Search

The Southern Annular Mode (SAM) is the most dominant natural mode of variability in the mid‐latitudes of the Southern Hemisphere (SH). However, both the sign and magnitude of the feedbacks from the diabatic processes, especially those associated with clouds, onto the SAM remain elusive. By applying the cloud locking technique to the Energy Exascale Earth System Model (E3SM) atmosphere model, this study isolates the positive feedback from the cloud radiative effect (CRE) to the SAM. Feedback analysis based on a wave activity‐zonal momentum interaction framework corroborates this weak but positive feedback. While the magnitude of the CRE feedback appears to be secondary compared to the feedbacks from the dry and other diabatic processes, the indirect CRE effects through the interaction with other dynamical and thermodynamical processes appear to play as important a role as the direct CRE in the life cycle of the SAM. The cross‐EOF analysis further reveals the obstructive effect of the interactive CRE on the propagation mode of the SH zonal wind directly through the CRE wave source and/or indirectly through modulating other diabatic processes. As a result, the propagation mode becomes more persistent and the SAM it represents becomes more predictable when the interactive CRE is disabled by cloud locking. Future efforts on inter‐model comparisons of CRE‐denial experiments are important to build consensus on the dynamical feedback of CRE. The annular mode is the most dominant mode of variability in the mid‐latitude atmospheric circulation system. Its origin, maintenance, and feedback mechanisms have long been the focus of atmospheric dynamics research. While its dry mechanisms are well understood, the role of the diabatic processes in the midlatitude storm tracks, especially the radiative effects of the cloud fields that evolve together with the storms, have not been quantified. Even the sign of the diabatic feedback to the annular mode remains a topic of debate. By disabling the cloud radiative feedback through a cloud‐locking technique that decorrelates the cloud fields with other dry and moist components in an atmospheric model, we are able to isolate the cloud radiative effect (CRE) throughout the life cycle of the Southern Annular Mode (SAM). Compared to the case with interactive CRE, the propagating SAM becomes more persistent and predictable in the cloud‐locking run, although the overall diabatic feedback to the standing dipole representation of the SAM is somewhat attenuated in this run. The diabatic and CRE feedbacks to the SAM identified here epitomize the sensitivity of the leading mode of the atmospheric variability to the model representation of the diabatic processes. Cloud locking technique is used to isolate the interactive cloud radiative effect (CRE) on the Southern Annular Mode (SAM)Cloud radiative effect boosts the persistence of SAM as the leading EOF of the Southern Hemisphere zonal windThe interactive CRE feedback acts to weaken the cross‐EOF interaction and counter the poleward propagation of the zonal wind Cloud locking technique is used to isolate the interactive cloud radiative effect (CRE) on the Southern Annular Mode (SAM) Cloud radiative effect boosts the persistence of SAM as the leading EOF of the Southern Hemisphere zonal wind The interactive CRE feedback acts to weaken the cross‐EOF interaction and counter the poleward propagation of the zonal wind

