Flux Feedback Research Articles

Aggregation–Disaggregation Cycles in ERA5 Reanalysis

Abstract Understanding convective aggregation is very important for understanding tropical climate and climate sensitivity. However, we still lack a full understanding of how aggregation evolves in the real world or what phenomena and scales are analogous to the self-aggregation observed in idealized models. In this study, we apply the moist static energy (MSE) variance budget framework to ERA5 reanalysis data to study the evolution of large-scale aggregation over tropical oceans at basinwide scales. Our novel phase space diagnostics focus on the variability of observed aggregation compared to most previous self-aggregation studies, which focus more on the aggregated mean state. We visualize observed aggregation to evolve anomalously around a mean state in a cyclical fashion forming aggregation–disaggregation cycles. We find horizontal advection of MSE to play the primary role in determining when the domain aggregates or disaggregates. In contrast, all advective, radiative, and surface flux feedbacks are found important for determining the magnitude of the aggregation anomalies. Surface fluxes and horizontal advection tend to dampen aggregation anomalies, while radiative fluxes and vertical advection tend to amplify aggregation anomalies. Looking deeper into the advection terms, we find that changes in vertical advection are dominated by enhanced low-level subsidence over the dry regions during the more aggregated states. This creates an anomalous drying tendency over the dry regions, which maintains aggregation anomalies. In contrast, horizontal advection changes are found to be dominated by increased moisture advection out of the moist columns with stronger aggregation. Significance Statement The purpose of this study is to characterize and understand the evolution of large-scale convective aggregation in the real world through reanalysis data. While most previous observational studies have focused on the evolution of clouds and cloud populations with aggregation, we focus on the energetics and the impact of aggregation on redistributing moisture throughout the domain. Our framework highlights that aggregation can be visualized as a continuously occurring cyclic feature at large scales in the tropics. Further, our work provides a deeper insight into the changes in large-scale circulation that accompany aggregation and characterizes the similarities and differences between the different regions in the tropics.

Weakened large-scale surface heat flux feedback at midlatitudes under global warming.

The surface heat flux feedback, which refers to the response of surface heat flux anomaly to the underlying sea surface temperature anomaly (SSTA), is one of the key processes in air-sea interaction. It plays an important role in regulating various aspects of the climate system, ranging from local SSTA persistence to the global overturning circulation and major climate modes. Yet its change under greenhouse gas-induced warming remains unknown. Here, using an ensemble of global climate simulations under a high radiative forcing scenario, we demonstrate that the intensity of surface heat flux feedback for spatially large-scale SSTA at the midlatitudes is projected to halve by the end of the 21st century, compared to pre-industrial levels. Such weakening is primarily attributed to a more stabilized marine atmospheric boundary layer, which diminishes the air-sea thermal disequilibrium caused by SSTA. In a warming climate, the variance of midlatitude SSTA at large spatial scales is expected to be significantly enhanced in response to the weakened surface heat flux feedback.

Research on Magnetic Levitation Control Method under Elastic Track Conditions Based on Backstepping Method

The vehicle–guideway coupled self-excited vibration of maglev systems is a common control instability problem in maglev traffic while the train is suspended above flexible girders, and it seriously affects the suspension stability of maglev vehicles. In order to solve this problem, a nonlinear dynamic model of a single-point maglev system with elastic track is established in this paper, and a new and more stable adaptive backstepping control method combined with magnetic flux feedback is designed. In order to verify the control effect of the designed control method, a maglev vehicle–guideway coupled experimental platform with elastic track is built, and experimental verifications under rigid and elastic conditions are carried out. The results show that, compared with the state feedback controller based on the feedback linearization controller, the adaptive backstepping control law proposed in this paper can achieve stable suspension under extremely low track stiffness, and that it shows good stability and anti-interference abilities under elastic conditions. This work has an important meaning regarding its potential to benefit the advancement of commercial maglev lines, which may significantly enhance the stability of the maglev system and reduce the cost of guideway construction.

