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This study examines the climatology and trends in climate in Guatemala, Honduras, and El Salvador over the past four decades, against the background of potential impacts on rainfed agriculture, livelihoods, and migration. The results show that there has been a significant warming of surface temperatures, an earlier start to the monsoon, a drier and longer mid‐summer drought, and a delay in the second peak of precipitation from September to October. These changes have led to an increase in vapour pressure deficit (VPD) in northern Guatemala and along the Pacific coast in winter. High VPD can stress plants and lead to reduced yields. The study also finds that the thresholds that cause a decline in coffee yield have already been reached in El Salvador, but the average VPD has also risen in Guatemala and Honduras over the past 42 years. Maize yields have also been negatively affected with an inverse relationship with daily maximum temperatures during the summer flowering season. Observed changes and trends in these climate factors are believed to have direct implications for crop yields and livelihoods, potentially driving shifts in migration patterns. [ABSTRACT FROM AUTHOR]

The cool season (November–March) of 2022–2023 was exceptional in the western United States (US), with the highest precipitation totals in ≥128 years in some areas. Recent precipitation extremes and expectations based on thermodynamics motivate us to evaluate the evidence for an anthropogenic intensification of western US cool‐season precipitation to date. Over cool seasons 1951–2023, trends in precipitation totals on the wettest cool‐season days were neutral or negative across the western US, and significantly negative in northern California and parts of the Pacific Northwest, counter to the expected net intensification effect from anthropogenic forcing. Multiple reanalysis data sets indicate a corresponding lack of increase in moisture transports into the western US, suggesting that atmospheric circulation trends over the North Pacific have counteracted the increases in atmospheric moisture expected from warming alone. The lack of precipitation intensification to date is generally consistent with climate model simulations. A large ensemble of 648 simulations from 35 climate models suggests it is too soon to detect anthropogenic intensification of precipitation across much of the western US. In California, the 35‐model median time of emergence for intensification of the wettest days is 2080 under a mid‐level emissions scenario. On the other hand, observed reductions of precipitation extremes in California and the Pacific Northwest are near the lower edge of the large ensemble of simulated trends, calling into question model representation of western US precipitation variability. Plain Language Summary: After warming, one of the best understood climate responses to greenhouse gasses is increased atmospheric moisture (from more evaporation) and resultant storm intensification. Given that hydrological infrastructure and policies are probably more attuned to past climate than future climate, intensified precipitation has important implications for management of flood hazards and water resources. In the western US, the very wet winter of 2023 and the extensive flooding that resulted indicated that this region may be highly vulnerable to increases in precipitation extremes. Are precipitation extremes already intensifying in the western US? We find no evidence for intensified western US winter precipitation thus far. In fact, precipitation intensity actually decreased in northern California and the Pacific Northwest over 1951–2023. In California, most climate models do not project intensified precipitation to become detectable from that region's wide range of natural variability until the 2060s even under heavy greenhouse‐gas emissions. By the end of this century, however, most climate models consistently project the strongest storms to be stronger than they were historically across most or all of the western US. Thus, the lack of precipitation intensification thus far should not dissuade planning for greater precipitation intensity and flood risks. Key Points: Over 1951–2023, precipitation on the wettest cool‐season days did not increase across most of the western US, and even declined in areasIn California, most climate models do not project human‐caused precipitation intensification to become detectable before the 2060sClimate models rarely simulate precipitation intensity to decline as much as it did in northern California and the Pacific Northwest [ABSTRACT FROM AUTHOR]

By summer 2021 moderate to exceptional drought impacted 28% of North America, focused west of the Mississippi, with serious impacts on fire, water resources, and agriculture. Here, using reanalyses and SST-forced climate models, we examine the onset and development of this southwestern drought from its inception in summer 2020 through winter and spring 2020/21. The drought severity in summer 2021 resulted from four consecutive prior seasons in which precipitation in the southwest United States was the lowest on record or, at least, extremely dry. The dry conditions in summer 2020 arose from internal atmospheric variability but are beyond the range of what the studied atmosphere models simulate for that season. From winter 2020 through spring 2021 the worsening drought conditions were guided by the development of a La Niña in the tropical Pacific Ocean. Decadal variability in the Pacific Ocean aided drought in the southern part of the region by driving the cool season to be drier during the last two decades. There is also evidence that the southern part of the region in spring is drying due to human-driven climate change. In sum the drought onset was driven by a combination of internal atmospheric variability and interannual climate variability and aided by natural decadal variability and human-driven climate change. [ABSTRACT FROM AUTHOR]

