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A sequence of torrential rainstorms pounded Pakistan in the summer of 2022, shattering records by massive margins (7 sigma). The severe socioeconomic damages underscore the urgency of identifying its dynamic drivers and relationship with human-induced climate change. Here, we find that the downpours were primarily initiated by the synoptic low-pressure systems, whose intensity and longevity far exceeded their counterparts in history as fueled by a historically-high cross-equatorial moisture transport over the Arabian Sea. The moisture transport has been trending upward since the 1960s and, in 2022, along with the anomalous easterly moisture influx caused by the combination of La Niña and negative Indian Ocean Dipole events, created a corridor of heavy rainfall extending from central India toward southern Pakistan. While it is not yet established whether the observed trend of the cross-equatorial moisture transport has exceeded natural variability, model-based analysis confirms that it is consistent with the fingerprint of anthropogenic climate warming and will raise the likelihood of such rare events substantially in the coming decades. [ABSTRACT FROM AUTHOR]

Rainfall extremes are one of the most difficult features to simulate using global climate models. Using pairs of high‐ and low‐resolution atmosphere‐only simulations from the High‐Resolution Model Intercomparison Project, we assess whether and how the horizontal grid spacing influences model's skill in simulating the Asian summer monsoon rainfall extremes. Despite the large inter‐model spread, improved model performance is seen across most models as resolution increases, both in terms of spatial distribution and intensity of monsoon rainfall extremes. Rainfall extremes are enhanced at the windward side of steep topography such as the foothills of Himalayas, and in the heavy monsoon regions such as central India, southern China, and western North Pacific. The improvements are found to be associated with a better representation of the vertical ascents and models' ability in simulating the rain‐producing synoptic low‐pressure systems, which are more frequent and stronger at higher resolutions. Plain Language Summary: Adequate simulations of rainfall extremes are essential for climate models to be useful for assessing regional climate impacts. However, notable biases exist in climate model simulations of Asian monsoon rainfall extremes and their spatial distributions. In the current work, we find that models with finer horizontal resolutions (smaller grid spacing) correct substantially the dry biases that are common in coarse resolution model simulations. The improvement is mainly through better simulations of rainfall extremes along the windward side of the steep topography such as the foothills of Himalayas, and over heavy monsoon regions including central India, southern China, the East Asian shorelines, and western North Pacific. Further analysis indicates that the enhanced extreme rainfall in high‐resolution models is associated with intensified vertical ascents during the occurrence of rainfall events and is attributable to more frequent and stronger monsoon low‐pressure systems, which are among the most important heavy rain‐bearing meteorological systems in the Asian summer monsoon regions. Key Points: Finer resolution models reduce dry biases in modeled Asian monsoon rainfall extremesImproved simulation of monsoon rainfall extremes attributed to better representation of monsoon low‐pressure systems (LPSs)Higher resolution models tend to produce more frequent and stronger LPSs [ABSTRACT FROM AUTHOR]

