A mixed-flux equilibrium asynchronous coupling scheme for accelerating convergence in ocean-atmosphere models

A coupled atmosphere/mixed-layer ocean energy balance model of intermediate complexity is used to examine the feasibility of a technique to accelerate the model's convergence rate while using equilibrium asynchronous coupling (EAC). EAC consists of an iterative sequence of integrations of an atmospheric model driven to equilibrium with fixed SSTs, the output of which is used to drive an ocean model to equilibrium. In an attempt to accelerate a radiatively perturbed climate to its final equilibrium state, we superimpose on the EAC scheme a mixed-flux condition, in which a portion of the turbulent air-sea heat flux from the most recent atmospheric leg is used in the present oceanic integration. In sensitivity tests using enhanced insolation, this mixed-flux approach strongly hastens the model's convergence rate, whose acceleration is regionally dependent: the tropics show the strongest sensitivity to the mixed-flux condition, while polar regions are least affected. This geographic variation stems from the presence of polar sea-ice, which promotes a temperature inversion and thereby causes a seasonal change in the sign of the total turbulent air-sea heat flux. Potential applications and limitations of this accelerated EAC method are also discussed.     

Simulation of long-term future climate changes with the green McGill paleoclimate model: the next glacial inception

The multi-component “green” McGill Paleoclimate Model (MPM), which includes interactive vegetation, is used to simulate the next glacial inception under orbital and prescribed atmospheric CO₂ forcing. This intermediate complexity model is first run for short-term periods with an increasing atmospheric CO₂ concentration; the model's response is in general agreement with the results of GCMs for CO₂ doubling. The green MPM is then used to derive projections of the climate for the next 100 kyr. Under a constant CO₂ level, the model produces three types of evolution for the ice volume: an imminent glacial inception (low CO₂ levels), a glacial inception in 50 kyr (CO₂ levels of 280 or 290 ppm), or no glacial inception during the next 100 kyr (CO₂ levels of 300 ppm and higher). This high sensitivity to the CO₂ level is due to the exceptionally weak future variations of the summer insolation at high northern latitudes. The changes in vegetation re-inforce the buildup of ice sheets after glacial inception. Finally, if an initial global warming episode of finite duration is included, after which the atmospheric CO₂ level is assumed to stabilize at 280, 290 or 300 ppm, the impact of this warming is seen only in the first 5 kyr of the run; after this time the response is insensitive to the early warming perturbation.     

Modeling the earth's climate

Mathematical models of the earth's climate provide intriguing opportunities to study a wide range of interdisciplinary problems involving processes within the climate system in a controlled and systematic manner. This paper is intended as a nontechnical review of climate modeling to enable researchers who are unfamiliar with the topic to better evaluate and judge the credibility of the model results. The types of climate models available for climate research are reviewed here, and four broad categories of climate models are identified. These range from the more simple energy balance models (EBMs) and radiative-convective models (RCMs), to the more complex statistical-dynamical models (SDMs), to the most powerful tools yet available for studying climate, the general circulation models (GCMs). This last category includes gridpoint and spectral GCMs. Four representations of the oceans which can be coupled to GCMs are described and include prescribed sea surface temperatures, an energy balance or swamp ocean, a mixed layer or slab ocean, or a fully computed ocean general circulation model. Selected examples considered representative of the types of studies possible with the various classes of models are given. Taken together, the spectrum of climate models provides a hierarchy of learning and research tools with which to effectively study the extremes of past climates, the vagaries of present-day climate, and possible climatic fluctuations well into the future.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Mechanism of formation of long-term directional climate changes in the past and current centuries

Climatic changes of the past century are diagnosed based on the generalized empirical data. Considered is a forcing impact of interannual inhomogeneity of the angular velocity of the Earth on the pressure field, atmospheric circulation, and radiation balance of the climate system. A physical mechanism of formation of global climate change tendencies as a response to the composition of greenhouse and rotational effects is described. Possible climate changes in the current century are assessed.     