Effects of confined distance and wire spacing on flame spread over double electrical wires for the layout design in buildings

In this study, the flame merging and flame spread behaviors over two identical parallel electrical wires (the ratios of copper core diameter to entire wire diameter are: 6mm/10 mm and 8mm/12 mm for typeⅠand typeⅡ, respectively) were experimentally investigated under different wire spacings (3∼18 mm) and confined distances(10∼30 mm). The results show that the smaller confined distance D will promote the merging, making the wire spacing s increase for the fully merging. With the increase of D, the flame width and flame spread rate Vf will increase.While the flame height will be stretched at smaller D. And Vf is larger than that of single wire at the same D for different wire spacings.The interesting finding is that a pool fire is formed at the middle of the wire spacing when s and D are small, which will enhance the flame spread.The heat flux feedback of components including of the conductive heat flux q˙c″, the first single flame heat flux q˙f1″, the second flame heat flux q˙f2″, the pool fire heat flux q˙pf″ and the gypsum board heat flux q˙g″ are quantitatively analyzed. At the intermittent and non merging stages, the convective and radiative heat fluxes q˙vf2″ and q˙rf2″ will decrease largely due to the interaction weakened greatly. Based on the heat transfer analysis, a flame spread model is established, which can well predict the flame spread rate.These findings can give the useful suggestions on the layout design of electrical wires in confined space in buildings.

Optimal Design and Development of Magnetic Field Detection Sensor for AC Power Cable.

The state detection of power cables is very important to ensure the reliability of the power supply. Traditional sensors are mostly based on electric field detection. The operation is complex, and its efficiency needs to be improved. This paper optimizes the design and development of the magnetic field detection sensor for AC power cables. First, through the establishment of the magnetic field sensor model, it is determined that permalloy is the material of the magnetic core, the optimal aspect ratio of the magnetic core is 20, and the ratio of coil length to core length is 0.3. Second, the coil-simulation model is established, and it is determined that the optimal number of turns of the coil is 11,000 turns, the diameter of the enameled copper wire is 0.08 mm, and the equivalent magnetic field noise of the sensor is 0.06 pT. Finally, the amplifying circuit based on negative magnetic flux feedback is designed, the sensor is assembled, and the experimental circuit is built for the sensitivity test. The results show that the sensitivity of the magnetic field sensor is 327.6 mV/μT. The sensor designed in this paper has the advantages of small size, high sensitivity, ease of carry, and high reliability.

Analysis of mixed rhythm and its dynamics in closed-loop respiratory control system driven by electromagnetic induction

The pre-Bötzinger complex is a crucial region for generating respiratory rhythms in mammals. Peripheral chemoreceptors have a significant influence on respiratory rhythm by monitoring changes in blood oxygen concentration and carbon dioxide concentration. This study introduces a closed-loop respiratory control model, which is driven by electromagnetic induction and based on the activation of pre-Bötzinger complex neurons. The model incorporates various factors including the motor pool, lung volume, lung oxygen, blood oxygen, and chemoreceptors. The response of the system subjected to the same hypoxic perturbation under different electromagnetic induction is studied, and the control effect of magnetic flux feedback coefficient on the recovery of mixed rhythms is investigated. Using bifurcation analysis and numerical simulations, it is found that the magnetic flux feedback coefficient has a significant influence on the ability to recover respiratory rhythm. The dynamic mechanism of the magnetic flux feedback coefficient on different hypoxic responses in closed-loop systems are revealed. Dynamic analysis indicates that under certain electromagnetic induction, the mixed bursting rhythm in the closed-loop system can autoresuscitate if the bifurcation structure before and after applying hypoxia perturbation are completely identical. However, when the bifurcation structure before and after applying hypoxia perturbation are different, the mixed bursting rhythm in the system cannot autoresuscitate. In addition, for the cases where automatic recovery is not achieved under mild electromagnetic induction, increasing the magnetic flux feedback coefficient appropriately can lead the system to autoresuscitate, which is closely related to the Hopf bifurcation and fold bifurcation of limit cycle. This study contributes to understanding the influence of the interaction between the central respiratory and peripheral chemoreceptive feedback on respiratory rhythm, as well as the control effect of external induction on the hypoxic response.

