Fast versus slow response in climate change: implications for the global hydrological cycle

Recent studies have shown that changes in global mean precipitation are larger for solar forcing than for CO₂ forcing of similar magnitude. In this paper, we use an atmospheric general circulation model to show that the differences originate from differing fast responses of the climate system. We estimate the adjusted radiative forcing and fast response using Hansen’s “fixed-SST forcing” method. Total climate system response is calculated using mixed layer simulations using the same model. Our analysis shows that the fast response is almost 40% of the total response for few key variables like precipitation and evaporation. We further demonstrate that the hydrologic sensitivity, defined as the change in global mean precipitation per unit warming, is the same for the two forcings when the fast responses are excluded from the definition of hydrologic sensitivity, suggesting that the slow response (feedback) of the hydrological cycle is independent of the forcing mechanism. Based on our results, we recommend that the fast and slow response be compared separately in multi-model intercomparisons to discover and understand robust responses in hydrologic cycle. The significance of this study to geoengineering is discussed.     

Relative roles of the equatorial upper ocean zonal current and thermocline in determining the timescale of the tropical climate system

Summary In this study, it is demonstrated that the amplitude of the equatorial upper-ocean zonal current anomaly induced by the fast-varying wind forcing (shorter than a year) is much greater than that induced by the slowly varying wind forcing (longer than 2 year), and the center of maximum zonal current anomaly shifts from the central Pacific to the western Pacific with an increase in the timescale of wind forcing. As a result, the zonal advective feedback (the zonal advection of mean sea surface temperature by anomalous current) in a slowly varying climate system becomes weaker and barely induces a low-frequency mode such as El Niño-Southern Oscillation. On the other hand, both amplitude and zonal location of the maximum thermocline anomaly are little changed by the change in the timescale of wind forcing – confined at the strong equatorial upwelling region of the eastern Pacific. Accordingly, the thermocline feedback (the vertical advection of anomalous subsurface temperature by the mean upwelling) is more favorable to generate a low-frequency mode.The relative roles of these two feedbacks are further explored under the coupled-system context. The eigen analysis of the stripped-down version of an intermediate ocean-atmosphere coupled model shows that by altering the regime space from the weakly coupled to the strongly coupled, the dominant process that leads the leading eigen mode changes from the zonal advective feedback to the thermocline feedback, and at the same time the frequency of the leading mode also changes from the high-frequency to the low-frequency. It implies that each feedback tends to favor the different timescale coupled mode.     

The impact of natural and anthropogenic forcings on climate and hydrology since 1550

A climate simulation of an ocean/atmosphere general circulation model driven with natural forcings alone (constant “pre-industrial” land-cover and well-mixed greenhouse gases, changing orbital, solar and volcanic forcing) has been carried out from 1492 to 2000. Another simulation driven with natural and anthropogenic forcings (changes in greenhouse gases, ozone, the direct and first indirect effect of anthropogenic sulphate aerosol and land-cover) from 1750 to 2000 has also been carried out. These simulations suggest that since 1550, in the absence of anthropogenic forcings, climate would have warmed by about 0.1 K. Simulated response is not in equilibrium with the external forcings suggesting that both climate sensitivity and the rate at which the ocean takes up heat determine the magnitude of the response to forcings since 1550. In the simulation with natural forcings climate sensitivity is similar to other simulations of HadCM3 driven with CO₂ alone. Climate sensitivity increases when anthropogenic forcings are included. The natural forcing used in our experiment increases decadal–centennial time-scale and large spatial scale climate variability, relative to internal variability, as diagnosed from a control simulation. Mean conditions in the natural simulation are cooler than in our control simulation reflecting the reduction in forcing. However, over certain regions there is significant warming, relative to control, due to an increase in forest cover. Comparing the simulation driven by anthropogenic and natural forcings with the natural-only simulation suggests that anthropogenic forcings have had a significant impact on, particularly tropical, climate since the early nineteenth century. Thus the entire instrumental temperature record may be “contaminated” by anthropogenic influences. Both the hydrological cycle and cryosphere are also affected by anthropogenic forcings. Changes in tree-cover appear to be responsible for some of the local and hydrological changes as well as an increase in northern hemisphere spring snow cover.

