Search for: All records

Abstract In January 2025, a rare and highly destructive wildfire devastated Southern California, becoming the costliest wildfire event in recorded history. The unusual timing of this wildfire suggests the possibility of unique remote, large-scale climatic precursors that differ from those of previous wildfires. Here, through observational analysis and large-ensemble numerical simulations, we identify that western Eurasian snow cover reduction is associated with weather conditions favorable for wildfires in Southern California in December-January, and simulations indicate that this can occur via an atmospheric teleconnection from western Eurasia, across the North Pacific and into North America. Both observations and simulations show that the reduced snow cover over western Eurasia contributes to the typical wintertime western warming-eastern cooling dipole pattern in North America. The main dynamical mechanisms involve downstream propagating Rossby wave trains triggered by the reduced snow cover in western Eurasia, as well as wave-mean flow interaction over the North Pacific. Our study suggests that Eurasian snow cover anomalies can be predictively linked to both subsequent wildfire risk in California and the wintertime North American zonal dipole temperature pattern, highlighting the broader impacts of Eurasian cryosphere variability on remote climate extremes. 

Abstract In the western United States (US), the annual wildfire extent increased by 306% over 1984–2022, largely due to a 1,320% increase in forest‐fire area. During this time, associated with the Pacific Decadal Oscillation (PDO), the tropical Pacific Ocean exhibited a “La‐Niña‐like” trend with an enhanced zonal sea‐surface temperature gradient (gSST) between its warm western and cool eastern sides. In contrast, most atmosphere‐ocean models simulate human‐caused climate change should force an “El‐Niño‐like” gSST trend, a discrepancy from observations that is the focus of vigorous investigation. To understand the relevance of uncertainties in projections of gSST to western US wildfire activity, we statistically estimate the contribution of the observed increase in gSST of the past four decades. We compare the observed burned area outcome against a counterfactual scenario in which the observed La Niña‐like gSST trend did not occur. We estimate that the strengthened gSST gradient accounted for 19% [11%, 33%] of the increase in western US wildfire area. In forest areas alone, we estimate the gSST trend accounted for 32% [21%, 45%] of the observed increase, due mostly to increased spring and summer vapor‐pressure deficit (VPD). Thus, the strengthened gSST was not the dominant driver of the observed increase in wildfire area, but it appeared to play an important secondary role. Should the positive gSST trends not reverse, western US climate may remain more conducive to forest fire than would be expected based on current climate projections. 

Abstract We perform a global assessment of dryness and wetness conditions using an ensemble of observational, satellite-informed, and reanalysis-based soil moisture datasets [soil moisture (SM) Global Land Evaporation Amsterdam Model (SM_GLEAM), SM_ERA5-LAND, and SM_MERRA-2] and drought indices [Palmer drought severity index (PDSI_CRU) and standardized precipitation evapotranspiration index (SPEI_CRU)]. Over the short term (1982–2023), four or more products consistently identify drying trends in western North America, South America, the Mediterranean, Europe, central Asia, and southern Australia. Wetting trends appear in southern and southeastern Asia, the Sahel, and parts of southern sub-Saharan Africa, while disagreements across products are most evident in central Africa, parts of South America, the Middle East, and Southeast Asia. The year 2023 ranks among the driest on record since 1982: driest in PDSI_CRUand SM_ERA5-LAND, second driest in SM_GLEAMand SPEI_CRU, and eighth in SM_MERRA-2. A longer-term analysis using PDSI_CRUfrom 1903 to 2023 estimates that 2023 was the most severe and spatially extensive drought year in over a century, affecting more than 27% of the global reporting land area. Moreover, the 2021–23 period marks the longest consecutive stretch of extreme drought conditions globally since the early twentieth century. The global patterns of droughts over 2021–23 align with long-term drying trends, consistent with the broader influence of anthropogenic climate change on global hydroclimate. Despite the presence of long-term wetting trends, the wetness of the past decades has not produced events of comparable magnitude to those in the mid-twentieth century, a time that remains unmatched in both the severity and extent of wet extremes. Significance StatementOn a global scale, the years 2021–23 have seen some of the most widespread and severe drought conditions in over a century. By analyzing five major datasets that characterize changes in soil moisture conditions, we find that 2023 was the driest year on record in over 120 years and that 2021–23 was the driest 3-yr period over the same time interval. We show that these recent twenty-first-century extremes are consistent with long-term drying trends and highlight a broad intensification of hydroclimate extremes across much of the globe. While previous work has emphasized regional drought events over shorter intervals, this study uses multiple datasets in a global framework to provide strong evidence that anthropogenic climate change is a key driver of an intensifying global pattern of more widespread and severe drought. 