Separating Direct Heat Flux Forcing and Freshwater Feedback on AMOC Change Under Global Warming

AbstractThe Atlantic meridional overturning circulation (AMOC) is predicted to weaken under global warming. Whether it is caused by heat flux or freshwater flux is under debate. Here we separate these two processes in changing the AMOC under global warming. The simulated AMOC is weakened during the first 600 years and then gradually recovered to its initial state, with heat flux and freshwater feedback dominating at different timescales. Global warming immediately puts freshwater into the Southern Ocean, which triggers the initial AMOC weakening via altering surface temperature. Concurrently, the extensive heat into the ocean surface increases the temperature over the subpolar North Atlantic, reducing the deep convection and thus the AMOC in the subsequent 50–150 years. Meanwhile, the Arctic sea ice melt leads to the AMOC shutdown. Subsequently, the salinity accumulation in the subtropical North Atlantic propagating northward to restart the North Atlantic deep convection is responsible for the AMOC recovery.

Investigation on unsteady thermal process of NEPE propellant based on an refined chemical kinetic model

This study presents a sophisticated investigation into the intricate heat and mass transfer mechanisms of NEPE composite propellant under realistic rocket motor conditions, with particular emphasis on the rapid depressurization effects. To accomplish this, we have developed an innovative particle packing algorithm coupled with a novel semi-global kinetics model. By integrating detailed chemical kinetics models for Ammonium perchlorate (AP) oxidizing powders, Cyclotetramethylene tetranitramine (HMX) oxidizing powders, and Nitroglycerin/1,2,4-Butane triol trinitrate (NG/BTTN) fuel-binder, we have constructed a meso-scale model of NEPE propellant based on comprehensive experimental data. Furthermore, a refined 9-step kinetic mechanism has been incorporated into the meso-scale model, along with a pressure drop model that accurately captures the intricate gas reaction domain. The predicted parameters of NEPE propellant have been meticulously compared with experimental results, demonstrating a remarkable level of agreement. Moreover, meticulous analyses have been carried out to investigate the complex thermal processes exhibited by this composite propellant under the challenging conditions of rapid depressurization combustion. These analyses primarily focus on unraveling the dynamic evolution of the gas-solid phase and the underlying thermal regression mechanism of the solid phase. Notably, our findings reveal intriguing temporal changes in local heat flux feedback spikes throughout the entire depressurization process, owing to the intricate topology structure of NEPE propellant, which comprises multiscale oxidizing micro-particles embedded within a fuel-binder matrix. This research significantly enhances our understanding of the thermal behavior and performance characteristics of NEPE composite propellant, thereby paving the way for future advancements in rocket propulsion technology.

Regimes of Convective Self-Aggregation in Convection-Permitting Beta-Plane Simulations

Abstract The spontaneous self-aggregation (SA) of convection in idealized model experiments highlights the importance of interactions between tropical convection and the surrounding environment. The authors have shown that SA fundamentally changes with the background rotation in previous f-plane simulations, in terms of both the resulting forms of organized convection and the relative roles of the physical feedbacks driving them. This study considers the dependence of SA on rotation in one large domain on the β plane, introducing an additional layer of complexity. Simulations are performed with uniform thermal forcing and explicit convection. Focuses include statistical and structural analysis of the convective modes, process-oriented diagnostics of how they develop, and resulting mean states. Two regimes of SA emerge within the first 15 days, separated by a critical zone where f is analogous to 10°–15° latitude. Organized convection at near-equatorial values of f primarily consists of convectively coupled Kelvin waves. Wind speed–surface enthalpy flux feedbacks are the dominant process driving moisture variability early on, then clear-sky shortwave radiative feedbacks are strongest in wave maintenance. In contrast, at higher f, numerous tropical cyclones develop and coexist, dominated by surface flux and longwave processes. Tropical cyclogenesis is most pronounced at intermediate f (analogous to 25°–40°), but are longer-lived at higher f. The resulting modes of SA at low f differ between these β-plane simulations (convectively coupled waves) and prior f-plane simulations (weak tropical cyclones or nonrotating clusters). Otherwise, these results provide further evidence for the changing roles of radiative, surface flux, and advective processes in influencing SA as f changes, as found in our previous study. Significance Statement In model simulations, convection often self-organizes due to interactions with its surrounding environment. These interactions are relevant in the real-world organization of rainfall and clouds, and may thus be useful to understand for improved prediction of tropical weather and climate. Previous work using a set of simple model experiments with constant Coriolis force showed that at different latitudes, different processes dominate, and different types of organized convection result. This study verifies that finding using a more complex and realistic model, where the Coriolis force varies within the domain to resemble different latitudes. Specifically, the convection here self-organizes into atmospheric waves (periodic disturbances) at low latitudes, and tropical cyclones at high latitudes.