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Efficient Anomalous Forcings for Linear Problems

For linear forcing problems, a method is developed to provide a set of forcing modes which form a complete orthonormal basis for the finite-time response to steady forcing in the energy inner product space. The forcing modes are found by calculating eigenvectors of a positive definite and symmetric matrix determined from given equations of motion. The amplitude of responses to forcing modes is given in terms of the associated eigenvalues. This method is used in a nondivergent barotropic model linearized about the 300 hPa zonally-varying climatological flow both for northern summertime and wintertime. The results show that the amplitude of response varies considerably with different forcing modes. Only a few of forcing modes associated with the leading eigenvalues, called efficient forcing mode, can excite significant response. The efficient forcing modes possess highly localized spatial structure with wavetrain appearance. Most of the efficient forcings are located to the south of regions of the jet cor     

Relative roles of climate sensitivity and forcing in defining the ocean circulation response to climate change

The response of the ocean’s meridional overturning circulation (MOC) to increased greenhouse gas forcing is examined using a coupled model of intermediate complexity, including a dynamic 3-D ocean subcomponent. Parameters are the increase in CO₂ forcing (with stabilization after a specified time interval) and the model’s climate sensitivity. In this model, the cessation of deep sinking in the north “Atlantic” (hereinafter, a “collapse”), as indicated by changes in the MOC, behaves like a simple bifurcation. The final surface air temperature (SAT) change, which is closely predicted by the product of the radiative forcing and the climate sensitivity, determines whether a collapse occurs. The initial transient response in SAT is largely a function of the forcing increase, with higher sensitivity runs exhibiting delayed behavior; accordingly, high CO₂-low sensitivity scenarios can be assessed as a recovering or collapsing circulation shortly after stabilization, whereas low CO₂-high sensitivity scenarios require several hundred additional years to make such a determination. We also systemically examine how the rate of forcing, for a given CO₂ stabilization, affects the ocean response. In contrast with previous studies based on results using simpler ocean models, we find that except for a narrow range of marginally stable to marginally unstable scenarios, the forcing rate has little impact on whether the run collapses or recovers. In this narrow range, however, forcing increases on a time scale of slow ocean advective processes results in weaker declines in overturning strength and can permit a run to recover that would otherwise collapse.     

Tropical precipitation regimes and mechanisms of regime transitions: contrasting two aquaplanet general circulation models

The atmospheric general circulation models ARPEGE-climate and LMDz are used in an aquaplanet configuration to study the response of a zonally symmetric atmosphere to a range of sea surface temperature (SST) forcing. We impose zonally-symmetric SST distributions that are also symmetric about the equator, with varying off-equatorial SST gradients. In both models, we obtain the characteristic inter-tropical convergence zone (ITCZ) splitting that separates two regimes of equilibrium (in terms of precipitations): one with one ITCZ over the equator for large SST gradients in the tropics, and one with a double ITCZ for small tropical SST gradients. Transition between these regimes is mainly driven by changes in the low-level convergence that are forced by the SST gradients. Model-dependent, dry and moist feedbacks intervene to reinforce or weaken the effect of the SST forcing. In ARPEGE, dry advective processes reinforce the SST forcing, while a competition between sensible heat flux and convective cooling provides a complex feedback on the SST forcing in the LMDz. It is suggested that these feedbacks influence the location of the transition in the parameter range.     

Response of the Asian summer Monsoons to a high-latitude thermal forcing: mechanisms and nonlinearities

This study investigates mechanisms and nonlinearities in the response of the Asian Summer Monsoons (ASM) to high-latitude thermal forcings of different amplitudes. Using a suite of runs carried out with an intermediate-complexity atmospheric general circulation model, we find that the imposed forcings produce a strong precipitation response over the eastern ASM but a rather weak response over the southern ASM. The forcing also causes a precipitation dipole with wet conditions over the eastern Tibetan Plateau (TP) and dry conditions over the Bay of Bengal (BoB) and southeast Asia. A moderate increase of precipitation along the southern margin of the TP is also produced. Simulations designed to isolate the causal mechanisms show that thermodynamic interactions involving the tropical surface oceans are far less important than the water-vapour feedback for the transmission of information from the high-latitudes to the ASM. Additionally, we assess the nonlinearity of the ASM precipitation response to the forcing amplitude using a novel application of the empirical orthogonal function method. The response can be decomposed in two overlapping patterns. The first pattern represents a precipitation dipole with wet conditions over the eastern TP and dry conditions over BoB, which linearly increases with forcing amplitude becoming quasi-stationary for large forcing amplitudes (i.e. amplitudes leading to Arctic temperature anomalies larger than 10 °C). The second pattern is associated with increased precipitation over the southeastern TP and is nonlinearly dependent on forcing, being most important for intermediate forcing amplitudes (i.e. amplitudes leading to Arctic temperature anomalies between 5 and 10 °C).     