The Intergovernmental Panel on Climate Change (IPCC) has confirmed that human-induced climate change is already impacting weather and climate extremes worldwide. This includes changes in heatwaves, heavy precipitation, droughts, and tropical cyclones, all of which are influenced by human activity. Global surface temperature will continue to rise until at least the mid-century, leading to increased moisture transport and changes in precipitation patterns. These changes have significant implications for water resources, agriculture, and ecosystems, and can result in events such as crop failure and infrastructure damage. It is important to deepen our understanding of these changes in order to make informed choices and adapt to a changing physical environment. The Special Issue of Atmosphere titled "Hydroclimate in a Changing World: Recent Trends, Current Progress and Future Directions" aims to contribute to this understanding. [Extracted from the article]

Future changes to the hydrological cycle are projected in a warming world, and any shifts in drought risk may prove extremely consequential for natural and human systems. In addition to long‐term moistening, drying, or warming trends, perturbations to the annual cycle of regional hydroclimate variables may also have substantial impacts. We analyze projected changes in several hydroclimate variables across the continental United States, along with shifts in the amplitude and phase of their annual cycles. We find that even in regions where no robust change in the annual mean is expected, coherent changes to the annual cycle are projected. In particular, we identify robust regional phase shifts toward earlier arrival of peak evaporation in the northern regions, and peak runoff and total soil moisture in the western regions. Changes in the amplitude of the annual cycle of total and surface soil moisture are also projected, and reflect changes to the annual cycle in surface water supply and demand. Whether changes become detectable above the background noise of internal variability depends strongly on the future scenario considered, and significant changes to the annual cycle are largely avoided in the lowest‐forcing scenario. Plain Language Summary: State‐of‐the‐art climate models project substantial changes to the climate of the continental United States (CONUS). Models project changes to the yearly average rainfall, soil moisture, and runoff in many regions. But crucially, they also project changes to the annual cycle, with peaks in evaporation, runoff, and soil moisture projected to shift earlier in the year even if the yearly averages of those quantities do not change. While different climate models disagree on the exact nature of many shifts, a large source of uncertainty is human behavior and policy choices, and many significant changes can be avoided if greenhouse gas emissions fall drastically in the future. Key Points: Coupled Model Intercomparison Project models project changes to the annual cycle of many hydroclimate variables, many of which are more significant than annual mean changesIn the continental United States, there are significant earlier shifts in the annual cycle in a high emissions scenarioSignificant changes to the annual cycle are largely avoided in the lowest‐emissions scenario [ABSTRACT FROM AUTHOR]

Mediterranean-type climates are defined by temperate, wet winters, and hot or warm dry summers and exist at the western edges of five continents in locations determined by the geography of winter storm tracks and summer subtropical anticyclones. The climatology, variability, and long-term changes in winter precipitation in Mediterranean-type climates, and the mechanisms for model-projected near-term future change, are analyzed. Despite commonalities in terms of location in the context of planetary-scale dynamics, the causes of variability are distinct across the regions. Internal atmospheric variability is the dominant source of winter precipitation variability in all Mediterranean-type climate regions, but only in the Mediterranean is this clearly related to annular mode variability. Ocean forcing of variability is a notable influence only for California and Chile. As a consequence, potential predictability of winter precipitation variability in the regions is low. In all regions, the trend in winter precipitation since 1901 is similar to that which arises as a response to changes in external forcing in the models participating in phase 5 of the Coupled Model Intercomparison Project. All Mediterranean-type climate regions, except in North America, have dried and the models project further drying over coming decades. In the Northern Hemisphere, dynamical processes are responsible: development of a winter ridge over the Mediterranean that suppresses precipitation and of a trough west of the North American west coast that shifts the Pacific storm track equatorward. In the Southern Hemisphere, mixed dynamic–thermodynamic changes are important that place a minimum in vertically integrated water vapor change at the coast and enhance zonal dry advection into Mediterranean-type climate regions inland. [ABSTRACT FROM AUTHOR]