Process-Oriented Diagnosis of Tropical Cyclones in Reanalyses Using a Moist Static Energy Variance Budget

Abstract Global models are frequently used for tropical cyclone (TC) prediction and climate projections but have biases in their representation of TCs that are not fully understood. The objective of this work is to assess how well and how robustly physical processes that are important for TC development are represented in modern reanalysis products and to consider whether reanalyses can serve as an observationally constrained reference against which model representation of these physical processes can be evaluated. Differences in the representation of large-scale environmental variables relevant to TC development do not readily explain the spread in TC climatologies across climate models, as found in prior work, or across reanalysis datasets, as shown here. This motivates the use of process-oriented diagnostics that focus on how convection, moisture, clouds, and related processes are coupled and can be used to identify areas to target for model improvement. Using the column-integrated moist static energy (MSE) variance budget, we analyze radiative and surface flux feedbacks across five different reanalyses. We construct an intensity-bin composite of the MSE variance budget to compare storms of similar intensity. Our results point to some fundamental differences across reanalyses in how they represent MSE variance and surface flux and radiative feedbacks in TCs, which could contribute to differences across reanalyses in how they represent TCs, but other factors also likely contribute. Any future work that evaluates these diagnostics in GCMs against reanalyses should do so cautiously, and efforts should be undertaken to provide a true observational estimate of these processes.

Realgar-Induced Neurotoxicity: Crosstalk Between the Autophagic Flux and the p62-NRF2 Feedback Loop Mediates p62 Accumulation to Promote Apoptosis.

Realgar is a traditional Chinese medicine that contains arsenic. It has been reported that the abuse of medicine-containing realgar has potential central nervous system (CNS) toxicity, but the toxicity mechanism has not been elucidated. In this study, we established an in vivo realgar exposure model and selected the end product of realgar metabolism, DMA, to treat SH-SY5Y cells in vitro. Many assays, including behavioral, analytical chemistry, and molecular biology, were used to elucidate the roles of the autophagic flux and the p62-NRF2 feedback loop in realgar-induced neurotoxicity. The results showed that arsenic could accumulate in the brain, causing cognitive impairment and anxiety-like behavior. Realgar impairs the ultrastructure of neurons, promotes apoptosis, perturbs autophagic flux homeostasis, amplifies the p62-NRF2 feedback loop, and leads to p62 accumulation. Further analysis showed that realgar promotes the formation of the Beclin1-Vps34 complex by activating JNK/c-Jun to induce autophagy and recruit p62. Meanwhile, realgar inhibits the activities of CTSB and CTSD and changes the acidity of lysosomes, leading to the inhibition of p62 degradation and p62 accumulation. Moreover, the amplified p62-NRF2 feedback loop is involved in the accumulation of p62. Its accumulation promotes neuronal apoptosis by upregulating the expression levels of Bax and cleaved caspase-9, resulting in neurotoxicity. Taken together, these data suggest that realgar can perturb the crosstalk between the autophagic flux and the p62-NRF2 feedback loop to mediate p62 accumulation, promote apoptosis, and induce neurotoxicity. Realgar promotes p62 accumulation to produce neurotoxicity by perturbing the autophagic flux and p62-NRF2 feedback loop crosstalk.

MJO in Different Orbital Regimes: Role of the Mean State in the MJO’s Amplitude during Boreal Winter

Abstract The Madden–Julian oscillation (MJO) exhibits pronounced seasonality, with one of the key unanswered questions being the following: what controls the maximum in MJO precipitation variance in the Southern Hemisphere during boreal winter? In this study, we examine a set of global climate model simulations in which the eccentricity and precession of Earth’s orbit are altered to change the boreal winter mean state in an attempt to reveal the processes that are responsible for the MJO’s amplitude in the boreal winter. In response to the forced insolation changes, the north–south asymmetry in sea surface temperature is amplified in boreal fall, which intensifies the Hadley circulation in boreal winter. The stronger Hadley circulation yields higher mean precipitation and stronger mean lower-tropospheric westerlies in the southern part of the Indo-Pacific warm pool. The MJO precipitation variability increases significantly where the mean precipitation and lower-tropospheric westerlies strengthen. In the column-integrated moisture budget of the simulated MJO, only surface latent heat flux feedback shows a trend that is consistent with the MJO’s amplitude, suggesting an important role for the surface latent heat flux feedback in the MJO’s amplitude during the boreal winter. An analysis of the moisture–precipitation relationship in the simulations shows that the increase in the mean precipitation lowers the convective moisture adjustment time scale, leading to the increase in precipitation variance. Our results suggest that the mean-state precipitation plays a critical role in the maintenance mechanism of the MJO.