Variation of surface temperature during the last millennium in a simulation with the FGOALS-gl climate system model

A reasonable past millennial climate simulation relies heavily on the specified external forcings, including both natural and anthropogenic forcing agents. In this paper, we examine the surface temperature responses to specified external forcing agents in a millennium-scale transient climate simulation with the fast version of LASG IAP Flexible Global Ocean-Atmosphere-Land System model (FGOALS-gl) developed in the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics (LASG/IAP). The model presents a reasonable performance in comparison with reconstructions of surface temperature. Differentiated from significant changes in the 20th century at the global scale, changes during the natural-forcing-dominant period are mainly manifested in the Northern Hemisphere. Seasonally, modeled significant changes are more pronounced during the wintertime at higher latitudes. This may be a manifestation of polar amplification associated with sea-ice-temperature positive feedback. The climate responses to total external forcings can explain about half of the climate variance during the whole millennium period, especially at decadal timescales. Surface temperature in the Antarctic shows heterogeneous and insignificant changes during the preindustrial period and the climate response to external forcings is undetectable due to the strong internal variability. The model response to specified external forcings is modulated by cloud radiative forcing (CRF). The CRF acts against the fluctuations of external forcings. Effects of clouds are manifested in shortwave radiation by changes in cloud water during the natural-forcing-dominant period, but mainly in longwave radiation by a decrease in cloud amount in the anthropogenic-forcing-dominant period.     

Climate response to the physiological impact of carbon dioxide on plants in the Met Office Unified Model HadCM3

The concentration of carbon dioxide in the atmosphere acts to control the stomatal conductance of plants. There is observational and modelling evidence that an increase in the atmospheric concentration of CO₂ would suppress the evapotranspiration (ET) rate over land. This process is known as CO₂ physiological forcing and has been shown to induce changes in surface temperature and continental runoff. We analyse two transient climate simulations for the twenty-first century to isolate the climate response to the CO₂ physiological forcing. The land surface warming associated with the decreased ET rate is accompanied by an increase in the atmospheric lapse rate, an increase in specific humidity, but a decrease in relative humidity and stratiform cloud over land. We find that the water vapour feedback more than compensates for the decrease in latent heat flux over land as far as the budget of atmospheric water vapour is concerned. There is evidence that surface snow, water vapour and cloudiness respond to the CO₂ physiological forcing and all contribute to further warm the climate system. The climate response to the CO₂ physiological forcing has a quite different signature to that from the CO₂ radiative forcing, especially in terms of the changes in the temperature vertical profile and surface energy budget over land.     

On aspects of the concept of radiative forcing

 The concept of radiative forcing has been extensively used as an indicator of the potential importance of climate change mechanisms. It allows a first order estimate of the global-mean surface temperature change; and it is possible to compare forcings from different mechanisms, on the assumption that similar global-mean forcings produce similar global-mean surface temperature changes. This study illustrates two circumstances where simple models show that the conventional definition of radiative forcing needs refining. These problems arise mainly with the calculation of forcing due to stratospheric ozone depletion. The first part uses simple arguments to produce an alternative definition of radiative forcing, using a time-dependent stratospheric adjustment method, which can give different forcings from those calculated using the standard definition. A seasonally varying ozone depletion can produce a quite different seasonal evolution of forcing than fixed dynamical heating arguments would suggest. This is especially true of an idealised and extreme “Antarctic ozone hole” type scenario where a sudden loss of ozone is followed by a sudden recovery. However, for observed ozone changes the annually averaged forcing is usually within 5% of the forcing calculated using the fixed dynamical heating approximation. Another problem with the accepted view of radiative forcing arises from the definition of the tropopause considered in the second part of this study. For a correct radiative forcing estimate the “tropopause” needs to separate the atmosphere into regions with a purely radiative response and those with a radiative-convective response. From radiative-convective model results it is found that radiative equilibrium conditions persist for several kilometres below the tropopause (the tropopause being defined as where the lapse rate reaches 2 K km^-1). This region needs to be included in stratospheric adjustment calculations for an accurate calculation of forcing, as it is only the region between the surface and the top of the convection that can be considered as a single, forced, system. Including temperature changes in this region has a very large effect on stratospheric ozone forcing estimates, and can reduce the magnitude of the forcing by more than a factor of two. Although these experiments are performed using simple climate models, the results are of equal importance for the analysis of forcing-response relationships using general circulation models. Received: 25 October 1996/Accepted: 14 April 1997     