The net surface water budget, precipitation minus evaporation (P = E), shows a clear seasonal cycle in the U.S. Southwest with a net gain of surface water (positive P = E) in the cold half of the year (October-March) and a net loss of water (negative P = E) in the warm half (April-September), with June and July being the driest months of the year. There is a significant shift of the summer drying toward earlier in the year under a CO2 warming scenario, resulting in substantial spring drying (March-May) of the U.S. Southwest from the near-term future to the end of the current century, with gradually increasing magnitude. While the spring drying has been identified in previous studies, its mechanism has not been fully addressed. Using moisture budget analysis, it was found that the drying is mainly due to decreased mean moisture convergence, partially compensated by the increase in transient eddy moisture flux convergence. The decreased mean moisture convergence is further separated into components as a result of changes in circulation (dynamic changes) and changes in atmospheric moisture content (thermodynamic changes). The drying is found to be dominated by the thermodynamic-driven changes in column-averaged moisture convergence, mainly due to increased dry zonal advection caused by the climatological land-ocean thermal contrast, rather than by the well-known "dry get drier" mechanism. Furthermore, the enhanced dry advection in the warming climate is dominated by the robust zonal mean atmospheric warming, leading to equally robust spring drying in the southwestern United States. [ABSTRACT FROM AUTHOR]

The recent California drought was associated with a persistent ridge at the west coast of North America that has been associated with, in part, forcing from warm SST anomalies in the tropical west Pacific. Here it is considered whether there is a role for human-induced climate change in favoring such a west coast ridge. The models from phase 5 of the Coupled Model Intercomparison Project do not support such a case either in terms of a shift in the mean circulation or in variance that would favor increased intensity or frequency of ridges. The models also do not support shifts toward a drier mean climate or more frequent or intense dry winters or to tropical SST states that would favor west coast ridges. However, reanalyses do show that over the last century there has been a trend toward circulation anomalies over the Pacific-North American domain akin to those during the height of the California drought. The trend has been associated with a trend toward preferential warming of the Indo-west Pacific, an arrangement of tropical oceans and Pacific-North American circulation similar to that during winter 2013/14, the driest winter of the California drought. These height trends, however, are not reproduced in SST-forced atmosphere model ensembles. In contrast, idealized atmosphere modeling suggests that increased tropical Indo-Pacific zonal SST gradients are optimal for forcing height trends that favor a west coast ridge. These results allow a tenuous case for human-driven climate change driving increased gradients and favoring the west coast ridge, but observational data are not sufficiently accurate to confirm or reject this case. [ABSTRACT FROM AUTHOR]

Through nine trace elements of the Shenmu section, Mu Us Desert is analyzed for the climate changes during Early Holocene. Except Sr, a migration process of P, Mn, Cr, Rb, Nb, Cu, Zn, and V is found in the dune sands, indicating the accumulation processes of the Asian winter monsoon-driven wind erosion and transport and deposition under the cold climatic phase. And accumulation of these elements in different layers of paleo sol dune sands is related to regional geomorphology and element properties. 8.5 winter-summer monsoon cycles in the early Holocene are observed. The duration time and climate of the stronger winter monsoon phases can be compared with the cold climate in the North Atlantic and other parts of China. The result of Red fit 3.8 power spectra shows 310a, 210a, 90a, and 70a centennial cycles for the element, which are consistent with the solar oscillation cycles. These suggest that the external solar activities of the Earth may have great influences on driving the winter and summer monsoon cycles during early Holocene. [ABSTRACT FROM AUTHOR]

Past cold climates are often thought to have been drier than today on land, which appears to conflict with certain recent studies projecting widespread terrestrial drying with near-future warming. However, other work has found that, over large portions of the continents, the conclusion of future drying versus wetting strongly depends on the physical property of interest. Here, it is shown that this also holds in simulations of the Last Glacial Maximum (LGM): the continents have generally wetter topsoils and higher values of common climate wetness metrics than in the preindustrial, as well as generally lower precipitation and ubiquitously lower photosynthesis (likely driven by the low CO2), with streamflow responses falling in between. Using a large existing global pollen and plant fossil compilation, it is also confirmed that LGM grasslands and open woodlands grew at many sites of present-day forest, seasonal forests at many sites of present-day rain forest, and so forth (116-144 sites out of 302), while changes in the opposite sense were very few (9-17 sites out of 302) and spatially confined. These vegetation changes resemble the model photosynthesis responses but not the hydroclimate responses, while published lake-level changes resemble the latter but not the former. Thus, confidence in both the model hydrologic and photosynthesis projections is increased, and there is no significant conflict. Instead, paleo- and modern climate researchers must carefully define 'wetting' and 'drying' and, in particular, should not assume hydrologic drying on the basis of vegetation decline alone or assume vegetation stress on the basis of declines in hydroclimatic indicators. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]