Abstract Apparent hydrological sensitivity ( η a ), the change in the global mean precipitation per degree K of global surface warming, is a key aspect of the climate system's response to increasing ...CO 2 forcing. To determine whether η a depends on the forcing amplitude we analyze idealized experiments over a broad range of abrupt CO 2 forcing, from 2× to 8× preindustrial values, with two distinct climate models. We find little change in η a between 2× and 4×CO 2 , and almost no change beyond 5×CO 2 . We validate this finding under transient CO 2 forcing at 1%‐per‐year, up to 8×CO 2 . We further corroborate this result by analyzing the 1%‐per‐year output of more than 15 CMIP5/6 models. Lastly, we examine the 1,000‐year long LongrunMIP model output, and again find little change in η a . This wealth of results demonstrates that η a is a very weak function of CO 2 forcing.
Plain Language Summary Hydrological sensitivity (HS) is defined as the change in globally‐averaged precipitation per degree K of surface temperature increase caused by increasing concentrations of greenhouse gasses, such as CO 2 . It is important to understand how HS changes with different levels of CO 2 in the atmosphere. To do this we analyzed model experiments with varying increases of CO 2 . We find little change in HS between 2× to 4× the pre‐industrial levels of CO 2 , and almost no change beyond 5×CO 2 . Additionally, we analyze model experiments where CO 2 concentrations increase by 1% per year and see similar results. Finally, we validate this finding with models with significantly longer run times. We thus conclude that HS is independent of the level of CO 2 in the atmosphere.
Key Points We examine the dependence of apparent hydrological sensitivity ( η a , defined as Δ P /Δ T ) on the magnitude of CO 2 forcing We find little change in η a in abrupt 2× to 8×CO 2 experiments and a transient 1%/year experiment up to 8×CO 2 Little change in η a , notably at large CO 2 , is also found in most CMIP5, CMIP6, and Longrun‐MIP models
Abstract This study investigates how climate sensitivity depends upon the spatial pattern of radiative forcing. Sensitivity experiments using a coupled ocean‐atmosphere model were conducted by adding ...anomalous incoming solar radiation over the entire globe, Northern Hemisphere mid‐latitudes, Southern Ocean, and tropics. The varied forcing patterns led to highly divergent climate sensitivities. Specifically, the climate is nearly twice as sensitive to Southern Ocean forcing as tropical forcing. Strong coupling between the surface and free troposphere in the tropics increases the inversion strength, leading to smaller cloud feedback in the tropical forcing experiments. In contrast, the extratropics exhibit weaker coupling, a decrease or near‐zero change in the inversion strength, and strong positive cloud feedback. These results contrast with the conventional SST‐pattern effect in which tropical surface temperature changes regulate climate sensitivity. They also have important implications for other potentially asymmetric forcings, such as those from geoengineering, volcanic eruptions, and paleoclimatic changes.
Abstract Air‐sea exchange of carbon dioxide (CO2) in the Southern Ocean plays an important role in the global carbon budget. Previous studies have suggested that flow around topographic features of ...the Southern Ocean enhances the upward supply of carbon from the deep to the surface, influencing air‐sea CO2exchange. Here, we investigate the role of seafloor topography on the transport of carbon and associated air‐sea CO2flux in an idealized channel model. We find elevated CO2outgassing upstream of a seafloor ridge, driven by anomalous advection of dissolved inorganic carbon. Argo‐like Lagrangian particles in our channel model sample heterogeneously in the vicinity of the seafloor ridge, which could impact float‐based estimates of CO2flux.
Abstract Significant imbalances in terrestrial water storage (TWS) and severe drought have been observed around the world as a consequence of climate changes. Improving our ability to monitor TWS and ...drought is critical for water‐resource management and water‐deficit estimation. We use continuous seismic ambient noise to monitor temporal evolution of near‐surface seismic velocity,dv/v, in central Oklahoma from 2013 to 2022. The deriveddv/vis found to be negatively correlated with gravitational measurements and groundwater depths, showing the impact of groundwater storage on seismic velocities. The hydrological effects involving droughts and recharge of groundwater occur on a multi‐year time scale and dominate the overall derived velocity changes. The thermoelastic response to atmospheric temperature variations occurs primarily on a yearly timescale and dominates the superposed seasonal velocity changes in this study. The occurrences of droughts appear simultaneously with local peaks ofdv/v, demonstrating the sensitivity of near‐surface seismic velocities to droughts.
Abstract The Pinatubo eruption in 1991 injected 10–20 Tg SO2into the stratosphere, which formed sulfate aerosols through oxidation. Our modeling results show that volcanic heating significantly ...perturbs the heterogeneous and homogeneous chemistry including NOxand HOxcatalytic cycles in the tropical stratosphere. The simulated tropical chemical ozone tendency is positive at 20 mb while negative at 10 mb in the tropics. The simulated ozone chemical tendency is of the same magnitude as the dynamical ozone tendency caused by the accelerated tropical upwelling, but with the opposite sign. Our study finds that the tropical ozone chemical tendency due to homogeneous chemistry becomes more important than heterogeneous chemistry 3 months after eruption. Sensitivity simulations further suggest that the tropical ozone tendency through heterogeneous chemistry is saturated when the injected amount exceeds 2 Tg.