CLIMBER-2: a climate system model of intermediate complexity. Part II: model sensitivity

 A set of sensitivity experiments with the climate system model of intermediate complexity CLIMBER-2 was performed to compare its sensitivity to changes in different types of forcings and boundary conditions with the results of comprehensive models (GCMs). We investigated the climate system response to changes in freshwater flux into the Northern Atlantic, CO₂ concentration, solar insolation, and vegetation cover in the boreal zone and in the tropics. All these experiments were compared with the results of corresponding experiments performed with different GCMs. Qualitative, and in many respects, quantitative agreement between the results of CLIMBER-2 and GCMs demonstrate the ability of our climate system model of intermediate complexity to address diverse aspects of the climate change problem. In addition, we used our model for a series of experiments to assess the impact of some climate feedbacks and uncertainties in model parameters on the model sensitivity to different forcings. We studied the role of freshwater feedback and vertical ocean diffusivity for the stability properties of the thermohaline ocean circulation. We show that freshwater feedback plays a minor role, while changes of vertical diffusivity in the ocean considerably affect the circulation stability. In global warming experiments we analysed the impact of hydrological sensitivity and vertical diffusivity on the long-term evolution of the thermohaline circulation. In the boreal and tropical deforestation experiments we assessed the role of an interactive ocean and showed that for both types of deforestation scenarios, an interactive ocean leads to an additional cooling due to albedo and water vapour feedbacks. Received: 28 May 2000 / Accepted: 9 November 2000     

Response of North Pacific eastern subtropical mode water to greenhouse gas versus aerosol forcing

Mode water is a distinct water mass characterized by a near vertical homogeneous layer or low potential vorticity, and is considered essential for understanding ocean climate variability. Based on the output of GFDL CM3, this study investigates the response of eastern subtropical mode water(ESTMW) in the North Pacific to two different single forcings: greenhouse gases(GHGs) and aerosol. Under GHG forcing, ESTMW is produced on lighter isopycnal surfaces and is decreased in volume. Under aerosol forcing, in sharp contrast, it is produced on denser isopycnal surfaces and is increased in volume.The main reason for the opposite response is because surface ocean-to-atmosphere latent heat flux change over the ESTMW formation region shoals the mixed layer and thus weakens the lateral induction under GHG forcing, but deepens the mixed layer and thus strengthens the lateral induction under aerosol forcing. In addition, local wind changes are also favorable to the opposite response of ESTMW production to GHG versus aerosol.     

A probabilistic quantification of the anthropogenic component of twentieth century global warming

This paper examines in detail the statement in the 2007 IPCC Fourth Assessment Report that “Most of the observed increase in global average temperatures since the mid-twentieth century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations”. We use a quantitative probabilistic analysis to evaluate this IPCC statement, and discuss the value of the statement in the policy context. For forcing by greenhouse gases (GHGs) only, we show that there is a greater than 90 % probability that the expected warming over 1950–2005 is larger than the total amount (not just “most”) of the observed warming. This is because, following current best estimates, negative aerosol forcing has substantially offset the GHG-induced warming. We also consider the expected warming from all anthropogenic forcings using the same probabilistic framework. This requires a re-assessment of the range of possible values for aerosol forcing. We provide evidence that the IPCC estimate for the upper bound of indirect aerosol forcing is almost certainly too high. Our results show that the expected warming due to all human influences since 1950 (including aerosol effects) is very similar to the observed warming. Including the effects of natural external forcing factors has a relatively small impact on our 1950–2005 results, but improves the correspondence between model and observations over 1900–2005. Over the longer period, however, externally forced changes are insufficient to explain the early twentieth century warming. We suggest that changes in the formation rate of North Atlantic Deep Water may have been a significant contributing factor.     