While some previous studies examined the contribution of Eastern Pacific (EP) hurricanes toward precipitation in the arid Southwest US (SWUS), their potential to influence wildfires in that region has not been explored. Here we show, using observations and simulations from the Energy Exascale Earth System Model (E3SM), that recurving EP hurricanes modulate the wildfire environment in the SWUS by increasing precipitation and soil moisture, and reducing the vapor pressure deficit. This is especially the case during late season months of September–October when the likelihood of storms to recurve and make landfall increases. Further, analysis of burnt area observations reveals that for the months of September–October, recurving EP hurricanes may significantly reduce the prevalence of wildfires in the SWUS. Finally, E3SM simulations indicate that late season EP hurricanes have been on the decline, with important implications for wildfires in the SWUS. A majority of Eastern Pacific (EP) hurricanes form and traverse westwards away from the North American coastline. Therefore, compared to tropical cyclones in some other basins, EP hurricanes have historically received less attention. However, some previous studies have shown that recurving EP hurricanes can contribute significantly to precipitation in the arid Southwest US (SWUS) region. Here, we build on these studies and demonstrate that recurving EP hurricanes significantly modulate the SWUS wildfire environment. During years with recurving EP hurricanes, the precipitation and soil moisture are enhanced, and the vapor pressure deficit is reduced, over the SWUS region. Therefore, during late season months of September–October, when recurving EP hurricanes occur, they make the environment less favorable for wildfires in the SWUS region. With climate models showing a long‐term decline in recurving EP hurricanes and the precipitation associated with them over the SWUS region, our study has substantial implications for autumn wildfires of that region. Eastern Pacific (EP) hurricanes that recurve tend to enhance precipitation and reduce vapor pressure deficit in the Southwest US regionDuring September–October, recurving EP hurricanes may make the environment less favorable for wildfires in the Southwest USClimate model simulations suggest that Southwest US precipitation associated with recurving EP hurricanes has decreased Eastern Pacific (EP) hurricanes that recurve tend to enhance precipitation and reduce vapor pressure deficit in the Southwest US region During September–October, recurving EP hurricanes may make the environment less favorable for wildfires in the Southwest US Climate model simulations suggest that Southwest US precipitation associated with recurving EP hurricanes has decreased

Tropical rainfall exhibits a prominent annual cycle, with characteristic amplitude and phase representing the range between wet and dry seasons and their onset timing, respectively. Previous studies note enhanced amplitude over ocean and delayed phase over land in model projections of global warming, underpinned by first-order physical principles. However, it is unclear whether these changes have emerged in observations. Here we use gridded precipitation datasets to report a seasonal delay of 4.1 ± 1.1 and 4.2 ± 0.9 days (P< 0.05) during 1979–2019 over the northern tropical land and Sahel, respectively. Most of the delay is driven by external forcings, dominated by greenhouse gases (GHG) and anthropogenic aerosols (AER). Increasing GHG and decreasing AER in the recent decades delay rainfall by producing a moister atmosphere, thus increasing its lag in response to seasonal solar forcing. As GHG increase and AER decrease, these seasonal delays are projected to further amplify in the future.

One of the grand challenges of climate science is understanding the changes of the tropical rain belts and monsoon systems owing to CO2‐induced warming. A promising path forward links the fluxes of energy and moisture to tropical circulation features. To this end, we make use of the Energy Exascale Earth System Model version 1, where the divergence of moist static energy and moisture have been calculated online, and employ a normalized gross moist stability (NGMS) diagnostic framework to understand the linkages between changes in the flow of energy and moisture within the monsoons. We focus on the Asian Summer Monsoon system and utilize a series of atmosphere‐land and atmosphere‐land‐ocean simulations to understand the connection between fluxes and monsoons. Uncoupled simulations with prescribed sea surface temperatures indicate that decreases in NGMS over land are important in explaining precipitation increases in response to both sea surface temperature and CO2increases. In fully coupled experiments, NGMS decreases remain an important contributor to the increase in P‐E, but the coupled simulations highlight the importance of consistent ocean and land responses in interpreting the monsoon changes. This study indicates that transient eddy fluxes play an important role in NGMS decreases and that a time‐mean view of the monsoon circulations is insufficient to quantify the link between future changes in the fluxes of energy and moisture. Compensation between dynamic and thermodynamic components of vertical moist static energy advection occurs, with the thermodynamic contribution dominating. The compensation is shown to be sensitive to relative humidity, with higher relative humidity leading to a stronger thermodynamic component. One of the challenges of climate science is understanding how warming will change monsoon rainfall. A promising path forward links the transport of energy to the transport of water vapor. This link, termed the normalized gross moist stability (NGMS), can be used to fingerprint monsoon rainfall changes coming from different energy sources as well as changes to NGMS itself. A fingerprint of the Asian Summer Monsoon rainfall response to warming is made for a general circulation model, which highlights the importance of the NGMS term for understanding the monsoon rainfall response to warming. By examining this NGMS term more carefully, evidence is shown for the importance of the ocean circulation toward explaining the monsoon rainfall response to warming. Further analysis of the NGMS term also suggests that the traditional time‐mean view of the monsoon is insufficient to explain its response to warming. The response of NGMS to warming is shown to be sensitive to the relative humidity of the atmosphere. Normalized gross moist stability framework diagnoses key factors for increase in Asian Summer Monsoon water cycle with warmingPrescribing sea surface temperature increases have an inconsistent circulation response to simulations with an active ocean modelTransient eddies contribute significantly to the changing normalized gross moist stability with warming