Abstract We perform quadrupled CO 2 climate simulations with the Community Earth System Model version 1 (CESM1) to study how air‐sea coupling affects the response of tropical rainfall under global ...warming. We use a hierarchy of ocean models to separate the effects of seasonal mixed‐layer entrainment, wind‐driven Ekman flows directed perpendicular to the wind, and the near‐equator frictional flows directed in the same direction as the wind. We show that the Pacific Ocean's enhanced equatorial warming pattern (EEW) and equatorward ITCZ contraction observed in previous climate simulations emerge when the ocean model includes wind‐driven Ekman and frictional flows. Furthermore, the near‐equator frictional flow contributes more than half of the heat convergence in the equatorial Pacific Ocean. Finally, we show that although Ekman flow and near‐equator frictional flow can both result in EEW, their coupled interactions with the Hadley circulation lead to opposite feedbacks on EEW's strength.
Plain Language Summary The ocean is important in modulating the atmospheric response to climate change. Here, we study how air‐sea coupling affects the response of tropical rainfall under global warming. To identify the importance of individual ocean processes, we use a hierarchy of ocean models to separate the effects of seasonal mixed‐layer entrainment, wind‐driven Ekman flows, and frictional flows. We show that including Ekman and frictional flows allows our simulation to produce the Pacific Ocean's enhanced equatorial warming pattern and equatorward ITCZ contraction noted in previous climate simulations. We also show that the frictional flow, which has yet to receive much attention, is as important as the Ekman flow in generating equatorial heat convergence.
Key Points Air‐sea momentum coupling enhances equatorial sea surface temperature (SST) response The frictional ocean flow, parallel with the surface wind stress, contributes half of the ocean heat convergence response near the equator The SST and vertical wind response to greenhouse gas forcing are modulated by the ocean's temperature and vertical velocity distribution
Abstract Understanding and forecasting Tropical Pacific Decadal‐scale Variability (TPDV) strongly rely on climate model simulations. Using a Linear Inverse Modeling (LIM) diagnostic approach, we ...reveal Coupled Model Intercomparison Project Phase 6 models have significant challenges in reproducing the spatial structure and dominant mechanisms of TPDV. Specifically, while the models' ensemble mean pattern of TPDV resembles that of observations, the spread across models is very large and most models show significant differences from observations. In observations, removing the coupling between extratropics and tropics reduces TPDV by ∼60%–70%, and removing the tropical thermocline variability makes the central tropical Pacific a key center of action for TPDV and El Niño Southern Oscillation variability. These characteristics are only confirmed in a subset of models. Differences between observations and simulations are outside the range of natural internal TPDV noise and pose important questions regarding our ability to model the impacts of natural internal low‐frequency variability superimposed on long‐term climate change.
Abstract We image the shallow seismic structure across the Southern San Andreas Fault (SSAF) using signals from freight trains and trucks recorded by a dense nodal array, with a linear component ...perpendicular to SSAF and two 2D subarrays centered on the Banning Fault and Mission Creek Fault (MCF). Particle motion analysis in the frequency band 2–5 Hz shows that the examined traffic sources can be approximated as moving single‐ or multi‐point sources that primarily induce Rayleigh waves. Using several techniques, we resolve strong lateral variations of Rayleigh wave velocities andQ‐values across the SSAF, including 35% velocity reduction across MCF toward the northeast and strong attenuation around the two fault strands. We further resolve 10% mass density reduction and 45% shear modulus decrease across the MCF. These findings suggest that the MCF is currently the main strand of the SSAF in the area with important implications for seismic hazard assessments.
Abstract Field and laboratory observations indicate that the frictional behaviors of faults depend on hydrothermal conditions. We extend the microphysical Chen‐Niemeijer‐Spiers (CNS) model to ...hydrothermal conditions by using the observed temperature variation of indentation hardness to infer the temperature dependence of a microphysical parameter . This parameter is assumed constant in previous versions of the CNS model. A simple spring‐slider system is used to simulate the fault system and investigate the steady‐state frictional behaviors of wet granite gouges. Our numerical results quantitatively reproduce experimental data showing the frictional‐plastic transition. The results also describe the transition from velocity‐strengthening at low temperatures (<160°C), to velocity‐weakening at intermediate temperatures (160°C–370°C), then back to velocity‐strengthening at high temperatures (>370°C). In our extended CNS model, these results suggest that the dominant shear deformation mechanism does transition from frictional granular flow to fully plastic creep with increasing temperature.
Abstract Volcanic and wildfire events between 2014 and 2022 injected ∼3.2 Tg of sulfur dioxide and 0.8 Tg of smoke aerosols into the stratosphere. With injections at higher altitudes and lower ...latitudes, the simulated stratospheric lifetime of the 2014–2022 injections is about 50% longer than the volcanic 2005–2013 injections. The simulated global mean effective radiative forcing (ERF) of 2014–2022 is −0.18 W m−2, ∼40% of the ERF of the period of 1991–1999 with a large‐magnitude volcanic eruption (Pinatubo). Our climate model suggests that the stratospheric smoke aerosols generate ∼60% more negative ERF than volcanic sulfate per unit aerosol optical depth. Studies that fail to account for the different radiative properties of wildfire smoke relative to volcanic sulfate will likely underestimate the negative stratospheric forcings. Our analysis suggests that stratospheric injections offset 20% of the increase in global mean surface temperature between 2014–2022 and 1999–2002.