Abstract The Southwestern U.S. and Eastern Australia are two of the most fire-prone regions of the world. These two regions also have significant and opposite-signed hydroclimate teleconnections to sea-surface temperature (SST) anomalies in the tropical Pacific, such that an El-Niño-like west-to-east tropical Pacific SST gradient is associated with hot, dry conditions in Eastern Australia and cool, wet conditions in the Southwestern U.S. Additionally, the tropical Pacific has experienced a pronounced La-Niña-like cooling over recent decades. Here, we investigate the roles of the tropical Pacific and anthropogenic climate change (ACC) as drivers of observed trends in burned forest area and vapor pressure deficit (VPD) in the Southwestern U.S. and Eastern Australia, using observations and Coupled Model Intercomparison Project Phase 6 model data. We characterize the strength and spatial structure of relationships between fire-favorable climate variables in these regions and SST anomalies in the Pacific. We show that in both regions, VPD has increased with a large contribution from ACC. In the Southwestern U.S., the recent trend in the tropical Pacific west–east sea surface temperature gradient positively contributed to the VPD-induced increase in burned forest area since 1984. However, in Eastern Australia, the tropical Pacific SST gradient trend has likely offset the ACC-induced tendency to increase burned forest area. 

Abstract The Mediterranean region has experienced declining precipitation during the cool season since the 1950s, which is broadly consistent with climate model future projections. The observed drying is focused on midwinter and the central to western Mediterranean. Observed winter drying in the western Mediterranean is at the very edge of the CMIP6 ensemble range of historical simulations for the same time period and, in the central to eastern Mediterranean, is in the lowest percentiles of model ensemble distributions. Moisture budget analysis shows the drying to be dynamic, driven by changes in circulation, rather than thermodynamically related to changes in humidity. An observed positive trend in the North Atlantic Oscillation drives the drying by placing subsiding air over the Mediterranean augmented by dry northerly advection. The more modest radiatively forced drying in the CMIP6 multimodel ensemble is also dynamic but related to a high pressure anomaly over the eastern North Atlantic with a different pattern to the North Atlantic Oscillation. A small number of individual runs of some CMIP6 models do show trends in Mediterranean drying, the North Atlantic Oscillation, and moisture budget similar to that observed. However, the observed Mediterranean drying and North Atlantic Oscillation trends are at the very limit of what climate model simulations can do from the combination of variability and forced response over this time period. The observed drying is either (i) an extremely rare manifestation of natural multidecadal variability akin to that in models, (ii) a result of variability not captured in models, or (iii) a forced response that models are missing. Significance StatementDrying of the Mediterranean region over the past decades has had significant impacts on water resources, agriculture, and fire. The drying has impacted the whole region but is most extreme in winter and the west. Understanding its causes will help improve predictions of whether this drying trend will continue, stop, or reverse. The drying is driven by a long-term trend to a more positive North Atlantic Oscillation that places northerly, subsiding drying airflow over the Mediterranean. Climate models simulate only weak Mediterranean drying as a response to radiative forcing, driven by different changes in atmospheric circulation. Individual runs of some models can come close to matching observations, but this is rare within the multimodel ensemble. We conclude the observed drying is either a case of internal variability that models struggle to capture or a forced response they are missing. 

