPS-induced if an LPS is within 500 km of a grid point within a ±1-day time window. A moisture budget analysis, based on the column-integrated moisture budget equation, is conducted to analyze the net moisture inflow into Pakistan, considering zonal and meridional components. The analysis also considers the combined effects of ENSO and IOD, examining the moisture transport anomalies during their negative and positive phases.

The 2022 Pakistan rainfall was unprecedented, exceeding the 42-year average by 283% and 7 times its interannual standard deviation. The heaviest rainfall occurred in the typically drier southern plains, unlike the 2010 flood. Six intense rainstorms cumulatively caused the flooding. These storms were triggered by unusually strong and long-lasting LPSs which originated near the Bay of Bengal and propagated farther westward than historical counterparts, residing over Pakistan for extended periods. The analysis reveals a strong link between cross-equatorial moisture transport from the South Indian Ocean and the anomalous LPS activity. Daily time series of moisture transport demonstrate the importance of this moisture influx in sustaining the LPSs and triggering extreme rainfall. On seasonal timescales, the increased cross-equatorial moisture transport remains a significant factor, exhibiting a positive correlation with the number of days LPSs influenced Pakistan. While La Niña and a negative IOD contributed to the rainfall, their impact was less significant than the unusually high cross-equatorial moisture transport. Analysis of MERRA-2 data indicates a significant long-term upward trend in cross-equatorial moisture transport since the 1960s. CMIP6 models confirm a moderate, spatially homogeneous increase in rainfall consistent with the observed enhancement in cross-equatorial moisture transport. The observed wetting trend is consistent with the effects of increased greenhouse gases. Models predict a substantial increase in the occurrence of extreme rainfall events in the coming decades, even under moderate emission scenarios (SSP2-4.5). While the frequency and intensity of La Niña and negative IOD events are projected to remain relatively unchanged, their combined effect on Pakistan rainfall will be amplified by climate change. The study attributes the increase in extreme rainfall primarily to the anthropogenically-driven enhancement in cross-equatorial moisture transport, allowing LPSs to propagate further into Pakistan before dissipation.

This multiscale analysis of the 2022 Pakistan floods reveals the combined impact of internal variability (La Niña, negative IOD) and anthropogenic climate change. While internal variability played a role in reducing eastward moisture outflow, the unprecedented cross-equatorial moisture transport, driven by anthropogenic warming, provided the necessary conditions for intense and prolonged LPS activity. The observed long-term upward trend in this moisture transport, supported by both reanalysis and climate models, highlights the significant influence of climate change. The study's findings strongly suggest that the likelihood of future extreme rainfall events in Pakistan will increase substantially, highlighting the urgency of climate change mitigation efforts. The results emphasize the importance of considering both internal climate variability and externally forced changes when assessing the risk of future extreme weather events.

The 2022 Pakistan floods resulted from a complex interplay between natural variability and anthropogenic climate change. The study demonstrates the crucial role of enhanced cross-equatorial moisture transport in intensifying and prolonging LPS activity, leading to record-breaking rainfall. Future research should focus on improving the representation of LPSs in climate models to reduce uncertainties in projections of extreme rainfall events and further explore the regional mechanisms contributing to enhanced moisture transport under a warming climate.

The study's reliance on reanalysis and model data introduces uncertainties. The models' ability to accurately simulate LPSs, particularly at finer scales, influences the precision of rainfall projections. While the study identifies a strong link between anthropogenic warming and increased moisture transport, disentangling the relative contributions of natural variability and forced changes remains challenging. Further research is needed to fully quantify the individual contributions of different factors to extreme rainfall events.