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Abstract The Pacific Walker circulation (PWC) has an outsized influence on weather and climate worldwide. Yet the PWC response to external forcings is unclear^1,2, with empirical data and model simulations often disagreeing on the magnitude and sign of these responses³. Most climate models predict that the PWC will ultimately weaken in response to global warming⁴. However, the PWC strengthened from 1992 to 2011, suggesting a significant role for anthropogenic and/or volcanic aerosol forcing⁵, or internal variability. Here we use a new annually resolved, multi-method, palaeoproxy-derived PWC reconstruction ensemble (1200–2000) to show that the 1992–2011 PWC strengthening is anomalous but not unprecedented in the context of the past 800 years. The 1992–2011 PWC strengthening was unlikely to have been a consequence of volcanic forcing and may therefore have resulted from anthropogenic aerosol forcing or natural variability. We find no significant industrial-era (1850–2000) PWC trend, contrasting the PWC weakening simulated by most climate models³. However, an industrial-era shift to lower-frequency variability suggests a subtle anthropogenic influence. The reconstruction also suggests that volcanic eruptions trigger El Niño-like PWC weakening, similar to the response simulated by climate models. 

Abstract In August 2022, Death Valley, the driest place in North America, experienced record flooding from summertime rainfall associated with the North American monsoon (NAM). Given the socioeconomic cost of these type of events, there is a dire need to understand their drivers and future statistics. Existing theory predicts that increases in the intensity of precipitation is a robust response to anthropogenic warming. Paleoclimatic evidence suggests that northeast Pacific (NEP) sea surface temperature (SST) variability could further intensify summertime NAM rainfall over the desert southwest. Drawing on this paleoclimatic evidence, we use historical observations and reanalyzes to test the hypothesis that warm SSTs on the southern California margin are linked to more frequent extreme precipitation events in the NAM domain. We find that summers with above-average coastal SSTs are more favorable to moist convection in the northern edge of the NAM domain (southern California, Arizona, New Mexico, and the southern Great Basin). This is because warmer SSTs drive circulation changes that increase moisture flux into the desert southwest, driving more frequent precipitation extremes and increases in seasonal rainfall totals. These results, which are robust across observational products, establish a linkage between marine and terrestrial extremes, since summers with anomalously warm SSTs on the California margin have been linked to seasonal or multi-year NEP marine heatwaves. However, current generation earth system models (ESMs) struggle to reproduce the observed relationship between coastal SSTs and NAM precipitation. Across models, there is a strong negative relationship between the magnitude of an ESM’s warm SST bias on the California margin and its skill at reproducing the correlation with desert southwest rainfall. Given persistent NEP SST biases in ESMs, our results suggest that efforts to improve representation of climatological SSTs are crucial for accurately predicting future changes in hydroclimate extremes in the desert southwest. 

Abstract Tropical cyclone (TC) impacts along the western Atlantic and Caribbean margin are not spatially uniform. Proxy based reconstructions of Common Era TC activity highlight this non‐uniform distribution at centennial‐millennial timescales. However, the sparse geographic scope of these reconstructions impedes our assessment of TC landfalls across broader spatial domains. This work presents a compilation of new and existing TC reconstructions from the Yucatan Peninsula for comparison with a contemporaneous compilation from New England, showing that these regions occupy distal nodes of a low‐frequency TC dipole. Increased Yucatan (New England) storminess is closely linked to intervals of Northern Hemisphere warming (cooling) and the expansion (contraction) of the Intertropical Convergence Zone, suggesting that secular shifts in the mean climate state mediate dipole orientation. 

Sinkholes develop on carbonate landscapes when caves collapse and can subsequently become lake-like environments if they are flooded by local groundwater. Sediment cores retrieved from sinkholes have yielded high-resolution reconstructions of past environmental change, hydroclimate, and hurricane activity. However, our understanding of the internal sedimentary processes of these systems remains incomplete. Here, we use a multiproxy approach including sedimentology (stratigraphy, coarse-grained particle density, bulk organic matter content), micropaleontology (ostracods), and geochemistry (δ13C and δ2H on n-alkanoic acids) to reconstruct evidence for paleolimnology and regional hydroclimate from a continuous stratigraphic record (Emerald Pond sinkhole) in the northern Bahamas that spans the middle to late Holocene. Basal peat at 8.9 m below modern sea level documents the maximum sea-level position at ~ 8200 cal. yr BP. Subsequent upward vertical migration of the local aquifer caused by regional sea-level rise promoted carbonate-marl deposition from ~ 8300 to 1700 cal. yr BP. A shift in coarse particle deposition and ostracods at 5500 cal. yr BP suggests some environmental change, which may be related to one or multiple internal or external drivers. Sapropel deposition from ~ 1700 to 1300 cal. yr BP indicates a fundamental change in limnology to promote increased organic matter preservation, perhaps related to the regional cooling during the Dark Ages Cold Period. We find δ2H28 values are largely invariant from 7700 to 6150 cal. yr BP suggesting a generally stable hydroclimate (mean − 133‰, 1σ = 5‰). The shift to more depleted values (− 156‰, 1σ = 19‰) at ~ 6000–4800 cal. yr BP may be linked to a weakened (eastern displaced) North Atlantic Subtropical High. Nevertheless, additional local hydroclimate records are needed to better disentangle uncertainties from either internal or external influences on the resultant measurements. 