Deep-ocean heat uptake and equilibrium climate response

We integrate the coupled climate model ECHAM5/MPIOM to equilibrium under atmospheric CO₂ quadrupling. The equilibrium global-mean surface-temperature change is 10.8 K. The surface equilibrates within about 1,200 years, the deep ocean within 5,000 years. The impact of the deep ocean on the equilibrium surface-temperature response is illustrated by the difference between ECHAM5/MPIOM and ECHAM5 coupled with slab ocean model (ECHAM5/SOM). The equilibrium global-mean surface temperature response is 11.1 K in ECHAM5/SOM and is thus 0.3 K higher than in ECHAM5/MPIOM. ECHAM5/MPIOM shows less warming over the northern-hemisphere mid and high latitudes, but larger warming over the tropical ocean and especially over the southern-hemisphere high latitudes. ECHAM5/MPIOM shows similar polar amplification in both the Arctic and the Antarctic, in contrast to ECHAM5/SOM, which shows stronger polar amplification in the northern hemisphere. The southern polar warming in ECHAM5/MPIOM is greatly delayed by Antarctic deep-ocean warming due to convective and isopycnal mixing. The equilibrium ocean temperature warming under CO₂ quadrupling is around 8.0 K and is near-uniform with depth. The global-mean steric sea-level rise is 5.8 m in equilibrium; of this, 2.3 m are due to the deep-ocean warming after the surface temperature has almost equilibrated. This result suggests that the surface temperature change is a poor predictor for steric sea-level change in the long term. The effective climate response method described in Gregory et al. (2004) is evaluated with our simulation, which shows that their method to estimate the equilibrium climate response is accurate to within 10 %.     

Atmospheric circulation climate changes

The role of the atmospheric circulation in climate change is examined. A review is given of the information available in the past record on the atmosheric circulation and its role in climate change, firstly at the surface via sea level pressure in both the northern and southern hemispheres and secondly for the free atmosphere. As with most climate information, the climate record is compromised by non-physical inhomogeneities arising from changes in observing and analyzing techniques and changes in data coverage. Problems with and threats to the rawinsonde network are discussed. Global analyses produced by the operational centers, U.S. National Meteorological Center (NMC) and the European Centre for Medium Range Weather Forecasts (ECMWF), for weather forecasting purposes contain many discontinuous changes in the analyses arising from improvements in the system used to produce them. A discussion is given of the prospects for and motivation behind an activity known as reanalysis in which the historical data are reanalyzed using a state-of-the-art system that is held constant for the entire record. The only sources of spurious change then are the changes in the observing system, such as the introduction of space-based observations. Recommendations are made on needed actions for better understanding and monitoring climate change.The role of the atmospheric circulation and the strong links to other variables such as temperature, precipitation and wind are established and illustrated with a survey of decadal variability, the evidence for it, and the way in which the observed atmospheric circulation is involved in the Pacific and Atlantic sectors. The importance of teleconnections is stressed, especially in the winter half year, for understanding local climate change. The likelihood that changes will be manifested in the frequency and intensity of preferred modes of behavior in the atmosphere, such as the El Niño-Southern Oscillation and Pacific-North American teleconnection patterns, rather than in changes in the modes is also emphasized. The recently observed climate changes and the tendency for an unprecedented prolonged El Niño are interpreted in this framework. The key coupled atmosphere-ocean character of decadal variability is noted with the atmosphere providing the spatial scales, the ocean the memory, but also with the need for collaborative, as opposed to destructive, interactions through the atmospheric circulation.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Critical issues for long-term climate monitoring

Climatic Change - Even after extensive re-working of past data, in many instances we are incapable of resolving important aspects concerning climate change and variability. Virtually every...     

Techniques for asynchronous and periodically synchronous coupling of atmosphere and ocean models

A periodically synchronous scheme suitable for coupling atmosphere and ocean models with high internal variability is presented. The performance of the scheme is tested by means of a simple zero-dimensional non-linear energy balance model with stochastic forcing. The equilibrium behaviour and the response to changes in the model parameters are analysed. The response experiments are similar to CO₂ doubling and transient CO₂ experiments. The best results are obtained using a method with weighted means of the air-sea fluxes which are calculated during the synchronously coupled periods.     

Cloud processes associated with past and future climate changes

 To investigate the cloud response during cold and warm periods, we have performed simulations of the Last Glacial Maximum (LGM-21ky BP) and of double CO₂ concentration using the LMD AGCM model. We observe that the thermal characteristics of these two climates are opposite, but the cloud response is more complex and does not display the same symmetry When doubling the CO₂, the warming of the troposphere and the cooling of the stratosphere are clearly linked with a reduction in low-level clouds and an increase of high-level clouds associated with relative humidity changes. For the LGM, the cloud response is more complex. In the inter tropical region, we show that the Hadley cell is reinforced during LGM (+20%) whereas it is reduced (−10%) for the double CO₂ experiments. The most important feature is that we observe an enlarged Hadley cell for LGM climate which strongly modifies the atmospheric dynamics and water transport. For LGM conditions, the cloud response is then mostly driven by these dynamical changes at low latitudes though at high latitudes the thermal changes explain a large part of the cloud response. Two different versions of the model, using different parametrizations for the precipitation show that cloud feedbacks may act differently for cold and warm climates; and that the cloud response may be more complex that previously expected, but also indicate that the details of these effects are model dependent.     