赤道高频纬向风强迫对ENSO事件强度的影响

ENSO强度的影响因子是一个具有争议性的问题.作者探讨了一种理想的赤道高频纬向风强迫对ENSO强度的影响.将该问题转化为一类关于模式参数扰动的非线性最优化问题;基于所用的理论ENSO模式,研究了赤道高频纬向风强迫在调制ENSO强度中的角色.结果表明,对于E1 Ni(n)o和La Ni(n)a事件,存在两类外强迫,一类促进E1 Ni(n)o事件的发展却抑制La Ni(n)a事件的发展,另一类则抑制E1 Ni(n)o而促进La Ni(n)a事件的发展.这两类外强迫的主要区别在于初始相位的不同.相位决定了外强迫对ENSO事件是促进的还是抑制的,而外强迫的振幅和周期则决定了外强迫影响ENSO强度的大小.这些外强迫主要是通过海洋波动对斜温层深度的调节来影响ENSO事件的强度的.     

A STUDY ON PHYSICAL MECHANISM OF INTERANNUAL VARIATIONS OF LARGE-SCALE FLOW PATTERNS*

The thermal forcings of annual and interannual periodic variations are introduced into the barotropic vorticity equation,by using low order spectral model of the equation,more than 40 numerical experiments whose integration time is larger than 100 model years are performed in order to investigate variations of large-scale flow patterns arising from both external interannual thermal forcing and internal dynamical processes.In certain parametric range,when the frequency of the forcing term with interannual period equals to the frequency which is created by the internal dynamical processes alone,the amplitude of interannual variations of flow patterns increases obviously,and the period becomes double.In other parametric range,the amplitude of interannual variations of flow patterns shows abrupt changes and other nonlinear behavior,along with gradual changes of interannual forcing parameters.     

Assessing the sensitivity of the North Atlantic Ocean circulation to freshwater perturbation in various glacial climate states

A striking characteristic of glacial climate in the North Atlantic region is the recurrence of abrupt shifts between cold stadials and mild interstadials. These shifts have been associated with abrupt changes in Atlantic Meridional Overturning Circulation (AMOC) mode, possibly in response to glacial meltwater perturbations. However, it is poorly understood why they were more clearly expressed during Marine Isotope Stage 3 (MIS3, ~60?C27?ka BP) than during Termination 1 (T1, ~18?C10?ka BP) and especially around the Last Glacial Maximum (LGM, ~23?C19?ka BP). One clue may reside in varying climate forcings, making MIS3 and T1 generally milder than LGM. To investigate this idea, we evaluate in a climate model how ice sheet size, atmospheric greenhouse gas concentration and orbital insolation changes between 56?ka BP (=56k), 21k and 12.5k affect the glacial AMOC response to additional freshwater forcing. We have performed three ensemble simulations with the earth system model LOVECLIM using those forcings. We find that the AMOC mode in the mild glacial climate type (56k and 12.5k), with deep convection in the Labrador Sea and the Nordic Seas, is more sensitive to a constant 0.15?Sv freshwater forcing than in the cold type (21k), with deep convection mainly south of Greenland and Iceland. The initial AMOC weakening in response to freshwater forcing is larger in the mild type due to an early shutdown of Labrador Sea deep convection, which is completely absent in the 21k simulation. This causes a larger fraction of the freshwater anomaly to remain at surface in the mild type compared to the cold type. After 200?years, a weak AMOC is established in both climate types, as further freshening is compensated by an anomalous salt advection from the (sub-)tropical North Atlantic. However, the slightly fresher sea surface in the mild type facilitates further weakening of the AMOC, which occurs when a surface buoyancy threshold (?0.6?kg?m^?3 surface density anomaly to the 56k reference state) is stochastically crossed in the Nordic Seas. While described details are model-specific, our results imply that a more northern location of deep convection sites during milder glacial times may have amplified frequency and amplitude of abrupt climate shifts.     