The core Indian monsoon region receives more than half of the rainfall extremes from low‐pressure systems (LPSs), which typically form over the Bay of Bengal and propagate upstream against the time‐mean low‐level westerlies. Yet, the relationship between the trends of LPSs and rainfall extremes remains uncertain. Using two tracking algorithms and reanalyses‐derived LPSs, we find that LPS activity and extreme rainfall exhibit coherent trends during the post‐1979 satellite era. Over time, the LPSs propagate preferentially into south‐central India rather than north‐central India, imparting a corresponding dipole footprint in rainfall extremes. Consistent with existing theories that the diabatic heating is instrumental in shifting the LPSs west‐southwestward, the LPSs traveling through south‐central India have stronger updrafts on their west‐southwestern flank than those passing through north‐central India. The increased frequency of LPSs propagating into south‐central India is likely due to a strengthened cross‐equatorial moisture transport, which favors stronger storm ascents. Plain Language Summary: The South Asian synoptic‐scale low‐pressure systems (LPSs), which typically form over the Bay of Bengal and propagate upstream against the time‐mean low‐level westerlies, produce more than half of the summer rainfall extremes over the densely populated central India. Given the vulnerability of societies in this region to rainfall extremes, investigating the connection between LPSs and extreme rainfall regarding their long‐term trends has important implications for climate adaptation. Using two different tracking algorithms and reanalyses‐derived LPS tracks, we find that the trends of extreme rainfall and LPS activity exhibit a strong coherence during the post‐1979 satellite era. Specifically, the LPSs prefer to propagate into south‐central India than north‐central India over time, imparting a corresponding dipole footprint in rainfall extremes. In agreement with previous studies that the LPS propagation is a combined effect of the northwestward‐propagating component due to horizontal nonlinear adiabatic advection and the southwestward‐propagating component due to diabatic heating, we notice that the LPSs migrating through the south‐central India have stronger updrafts on their west‐southwestern flank compared to those passing through north‐central India. Our results indicate that the increasing number of LPSs propagating into south‐central India is likely due to a strengthened cross‐equatorial moisture transport, which provides a wetter environment and favors stronger storm ascents. Key Points: An extreme rainfall dipole with positive trends over south‐central India and negative trends over north‐central India is observed since 1979The extreme rainfall dipole aligns with the trends in the number of Indian monsoon low‐pressure systems passing through the two regionsThe changing LPS translation is likely associated with a wetter environment owing to a strengthened cross‐equatorial moisture transport [ABSTRACT FROM AUTHOR]

Wet and dry periods within the South Asian summer monsoon season can have acute societal impacts. Recent studies have identified changes in daily rainfall characteristics of the monsoon, but the underlying causes are poorly understood. In particular, although the dominant role of anthropogenic aerosols in shaping historical changes in seasonal-mean monsoon rainfall has been documented, their influence on daily-scale rainfall remains unconstrained. Using an ensemble of single-forcing climate simulations, we find that anthropogenic aerosols have a stronger influence on late-twentieth century changes in the frequency of wet events, dry events and rainless days, compared with other climate forcings. We also investigate the role of aerosol-cloud interactions ("indirect effects") in the total aerosol response, and the contribution of aerosols emitted from South Asia versus from remote sources. Based on additional simulations with the GFDL-CM3 climate model, we find that the simulated aerosol response over South Asia is largely associated with aerosol-indirect effects. In addition, local aerosols suppress wet-event frequency and enhance dry-event frequency over eastern-central India, where increases in aerosol loading are the largest. Remote aerosols cause a north–south dipole pattern of change in mean rainfall over India and fewer rainless days over western India. However, the overall spatial response of South Asian rainfall characteristics to total aerosol forcing is substantially influenced by the combined non-linear climate response to local and remote aerosols. Together, our results suggest that understanding the influence of different aerosol emissions trajectories on the regional climate dynamics is critical for effective climate-risk management in this populated, vulnerable region. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP) has long been regarded as a key driver for the formation and variations of the Indian summer monsoon (ISM). Recent studies, however, have indicated that the ISM also exerts a considerable impact on rainfall variations in the TP, suggesting that the ISM and the TP should be considered as an interactive system. From this perspective, the covariability of the July-August mean rainfall across the Indian subcontinent (IS) and the TP is investigated. It is found that the interannual variation of IS and TP rainfall exhibits a dipole pattern in which rainfall in the central and northern IS tends to be out of phase with that in the southeastern TP. This dipole pattern is associated with significant anomalies in rainfall, atmospheric circulation, and water vapor transport over the Asian continent and nearby oceans. Rainfall anomalies and the associated latent heating in the central and northern IS tend to induce changes in regional circulation that suppress rainfall in the southeastern TP and vice versa. Furthermore, the sea surface temperature anomalies in the tropical southeastern Indian Ocean can trigger the dipole rainfall pattern by suppressing convection over the central IS and the northern Bay of Bengal, which further induces anomalous anticyclonic circulation to the south of TP that favors more rainfall in the southeastern TP by transporting more water vapor to the region. The dipole pattern is also linked to the Silk Road wave train via its link to rainfall over the northwestern IS. [ABSTRACT FROM AUTHOR]