Abstract Natural decadal climate variability in the Pacific, such as the Pacific decadal oscillation (PDO) or the interdecadal Pacific oscillation (IPO), plays a powerful role in evolving global hydroclimate on decadal time scales. Recent generations of general circulation models (GCMs) have been found to simulate the spatial pattern of the PDO well but struggle to capture temporal variability on decadal time scales. To use GCMs to project future climate, we must understand the degree to which climate models can successfully reproduce historical PDO and IPO spatial patterns, temporal behavior, and influence on hydroclimate. We calculate PDO and IPO spatial patterns and time series using 16 models within the CMIP6 archive, all with large (n≥ 10) ensembles, and compare them to observations in an integrated assessment of models’ ability to represent Pacific decadal variability spatiotemporally. All models underestimate decadal variability in the PDO and IPO and have a westward bias in their PDO and IPO North Pacific SST anomalies. We also evaluate hydroclimate teleconnections of the PDO and IPO in models using PDO- and IPO-associated precipitation, circulation, low-cloud, and vapor pressure deficit anomalies. We show that models’ underpowered decadal variability in the Pacific is consistent with their inability to reproduce large-amplitude decadal swings in precipitation in southwestern North America and that models are virtually unable to produce a 30-yr precipitation trend in the southwest of the magnitude observed from 1982 to 2011. We emphasize the importance of model fidelity in simulating Pacific decadal variability for accurate representation of decadal-scale hydroclimate change in Pacific-teleconnected land regions. 

Abstract Wildfires pose a significant threat to human society as severe natural disasters. The western United States (US) is one hotspot that has experienced dramatic influences from autumn wildfires especially after 2000, but what has caused its year‐to‐year variations remains poorly understood. By analyzing observational and atmospheric reanalysis datasets, we found that the West Pacific (WP) pattern centered in the western North Pacific acted as a major climatic factor to the post‐2000 autumn wildfire activity by inducing anomalous high pressure over the western US via teleconnections with increased surface temperature, decreased precipitation, and reduced relative humidity. The WP pattern explains about one‐third of the post‐2000 years‐to‐year variance of the western US autumn wildfires. These effects were found to be much weaker in the 1980–1990s, as the active region of WP‐associated high pressure was confined to the eastern North Pacific. Such eastward shift of the WP teleconnection pattern and its resultant, enhanced influence on the weather conditions of western US autumn wildfire after 2000 are also captured by the sea surface temperature (SST)‐forced atmospheric model simulations with the Community Atmosphere Model version 6 (CAM6). The CAM6 ensemble‐mean changes in the WP teleconnection pattern at 2000 is about half of the observed changes, which implies that external radiative forcing and/or SST changes have played an important role in the WP pattern shift. Our results highlight a pressing need to consider the joint impacts of atmospheric internal variability and externally forced climate changes when studying the interannual variations of wildfire activity. 

Abstract On seasonal time scales, vapor pressure deficit (VPD) is a known predictor of burned area in the southwestern United States (“the Southwest”). VPD increases with atmospheric warming due to the exponential relationship between temperature and saturation vapor pressure. Another control on VPD is specific humidity, such that increases in specific humidity can counteract temperature-driven increases in VPD. Unexpectedly, despite the increased capacity of a warmer atmosphere to hold water vapor, near-surface specific humidity decreased from 1970 to 2019 in much of the Southwest, particularly in spring, summer, and fall. Here, we identify declining near-surface humidity from 1970 to 2019 in the southwestern United States with both reanalysis and in situ station data. Focusing on the interior Southwest in the months preceding the summer forest fire season, we explain the decline in terms of changes in atmospheric circulation and moisture fluxes between the surface and the atmosphere. We find that an early spring decline in precipitation in the interior region induced a decline in soil moisture and evapotranspiration, drying the lower troposphere in summer. This prior season precipitation decline is in turn related to a trend toward a Northern Hemisphere stationary wave pattern. Finally, using fixed humidity scenarios and the observed exponential relationship between VPD and burned forest area, we estimate that with no increase in temperature at all, the humidity decline alone would still lead to nearly one-quarter of the observed VPD-induced increase in burned area over 1984–2019. Significance StatementBurned forest area has increased significantly in the southwestern United States in recent decades, driven in part by an increase in atmospheric aridity [vapor pressure deficit (VPD)]. Increases in VPD can be caused by a combination of increasing temperature and decreasing specific humidity. As the atmosphere warms with climate change, its capacity to hold moisture increases. Despite this, there is a decrease in near-surface air humidity in the interior southwestern United States over 1970–2019, which during the summer is likely caused by a decline in early spring precipitation leading to limited soil moisture and evaporation in spring and summer. We estimate that this declining humidity alone, without an increase in temperature, would cause about one-quarter of the VPD-induced increase in burned forest area in this region over 1984–2019. 