Couplings between land and the near surface atmosphere are modulated by interactions between soil conditions, vegetation dynamics, turbulent fluxes, and atmospheric properties. How the land‐atmosphere coupling responds to warming and elevated CO2are important for understanding the land surface carbon, energy, and water cycles. In this work, we documented this coupled land‐atmosphere network based on observations and the Energy Exascale Earth System Model (E3SM) simulations over extratropical forest ecosystems. We employed a transfer entropy approach and novel network metrics to reveal patterns and strength of the land‐atmosphere coupling under historical conditions and a future high emission scenario (SSP585). We found that, in observations, the present‐day extratropical forest coupling network has high network connectivity (72%–88% of the targeted processes are coupled). E3SM reasonably captured the extratropical forest coupling network (modeled network connectivity was 81%–96%) and predicted that the coupling strength would significantly increase by 28% (±3%) under warming and elevated CO2conditions. Furthermore, E3SM factorial coupled experiments suggested that warming enhanced soil nitrogen mineralization favoring plant nitrogen uptake and vegetation growth were responsible for the strengthening future land‐atmosphere coupling. This work provides new metrics to analyze and document complex couplings for coupled earth system processes and highlights the important roles soil nutrient availability and biogeochemistry have on land‐atmosphere coupling. Land exchanges a vast amount of water and energy with atmosphere that drive the coupling of atmosphere conditions (e.g., temperature), vegetation activities (e.g., growth), and soil properties (e.g., soil moisture). However, such land‐atmosphere coupling is likely modified due to climate warming and rising atmospheric CO2concentrations in the future. This work used observational datasets and coupled earth system model experiments to address the important questions of how and why the land atmosphere coupling change over time. We found strong evidence that elevated CO2concentrations could dampen while warming would largely enhance the land‐atmosphere coupling strength, leading to a net increase of land‐atmosphere coupling until the end of 21st century under the SSP585 high emission scenario. Using novel network metrics, we found high connectivity of land‐atmosphere coupling of water and energy over extratropical forest regionsEnergy Exascale Earth System Model coupled experiments revealed that the land‐atmosphere coupling strength would increase by 28% under SSP585 high emission scenario Using novel network metrics, we found high connectivity of land‐atmosphere coupling of water and energy over extratropical forest regions Energy Exascale Earth System Model coupled experiments revealed that the land‐atmosphere coupling strength would increase by 28% under SSP585 high emission scenario