ENSO‐induced decadal variability in the tropical Pacific subsurface in CMIP6 models

AbstractThe tropical Pacific decadal variability (TPDV) is an important component of the global interdecadal variability. Previous studies have shown that the TPDV in the sea surface temperature (SST) has two clear patterns, the El Niño–Southern Oscillation (ENSO)‐like and ENSO‐induced patterns, but the pattern of TPDV in the ocean subsurface is still a matter of debate in observations and models. The present study analyses the subsurface TPDV in the simulations of 26 CMIP6 models. The ENSO‐like and ENSO‐induced TPDVs in the subsurface are defined by the regression of the interdecadal anomalies of the oceanic subsurface temperature (Tsub) onto the PCs of two leading EOF modes of the interdecadal SST anomalies in the tropical Pacific. The pattern of the ENSO‐like TPDV in the subsurface shows high model consistency, whereas the ENSO‐induced TPDV in the subsurface has two distinct modes among the models, one with a centre in the central Pacific and the other showing a zonal dipole in the equatorial Pacific. The zonal pattern of the ENSO‐induced TPDV in the subsurface is mainly induced by the SST skewness in the equatorial eastern Pacific, which is further related to the surface heat flux feedback during La Niña.

Tropical instability waves in a high-resolution oceanic and coupled GCM

Tropical instability waves (TIWs) are the dominant mesoscale variability in the eastern equatorial Pacific Ocean. TIWs have direct impacts on the local hydrology, biochemistry and atmospheric boundary layer, and feedback on ocean circulations and climate variability. In this study, the basic characteristics of Pacific Ocean TIWs simulated by an eddy-resolving ocean model and a coupled general circulation model are evaluated. The simulated TIW biases mainly result from the mean climatology state, as TIWs extract eddy energy from the mean potential and kinetic energy. Both the oceanic and coupled models reproduce the observed westward propagating large-scale Rossby waves between approximately 2°–8°N, but the simulated TIWs have shorter wavelengths than the observed waves due to the shallower thermocline. Meanwhile, the weak meridional shears of background zonal currents and the less-tilted pycnocline in these two models compared to the observations causes weak barotropic and baroclinic instability, which decreases the intensity of the simulated TIWs. We then contrast the TIWs from these two models and identify the roles of atmospheric feedback in modulating TIWs. The latent heat flux feedback is similar to observation in the coupled model but absent in the ocean model, contributing to the stronger standard deviation (STD) of the TIW SST in the ocean model. The ocean model is not able to capture realistic air–sea interaction processes when forced with prescribed atmospheric forcing. However, the misrepresented atmospheric feedback in the ocean model tends to decrease the sea surface height (SSH) variability, and the current feedback damping effect is stronger in the ocean model than in the coupled model. Combined with weaker barotropic conversion rate and baroclinic conversion rate in the ocean model than in the coupled model, the STD of the TIW SSH in the ocean model is weaker.

Spatio-temporal variability of surface turbulent heat flux feedback for mesoscale sea surface temperature anomaly in the global ocean

The surface turbulent heat flux feedback αT plays an important role in the atmosphere–ocean coupling. However, spatio-temporal variability of αT for sea surface temperature anomaly (SSTA) at oceanic mesoscales in the global ocean remains poorly assessed. In this study, we tackle this issue using an advanced statistical model, i.e., the geographically and temporally weighted regression model. The estimated time-mean αT for mesoscale SSTA generally ranges from 10 to 50 W/(m2 K) within 70°S–70°N, except in the Antarctic coastal region where its value drops to zero. The αT is larger in the tropics than in off-tropical regions and locally enhanced in the equatorial cold tongues, western boundary currents, and their extensions. The spatial structure αT is primarily attributed to the non-linearity in the Clausius–Clapeyron relation and inhomogeneity in the background wind speed, whereas adjustment of surface wind speed, air temperature, or moisture to mesoscale SSTA plays an important role in the regional variability. There is an evident seasonal cycle of αT in the tropics and under the northern hemisphere’s storm tracks. The former is due to the seasonally varying response of surface wind speed to mesoscale SSTA, and the latter results from the seasonality of atmospheric and oceanic background states. Our analysis reveals prominent spatio-temporal variability of αT for mesoscale SSTA governed by complicated dynamics.

