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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 Droughts over the last century in Southwestern North America (SWNA) have had severe consequences for people and ecosystems across the region, most recently during the early 21st‐century megadrought (2000–2022). The 20^thcentury, however, was bracketed by two extended pluvials that also had significant impacts in the region. We use a 1,224 years (800–2023 CE) record of observed and reconstructed soil moisture, in concert with a paleoclimate reanalysis product, to place the 20th‐century pluvials in a longer‐term context and investigate the occurrence and dynamics of similar events in the Common Era. Analyses of the soil moisture reconstruction demonstrate that pluvials and megapluvials are as ubiquitous as droughts and megadroughts over the last millennium. The early (19 years; 1905–1923) and late (22 years; 1978–1999) 20th‐century pluvials rank as the second and first wettest in the record, respectively, positioning these as events on par with the most extreme megadroughts. Pluvials show a strong association with tropical Pacific (warm) sea surface temperatures (SSTs) during the 20^thcentury and over the prior millennium, though the role of the tropical Atlantic is much more uncertain and ambiguous. Using a Bayesian hierarchical modeling approach trained on the pre‐industrial period (800–1849 CE), we find that the record setting late 20^th‐century megapluvial likely occurred as a consequence of anomalously strong Pacific sea surface temperature forcing. This work establishes pluvial and megapluvial events as intrinsic components of Common Era hydroclimate variability in SWNA, comparable in importance to droughts and megadroughts. 

Abstract In response to greenhouse gas forcing, most coupled global climate models project the tropical Pacific SST trend toward an “El Niño–like” state, with a reduced zonal SST gradient and a weakened Walker circulation. However, observations over the last five decades reveal a trend toward a more “La Niña–like” state with a strengthening zonal SST gradient. Recent research indicates that the identified trend differences are unlikely to be entirely due to internal variability and probably result, at least in part, from systematic model biases. In this study, Community Earth System Model, version 2 (CESM2), is used to explore how mean-state biases within the model may influence its forced response to radiative forcing in the tropical Pacific. The results show that using flux adjustment to reduce the mean-state bias in CESM2 over the tropical regions results in a more La Niña–like trend pattern in the tropical Pacific, with a strengthening of the tropical Pacific zonal SST gradient and a relatively enhanced Walker circulation, as hypothesized to occur if the ocean thermostat mechanism is stronger than the atmospheric mechanisms which by themselves would weaken the Walker circulation. We also find that the historical strengthening of the tropical Pacific zonal gradient is transient but persists into the near term in a high-emissions future warming scenario. These results suggest the potential of flux adjustment as a method for developing alternative projections that represent a wider range of possible future tropical Pacific warming scenarios, especially for a better understanding of regional patterns of climate risk in the near term. 

Abstract Climate models suffer from longstanding precipitation biases, much of which has been attributed to their atmospheric component owing to unrealistic parameterizations. Here we investigate precipitation biases in 37 Atmospheric Model Intercomparison Project Phase 6 (AMIP6) models, focusing on the Indo‐Pacific region during boreal summer. These models remain plagued by considerable precipitation biases, especially over regions of strong precipitation. In particular, 22 models overestimate the Asian‐Pacific monsoon precipitation, while 28 models underestimate the southern Indian Ocean Intertropical Convergence Zone precipitation. The inter‐model spread in summer precipitation is decomposed into Empirical Orthogonal Functions (EOFs). The leading EOF mode features an anomalous anticyclone circulation spanning the Indo‐northwest Pacific oceans, which we show is energized by barotropic conversion from the confluence of the background monsoonal westerlies and trade‐wind easterlies. Our results suggest precipitation biases in atmospheric models, though caused by unrealistic parameterizations, are organized by dynamical feedbacks of the mean flow. 

Abstract Understanding how the tropical Pacific responds to rising greenhouse gases in recent decades is of paramount importance given its central role in global climate systems. Extensive research has explored the long-term trends of tropical Pacific sea surface temperatures (SSTs) and the overlying atmosphere, yet the historical change in the upper ocean has received far less attention. Here, we present compelling evidence of a prominent subsurface cooling pattern along the thermocline in the central-to-eastern tropical Pacific since 1958. This subsurface cooling has been argued to be contributing to the observed cooling or lack of warming of the equatorial cold tongue SST. We further demonstrate that different mechanisms are responsible for different parts of the subsurface cooling. In the central-to-eastern equatorial Pacific and the southeastern off-equatorial Pacific, where zonal wind stress strengthens, a pronounced subsurface cooling trend emerges just above the thermocline that is closely tied to increased Ekman pumping. In the eastern equatorial Pacific where zonal wind stress weakens, the westward surface current and eastward Equatorial Undercurrent weaken as well, resulting in reduced vertical current shear and increased ocean stability, which suppresses vertical mixing and leads to local cooling. We conclude that the historical subsurface cooling is primarily linked to dynamical adjustments of ocean currents to tropical surface wind stress changes. 