大尺度流型年际变化可能机制的研究

本文将年变周期和非年变周期热力强迫项引进正压涡度方程,用方程的截谱形式实施了几十组时间长度为100年的数值积分,研究了非年变周期热力强迫和大气内部动力过程共同激发的大尺度流型的年际变化问题。在一定的参数集合,当非年变外源强迫的振荡周期与大气内部动力过程单独激发出来的振荡的周期相同时,流型年际振动的振幅明显加大,振动的周期则为倍化。在另外的参数集合,随着非年变强迫参数的渐变,流型年际变化的幅度显示出清楚的突变以及其它富有非线性特色的行为。     

Long-term climate response to stabilized and overshoot anthropogenic forcings beyond the twenty-first century

From multi-ensembles of climate simulations using the Community Climate System Model version 3, global climate changes have been investigated focusing on long-term responses to stabilized anthropogenic forcings. In addition to the standard forcing scenarios for the current international assessment, an overshoot scenario, where radiative forcings are decreased from one stabilized level to another, is also considered. The globally-averaged annual surface air temperature increases during the twenty-first century by 2.58 and 1.56°C for increased forcings under two future scenarios denoted by A1B and B1, respectively. These changes continue but at much slower rates in later centuries under forcings stabilized at year 2100. The overshoot scenario provides a different pathway to the lower B1 level by way of the greater A1B level. This scenario results in a surface climate similar to that in the B1 scenario within 100 years after the forcing reaches the B1 level. Contrasting to the surface changes, responses in the ocean are significantly delayed. It is estimated from the linear response theory that temperature changes under stabilized forcings to a final equilibrium state in the A1B (B1) scenario are factors of 0.3–0.4, 0.9, and 17 (0.3, 0.6, and 11) to changes during the twenty-first century, respectively, for three ocean layers of the surface to 100, 100–500, and 500 m to the bottom. Although responses in the lower ocean layers imply a nonlinear behavior, the ocean temperatures in the overshoot and B1 scenarios are likely to converge in their final equilibrium states.     

Importance of instantaneous radiative forcing for rapid tropospheric adjustment

To better understand CFMIP/CMIP inter-model differences in rapid low cloud responses to CO₂ increases and their associated effective radiative forcings, we examined the tropospheric adjustment of the lower tropospheric stability (LTS) in three general circulation models (GCMs): HadGEM2-A, MIROC3.2 medres, and MIROC5. MIROC3.2 medres showed a reduction in LTS over the sub-tropical ocean, in contrast to the other two models. This reduction was consistent with a temperature decrease in the mid-troposphere. The temperature decrease was mainly driven by instantaneous radiative forcing (RF) caused by an increase in CO₂. Reductions in radiative and latent heating, due to clouds, and in adiabatic and advective heating, also contribute to the temperature decrease. The instantaneous RF in the mid-troposphere in MIROC3.2 medres is inconsistent with the results of line-by-line (LBL) calculations, and thus it is considered questionable. These results illustrate the importance of evaluating the vertical profile of instantaneous RF with LBL calculations; improved future model performance in this regard should help to increase our confidence in the tropospheric adjustment in GCMs.     

Volcanos and El Niño: signal separation in Northern Hemisphere winter

The frequent coincidence of volcanic forcing with El Niño events disables the clear assignment of climate anomalies to either volcanic or El Niño forcing. In order to select the signals, a set of four different perpetual January GCM experiments was performed (control, volcano case, El Niño case and combined volcano/El Niño case) and studied with advanced statistical methods for the Northern Hemisphere winter. The results were compared with observations. The signals for the different forcings are discussed for three variables (temperature, zonal wind and geopotential height) and five levels (surface, 850 hPa, 500 hPa, 200 hPa and 50 hPa). The global El Niño signal can be selected more clearly in the troposphere than in the stratosphere. In contrast, the global volcano signal is strongest in the stratospheric temperature field. The amplitude of the perturbation for the volcano case is largest in the Atlantic region. The observed effect of local cooling due to the volcanic reduction of shortwave radiation over large land areas (like Asia) in subtropical regions, the observed advective warming over Eurasia and the advective cooling over Greenland are well simulated in the model. The radiative cooling near the surface is important for the volcano signal in the subtropics, but it is weak in high latitudes during winter. A statistically significant tropospheric signal of El Niño forcing occurs in the subtropics and in the midlatitudes of the North Pacific. The local anomalies in the El Niño forcing region in the tropics, and the warming over North America in middle and high latitudes are simulated as observed. The combined signal is different from a simple linear combination of the separate signals. It leads to a climate perturbation stronger than for forcing with El Niño or stratospheric aerosol alone and to a somewhat modified pattern.