Abstract Change over recent decades in the world's five Mediterranean Climate Regions (MCRs) of quantities of relevance to water resources, ecosystems and fire are examined for all seasons and placed in the context of changes in large‐scale circulation. Near‐term future projections are also presented. It is concluded that, based upon agreement between observational data sets and modelling frameworks, there is strong evidence of radiatively‐driven drying of the Chilean MCR in all seasons and southwest Australia in winter. Observed drying trends in California in fall, southwest southern Africa in fall, the Pacific Northwest in summer and the Mediterranean in summer agree with radiatively‐forced models but are not reproduced in a model that also includes historical sea surface temperature (SST) forcing, raising doubt about the human‐origin of these trends. Observed drying in the Mediterranean in winter is stronger than can be accounted for by radiative forcing alone and is also outside the range of the SST‐forced ensemble. It is shown that near surface vapour pressure deficit (VPD) is increasing almost everywhere but that, surprisingly, this is contributed to in the Southern Hemisphere subtropics to mid‐latitudes by a decline in low‐level specific humidity. The Southern Hemisphere drying, in terms of precipitation and specific humidity, is related to a poleward shift and strengthening of the westerlies with eddy‐driven subsidence on the equatorward side. Model projections indicate continued drying of Southern Hemisphere MCRs in winter and spring, despite ozone recovery and year‐round drying in the Mediterranean. Projections for the North American MCR are uncertain, with a large contribution from internal variability, with the exception of drying in the Pacific Northwest in summer. Overall the results indicate continued aridification of MCRs other than in North America with important implications for water resources, agriculture and ecosystems. 

As a climate change hotspot, the Mediterranean is often thought to be directly influenced by the North Atlantic. However, growing evidence highlights remote effects of the tropical oceans on the North Atlantic, complicating our understanding of the mechanisms driving Mediterranean climate change. The Last Interglacial (LIG; ∼127 ka) and the mid-Holocene (MH; ∼6 ka), both characterized by higher boreal summer insolation and lower winter insolation compared to the preindustrial climate, provide valuable opportunities to investigate the Mediterranean climate’s response to global-scale forcings. Here, we show that multimodel simulations from the Paleoclimate Modelling Intercomparison Project phase 4 (PMIP4), consistent with proxy data, reveal that the Mediterranean experienced wetter conditions during the LIG and MH compared to the preindustrial period. The simulated wetting is most pronounced in late winter and early spring (February–April), with circulation anomalies resembling a negative phase of the North Atlantic Oscillation and the North Atlantic storm track shifting southward. Stand-alone atmospheric experiments reveal both local and teleconnected effects during the LIG. A reduced meridional temperature gradient in the North Atlantic contributes about one-quarter of the increase in Mediterranean precipitation. More importantly, suppressed convection over the Indian Ocean forced by the local surface cooling induces roughly four-fifths of the simulated Mediterranean wetting. This physical link between Mediterranean wetting and Indian Ocean drying is consistently reproduced across the intermodel spread during the LIG. Our findings unravel the dominant role of the Indian Ocean in Mediterranean wetting during interglacial intervals, with implications for the ongoing Mediterranean drying under global warming.