Understanding how the connection between rainfall and tropical forests will respond to increasing CO2concentrations is a key element in understanding how the tropical water cycle will respond to increasing CO2. The plant physiological and radiative impacts of CO2on rainfall patterns over tropical forest regions are examined in the Energy Exascale Earth System Model version 1.1 (E3SMv1.1‐BGC) biogeochemistry experiments. Composite analysis reveals a dampening of the diurnal cycle of rainfall over the Amazon, Congo, and Maritime Continent in response to rising CO2levels, regardless of the sign of total rainfall change. A full factorial model experiment confirms that the CO2radiative and CO2plant physiological effects can individually or jointly reduce the magnitude of the rainfall diurnal cycle, though the physical pathway giving rise to the reduction differs between the two effects. For the physiological response, stomatal closure reduces evapotranspiration, which dries the boundary layer and raises the lifting condensation level. These effects combine to reduce deep convective rainfall during its peak occurrence in the late daytime to early nighttime period. For the radiative response, a relative reduction in daytime Convective Available Potential Energy (consistent with a reduction in the diurnal temperature range) leads to less frequent triggering of deep convection and a reduction of rainfall diurnal amplitude. These diurnal rainfall changes are structurally similar across seasons, and show little sensitivity to representation of nutrient coupling for the land biogeochemistry. In agreement with previous findings, the physiological response has only minor impact on extreme rainfall relative to the radiative response. The distribution of rainfall in time and space is one of the key features of interest in the climate system. Simulations using the biogeochemistry‐enabled version of the U.S. Department of Energy's Energy Exascale Earth System Model are used to explore how increasing atmospheric CO2concentrations impact the daily cycle of rainfall over the tropical forest regions. Increasing CO2not only warms the climate system, but also reduces the amount of transpiration from plants. These two effects are examined in isolation and are both found to reduce the daily peak in rainfall, dampening the cycle overall. The warming response is shown to make the atmosphere less favorable to convection, which reduces rainfall, while the reduced transpiration dries out the lower atmosphere, which also reduces rainfall. Physiological and radiative effects of CO2both dampen the diurnal cycle of rainfall in tropical forest regionsPhysiological effect of CO2reduces transpiration, dries boundary layer, and raises the lifting condensation level, decreasing deep convective rainfallRadiative effect of CO2reduces Convective Available Potential Energy leading to reductions in deep convective frequency and rainfall Physiological and radiative effects of CO2both dampen the diurnal cycle of rainfall in tropical forest regions Physiological effect of CO2reduces transpiration, dries boundary layer, and raises the lifting condensation level, decreasing deep convective rainfall Radiative effect of CO2reduces Convective Available Potential Energy leading to reductions in deep convective frequency and rainfall

Monsoon rainfall anomalies are often organized into dynamical modes that are more predictable than rainfall at specific locations. Here, a linear response function (LRF) is constructed to extract the neutral modes (NM) of the dynamical atmospheric system and their optimal forcing that govern precipitation variability, providing a dynamical underpinning for the empirically derived, recurrent rainfall patterns. The leading mode (NM1) features a shift in the Atlantic ITCZ, a drying over the Indian Ocean and Indian subcontinent, and a “South‐Flood‐North Drought (SFND)” pattern over Eastern China, a pattern inferable from changes in the atmospheric energy flow forced by the corresponding optimal forcing and further amplified by the water vapor and cloud feedbacks. Moreover, the slow evolving component of the NM1 in response to CO2forcing can be skillfully predicted by running the ocean heat uptake through the LRF, implicating a deterministic ocean dynamic source for the NM1‐like trend under climate warming. The Asian monsoon has long been observed to exhibit coherent patterns of variability, but it remains to be seen if they represent the true dynamical modes of the climate system. By probing the ocean heat flux forcing for a climate model from all representative locations of the global ocean, this study constructs the systematic forcing‐response relationship for the boreal summer precipitation, therefrom the leading dynamical modes of variability known as neutral modes can be extracted. The result confirms that the previous empirically derived monsoon rainfall patterns, particularly the one comprising the “South‐Flood‐North Drought (SFND)” dipole over Eastern China, are dynamically based and explainable by the shift and/or pulsing of the global atmospheric energy flow. The dynamical mode with the SFND dipole as its integral part is found to be acutely responsive to an interhemispherically contrasting ocean heat flux, a likely consequence of the slowly evolving ocean heat uptake (OHU) under increased CO2forcing. The recurrent patterns of the Asian summer‐monsoon rainfall are shown to represent the dynamical modes of the climate systemThe patterns of the leading dynamical modes emerge as the result of the variations in the atmospheric energy flowThe projected long‐term trend towards the “South‐Flood‐North Drought” pattern in China may have a deterministic oceanic origin The recurrent patterns of the Asian summer‐monsoon rainfall are shown to represent the dynamical modes of the climate system The patterns of the leading dynamical modes emerge as the result of the variations in the atmospheric energy flow The projected long‐term trend towards the “South‐Flood‐North Drought” pattern in China may have a deterministic oceanic origin