Variability in hydroclimate impacts natural and human systems worldwide. In particular, both decadal variability and extreme precipitation events have substantial effects and are anticipated to be strongly influenced by climate change. From a practical perspective, these impacts will be felt relative to the continuously evolving background climate. Removing the underlying forced trend is therefore necessary to assess the relative impacts, but to date, the small size of most climate model ensembles has made it difficult to do this. Here we use an archive of large ensembles run under a high-emissions scenario to determine how decadal “megadrought” and “megapluvial” events—and shorter-term precipitation extremes—will vary relative to that changing baseline. When the trend is retained, mean state changes dominate: In fact, soil moisture changes are so large in some regions that conditions that would be considered a megadrought or pluvial event today are projected to become average. Time-of-emergence calculations suggest that in some regions including Europe and western North America, this shift may have already taken place and could be imminent elsewhere: Emergence of drought/pluvial conditions occurs over 61% of the global land surface (excluding Antarctica) by 2080. Relative to the changing baseline, megadrought/megapluvial risk either will not change or is slightly reduced. However, the increased frequency and intensity of both extreme wet and dry precipitation events will likely present adaptation challenges beyond anything currently experienced. In many regions, resilience against future hazards will require adapting to an ever-changing “normal,” characterized by unprecedented aridification/wetting punctuated by more severe extremes. 

Abstract Characterizing variability in the global water cycle is fundamental to predicting impacts of future climate change; understanding the role of the Pacific Walker circulation (PWC) in the regional expression of global water cycle changes is critical to understanding this variability. Water isotopes are ideal tracers of the role of the PWC in global water cycling because they retain information about circulation-dependent processes including moisture source, transport, and delivery. We collated publicly available measurements of precipitation δ 18 O ( δ 18 O P ) and used novel data processing techniques to synthesize long (34 yr), globally distributed composite records from temporally discontinuous δ 18 O P measurements. We investigated relationships between global-scale δ 18 O P variability and PWC strength, as well as other possible drivers of global δ 18 O P variability—including El Niño–Southern Oscillation (ENSO) and global mean temperature—and used isotope-enabled climate model simulations to assess potential biases arising from uneven geographical distribution of the observations or our data processing methodology. Covariability underlying the δ 18 O P composites is more strongly correlated with the PWC ( r = 0.74) than any other index of climate variability tested. We propose that the PWC imprint in global δ 18 O P arises from multiple complementary processes, including PWC-related changes in moisture source and transport length, and a PWC- or ENSO-driven “amount effect” in tropical regions. The clear PWC imprint in global δ 18 O P implies a strong PWC influence on the regional expression of global water cycle variability on interannual to decadal time scales, and hence that uncertainty in the future state of the PWC translates to uncertainties in future changes in the global water cycle. 

Abstract The paleoclimatic record from Mexico and Central America, or Mesoamerica, documents dramatic swings in hydroclimate over the past few millennia. However, the dynamics underlying these past changes remain obscure. We use proxy indicators of hydroclimate to show that last millennium hydroclimate variability was dominated by opposite‐signed moisture anomalies in northern and southern Mesoamerica. This pattern results from changes in moisture convergence driven by Atlantic‐Pacific interbasin temperature gradients. While this pattern is reproduced by several models and multiple experiments with a single model, models appear to disagree about the underlying dynamics of this interbasin gradient. Moreover, disagreement about the interbasin gradient, and associated hydroclimate pattern, dominates spread in 21st century regional hydroclimate projections. These results emphasize the role of interbasin asymmetries in governing past and future regional climate change. They also demonstrate that paleoclimate studies can elucidate mechanisms directly relevant to projecting future hydroclimate in climate change hot spots like Mesoamerica. 

Abstract Event‐based paleohurricane reconstructions of the last millennium indicate dramatic changes in the frequency of landfalling hurricanes on centennial timescales. It is difficult to assess whether the variability captured in these paleorecords is related to changing climate or randomness. We assess whether centennial‐scale active and quiet intervals of intense hurricane activity occur in a set of synthetic storms run with boundary conditions from an earth system model simulation of the last millennium. We generate 1,000 pseudo sedimentary records for a site on South Andros Island using a Poisson random draw from this synthetic storm data set. We find that any single pseudo sedimentary record contains active and quiet intervals of hurricane activity. The 1,000‐record ensemble average, which reflects the common signal of climate variability, does not. This suggests that the record of paleohurricane activity from The Bahamas reflects variability in hurricane frequency dominated by randomness and not variability in the climatic conditions.