ENSO Asymmetry in CMIP6 Models

Abstract An interesting aspect of the El Niño–Southern Oscillation (ENSO) phenomenon is the asymmetry between its two phases. This paper evaluates the simulations of this property of ENSO by the Coupled Model Intercomparison Project phase 6 (CMIP6) models. Both the surface and subsurface signals of ENSO are examined for this purpose. The results show that the models still underestimate ENSO asymmetry as shown in the SST field, but do a better job in the subsurface. A much weaker negative feedback from the net surface heat flux during La Niña in the models is identified as a factor causing the degradation of the ENSO asymmetry at the surface. The simulated asymmetry in the subsurface is still weaker than the observations owing to a weaker dynamic coupling between the atmosphere and ocean. Consistent with the finding of a weaker dynamic coupling strength, the precipitation response to the SST changes is also found to be weaker in the models. The results underscore that a more objective assessment of the simulation of ENSO by climate models may have to involve the examination of the subsurface signals. Future improvements in simulating ENSO will likely require a better simulation of the surface heat flux feedback from the atmosphere as well as the dynamical coupling strength between the atmosphere and ocean. Significance Statement The ENSO phenomenon affects weather and climate worldwide. An interesting aspect of this phenomenon is the asymmetry between its two phases. Previous studies have reported a weaker asymmetry in the simulations by climate models. But these studies have focused on the ENSO asymmetry at the surface. Here by examining the ENSO asymmetry at the surface and the subsurface, we have found that ENSO asymmetry is better simulated in the subsurface than at the surface. We have also identified factors that are responsible for the degradation of the ENSO asymmetry at the surface as well as the remaining weakness in the subsurface, pointing out specific pathways to take to further improve ENSO simulations by coupled climate models.

Moisture Recharge–Discharge Cycles: A Gross Moist Stability–Based Phase Angle Perspective

Abstract Moist static energy (MSE) budgets and gross moist stability (GMS) have been widely used as a diagnostic tool to study the evolution of moisture and convection at different time scales. However, use of GMS is limited at shorter time scales because many points in the tropics have close-to-zero large-scale vertical motion at a given time. This is particularly true in the case of convective life cycles, which have been shown to exist with noise-like ubiquity throughout the tropics at intraseasonal time scales. This study proposes a novel phase angle–based framework as a process-level diagnostic tool to study the MSE budgets during these cycles. Using the GMS phase plane, a phase angle parameter is defined, which converts the unbound GMS into a finite ranged variable. The study finds that the convective life cycles are closely linked to evolution of moisture and effectively behave as moisture recharge–discharge cycles. Convective cycles in different datasets are studied using TOGA COARE, a mix of different satellite products and ERA-Interim. Analysis of the MSE budget reveals that the cyclic behavior is a result of transitions between wet and dry equilibrium states and is similar across different regions. Further, vertical and horizontal advection of MSE are found to act as the primary drivers behind this variability. In contrast, nonlinearities in the radiative and surface flux feedbacks are found to resist the convective evolution. A linearized model consistent with moisture mode dynamics is able to replicate the recharge–discharge cycle variability in TOGA COARE data. Significance Statement In the tropics, variability of moisture and rainfall are closely linked to each other. Through this study we aim to better understand the evolution of moisture in observed daily time series data. We present a novel phase angle–based diagnostic tool to represent and study the energy budget of the system at this time resolution. Our results suggest that similar processes and mechanisms are relevant across different regions and at different scales in the tropics with moisture dynamics being important for these processes. Further, a key role is played by the energy transport associated with the large-scale circulation that drives moisture evolution in a cyclic pattern.