Abstract The equatorial cold tongue region has not warmed up in response to historical radiative forcing in the real world, contrary to the strong warming often simulated by climate models. Here we demonstrate that climate models fail to represent one or both of the key processes driving observed sea surface temperature (SST) pattern formation: a realistic surface wind stress pattern shaping subsurface cooling through wind‐driven circulation changes, and effective connectivity between subsurface and surface temperatures via upwelling and mixing. Consequently, none of the models approximate the observed lack of cold tongue SST warming and strengthening of zonal SST gradient across the equatorial Pacific. Furthermore, those that come closest achieve this due to interhemispheric warming differences rather than equatorial dynamics as observed. Addressing different origins of subsurface cooling in observations and simulations, and how they connect to SST, will lead to improved understanding of tropical Pacific SST changes to date and how they will evolve in the future. 

Arid and semi-arid regions of the world are particularly vulnerable to greenhouse gas–driven hydroclimate change. Climate models are our primary tool for projecting the future hydroclimate that society in these regions must adapt to, but here, we present a concerning discrepancy between observed and model-based historical hydroclimate trends. Over the arid/semi-arid regions of the world, the predominant signal in all model simulations is an increase in atmospheric water vapor, on average, over the last four decades, in association with the increased water vapor–holding capacity of a warmer atmosphere. In observations, this increase in atmospheric water vapor has not happened, suggesting that the availability of moisture to satisfy the increased atmospheric demand is lower in reality than in models in arid/semi-arid regions. This discrepancy is most clear in locations that are arid/semi-arid year round, but it is also apparent in more humid regions during the most arid months of the year. It indicates a major gap in our understanding and modeling capabilities which could have severe implications for hydroclimate projections, including fire hazard, moving forward. 

Abstract Southwest North America is projected by models to aridify, defined as declining summer soil moisture, under the influence of rising greenhouse gases. Here, we investigate the driving mechanisms of aridification that connect the oceans, atmosphere, and land surface across seasons. The analysis is based on atmosphere model simulations forced by imposed sea surface temperatures (SSTs). For the historical period, these are the observed ones, and the model is run to 2041 using SSTs that account for realistic and plausible evolutions of Pacific Ocean and Atlantic Ocean interannual to decadal variability imposed on estimates of radiatively forced SST change. The results emphasize the importance of changes in precipitation throughout the year for declines in summer soil moisture. In the worst-case scenario, a cool tropical Pacific and warm North Atlantic lead to reduced cool season precipitation and soil moisture. Drier soils then persist into summer such that evapotranspiration reduces and soil moisture partially recovers. In the best-case scenario, the opposite states of the oceans lead to increased cool season precipitation but higher evapotranspiration prevents this from increasing summer soil moisture. Across the scenarios, atmospheric humidity is primarily controlled by soil moisture: drier soils lead to reduced evapotranspiration, lower air humidity, and higher vapor pressure deficit (VPD). Radiatively forced change reduces fall precipitation via anomalous transient eddy moisture flux divergence. Fall drying causes soils to enter winter dry such that, even in the best-case scenario of cool season precipitation increase, soil moisture remains dry. Radiative forcing reduces summer precipitation aided by reduced evapotranspiration from drier soils. Significance StatementSouthwest North America has long been projected to undergo aridification under rising greenhouse gases. In this model-based paper, we examine how coupling across seasons between the atmosphere and land system moves the region toward reduced summer soil moisture. The results show the dominant control on summer soil moisture by precipitation throughout the year. It also shows that even in best-case scenarios when changes in decadal modes of ocean variability lead to increases in cool season precipitation, rising spring and summer evapotranspiration means this does not translate into increased summer soil moisture. The work places projections of regional aridification on a firmer basis of understanding of the ocean driving of the atmosphere and its coupling to the land system. 

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.