The atmospheric component of Energy Exascale Earth System Model version 1 has included many new features in the physics parameterizations compared to its predecessors. Potential complex nonlinear interactions among the new features create a significant challenge for understanding the model behaviors and parameter tuning. Using the one‐at‐a‐time method, the benefit of tuning one parameter may offset the benefit of tuning another parameter, or improvement in one target variable may lead to degradation in another target variable. To better understand the Energy Exascale Earth System Model version 1 model behaviors and physics, we conducted a large number of short simulations (three days) in which 18 parameters carefully selected from parameterizations of deep convection, shallow convection, and cloud macrophysics and microphysics were perturbed simultaneously using the Latin hypercube sampling method. From the perturbed parameter ensemble simulations and use of different skill score functions, we identified the most sensitive parameters, quantified how the model responds to changes of the parameters for both global mean and spatial distribution, and estimated the maximum likelihood of model parameter space for a number of important fidelity metrics. Comparison of the parametric sensitivity using simulations of two different lengths suggests that perturbed parameter ensemble using short simulations has some bearing on understanding parametric sensitivity of longer simulations. Results from this analysis provide a more comprehensive picture of the Energy Exascale Earth System Model version 1 behavior. The difficulty in reducing biases in multiple variables simultaneously highlights the need of characterizing model structural uncertainty (so‐called embedded errors) to inform future development efforts. Short PPE simulations provide a comprehensive picture of the behaviors of a new Earth system model associated with uncertain parametersPPE using short simulations has some bearing on understanding parametric sensitivity of longer simulationsThe difficulty in reducing biases in multiple variables simultaneously highlights the need of characterizing model embedded errors

The sensitivity of the intertropical convergence zone (ITCZ) position to forcing patterns is important for understanding changes in tropical rainfall. A set of Green's function experiments reveals hat this sensitivity is asymmetric depending on which hemisphere (northern or southern) the forcing is applied. Northern hemisphere forcings produce a much larger response than southern hemisphere forcings of similar magnitude. The response of the ITCZ position to forcing can be broken into linear and nonlinear components, and it is shown that the asymmetry arises from the nonlinear component. The linear and nonlinear response components have similar magnitudes, but the pattern of the nonlinear component is insensitive to the location of the forcing, such that it amplifies the response to northern hemisphere forcings and dampens the response to southern hemisphere forcings. This asymmetry hints at an intrinsic mode of the climate system, depending on the current climate state. The tropical rain belt is a key component of the climate system, and understanding its response to heating and cooling patterns is a necessary step toward understanding tropical rainfall changes. When the atmosphere is heated or cooled in one hemisphere, flow of energy across the equator in the atmosphere shifts the tropical rain belt. Warming the northern hemisphere and cooling the southern hemisphere produces a much stronger (northward) shift of the rain belt than warming the southern hemisphere and cooling the northern hemisphere. The inclination toward a northward shift of the rain belt, irrespective of the location of heating (or cooling), likely arises from the intrinsic nonlinearity of the current climate. In other words, the asymmetry arises because the rain belt response depends on the state of the current climate, the interhemispheric asymmetry of which underpins the asymmetric response to external energy perturbations. The response of the ITCZ position to forcing is asymmetric depending on which hemisphere the forcing is applied inThe asymmetric ITCZ response comes from the nonlinear component of the response, which has a similar magnitude to the linear responseThe nonlinear response pattern is insensitive to the location of the forcing, but its magnitude is greatest for high‐latitude forcings