Asymmetric daytime and nighttime surface temperature feedback induced by crop greening across Northeast China

Mid-high latitude Northeast China witnessed significant crop greening from 2001 to 2020, as evidenced by satellite records and field observations. The land surface temperature of croplands during the growing season showed a decreasing trend, suggesting negative surface temperature feedback to crop greening of agricultural ecosystems in mid-high latitude Northeast China. Using time-series remote sensing products and long-term scenario simulations, the present study highlights that crop greening can slow climate warming. Our study noted a stronger surface cooling effect induced by crop greening during the growing season in the day than at the night, which contributed to asymmetric diurnal temperature cycle changes in Northeast China. In addition, our biophysical mechanism analysis revealed aerodynamic and surface resistances as the major driving factors for the daytime land surface temperature (LST) cooling effect induced by crop greening, while the ground heat flux and ambient temperature feedback as the major attributes of the nighttime LST cooling impact due to crop greening.

ENSO atmospheric feedbacks under global warming and their relation to mean-state changes

Two atmospheric feedbacks play an important role in the dynamics of the El Niño/Southern Oscillation (ENSO), namely the amplifying zonal wind feedback and the damping heat flux feedback. Here we investigate how and why both feedbacks change under global warming in climate models participating in the 5th and 6th phase of the Coupled Model Intercomparison Project (CMIP5 and CMIP6) under the business-as-usual scenario (RCP8.5 and SSP5-8.5, respectively). The amplifying zonal wind feedback over the western equatorial Pacific (WEP) becomes significantly stronger in two third of the models, on average by 12 ± 7% in these models. The heat flux damping feedback over the eastern and central equatorial Pacific (EEP and CEP, respectively) increases as well in nearly all models, with the damping effect increasing on average by 18 ± 11%. The simultaneous strengthening of the two feedbacks can be explained by the stronger warming in the EEP relative to the WEP and the off-equatorial regions, which shifts the rising branch of the Pacific Walker Circulation to the east and increases the mean convection over the CEP. This in turn leads to a stronger vertical wind response during ENSO events over the CEP that strengthens both atmospheric feedbacks. We separate the climate models into sub-ensembles with STRONG and WEAK ENSO atmospheric feedbacks, as 2/3 of the models underestimate both feedbacks under present-day conditions by more than 40%, causing an error compensation in the ENSO dynamics. The biased mean state in WEAK in 20C constrains the ENSO atmospheric feedback projection in 21C, as the models of the WEAK sub-ensemble also have weaker ENSO atmospheric feedbacks in 21C. Further, due to the more realistic dynamics and teleconnections, we postulate that one should have more confidence in the ENSO predictions with models belonging to the STRONG sub-ensemble. Finally, we analyze the relation between ENSO amplitude change and ENSO atmospheric feedback change. We find that models simulating an eastward shift of the zonal wind feedback and increasing precipitation over the EEP during Eastern Pacific El Niño events tend to predict a larger ENSO amplitude in response to global warming.

Greenhouse-gas forced changes in the Atlantic meridional overturning circulation and related worldwide sea-level change

The effect of anthropogenic climate change in the ocean is challenging to project because atmosphere-ocean general circulation models (AOGCMs) respond differently to forcing. This study focuses on changes in the Atlantic Meridional Overturning Circulation (AMOC), ocean heat content (DeltaOHC), and the spatial pattern of ocean dynamic sea level (Delta zeta). We analyse experiments following the FAFMIP protocol, in which AOGCMs are forced at the ocean surface with standardised heat, freshwater and momentum flux perturbations, typical of those produced by doubling hbox {CO}_{{2}}. Using two new heat-flux-forced experiments, we find that the AMOC weakening is mainly caused by and linearly related to the North Atlantic heat flux perturbation, and further weakened by a positive coupled heat flux feedback. The quantitative relationships are model-dependent, but few models show significant AMOC change due to freshwater or momentum forcing, or to heat flux forcing outside the North Atlantic. AMOC decline causes warming at the South Atlantic-Southern Ocean interface. It does not strongly affect the global-mean vertical distribution of DeltaOHC, which is dominated by the Southern Ocean. AMOC decline strongly affects Delta zeta in the North Atlantic, with smaller effects in the Southern Ocean and North Pacific. The ensemble-mean Delta zeta and DeltaOHC patterns are mostly attributable to the heat added by the flux perturbation, with smaller effects from ocean heat and salinity redistribution. The ensemble spread, on the other hand, is largely due to redistribution, with pronounced disagreement among the AOGCMs.