Techniques for asynchronous and periodically synchronous coupling of atmosphere and ocean models

Asynchronous and periodically-synchronous schemes for coupling atmosphere and ocean models are presented. The performance of the schemes is tested by simulating the climatic response to a step function forcing and to a gradually increasing forcing with a simple zero-dimensional non-linear energy balance model. Both the initial transient response and the asymptotic approach of the equilibrium state are studied. If no annual cycle is allowed the asynchronous coupling technique proves to be a suitable tool. However, if the annual cycle is retained, the periodically synchronous coupling technique reproduces the results of the synchronously coupled runs with smaller bias. In this case it is important that the total length of one synchronous period and one ocean only period is not a multiple of 6 months.     

具有热力-动力耦合的二维能量平衡气候模式

本文设计了一个热力—动力耦合的理论模式,所得到温度场和运动场的平均气候状态基本上与实况接近.分析表明,温度场和速度场是相互制约的,温度分布在纬向上的不均匀形成了垂直经圈环流,此种流场又把赤道的热量往极地输送,其结果与没有运动场耦合的情况相比,可使赤道温度降低而极地温度升高. 研究模式气候对太阳常数变化的敏感性后指出,要使冰界线从现在的72°N南移到冰河期的50°N,太阳常数要减小15％左右.这在考虑或不考虑流场耦合的模式中都是差不多的.     

Atmosphere-ocean coupling schemes in a one-diemensional climate model

In this paper, the coupling schemes of atmosphere-ocean climate models are discussed with one-dimensional advection equations. The convergence and stability for synchronous and asynchronous schemes are demonstrated and compared. Conclusions inferred from the analysis are given below. The synchronous scheme as well as the asynchronous-implicit scheme in this model are stable for arbitrary integrating time intervals. The asynchronous explicit scheme is unstable under certain conditions, which depend upon advection velocities and heat exchange parameters in the atmosphere and oceans. With both synchronous and asynchronous stable schemes the discrete solutions converge to their unique exact ones. Advections in the atmosphere and ocean accelerate the rate of convergence of the asynchronous-implicit scheme. It is suggusted that the asynchronous-implicit coupling scheme is a stable and efficient method for most climatic simulations.     

A study of long-term climate change in a simple seasonal nonlinear climate model

Solar radiation cycles, earth-orbital changes, and continental drift drive long to very long term (10³–10⁶ years) climatic changes. Lin and North used the stationary solutions of a simple energy balance model (EBM) to study the equilibrium climatic stages. In this paper, we study time dependent solutions and, in particular, transition processes. We make use of two time scales: a seasonal cycle (fast variation) and a long term time change (slow variation). Variations over short time scales are solved using a Fourier transform in time and long term variations are studied using a 4th order Runge-Kutta method. The energy balance equation is a parabolic type equation and it is well posed. Climate changes depend mainly on external forcing and the state of the climate is determined by the slow time scale forcing. In other words, transitions from one climate stage (snow-covered) to another (snow-free) at bifurcation points are monotonic, despite 20% to 50% shortperiod random fluctuations in the solar energy. This smooth transition is especially noticeable when the land bands lie close to the north pole (70° N to 90° N) or at high latitudes (50° N to 75° N).Now at Applied Physics Laboratory, The Johns Hopkins University, Laurel, MD 20723, USA     

The coupling of optimal economic growth and climate dynamics

In this paper, we study optimal economic growth programs coupled with climate change dynamics. The study is based on models derived from MERGE, a well established integrated assessment model (IAM). We discuss first the introduction in MERGE of a set of “tolerable window” constraints which limit both the temperature change and the rate of temperature change. These constraints, obtained from ensemble simulations performed with the Bern 2.5-D climate model, allow us to identity a domain intended to preserve the Atlantic thermohaline circulation. Next, we report on experiments where a two-way coupling is realized between the economic module of MERGE and an intermediate complexity “3-D-” climate model (C-GOLDSTEIN) which computes the changes in climate and mean temperature. The coupling is achieved through the implementation of an advanced “oracle based optimization technique” which permits the integration of information coming from the climate model during the search for the optimal economic growth path. Both cost-effectiveness and cost-benefit analysis modes are explored with this combined “meta-model” which we refer to as GOLDMERGE. Some perspectives on future implementations of these approaches in the context of “collaborative” or “community” integrated assessment modules are derived from the comparison of the different approaches.     

Backcasting long-term climate data: evaluation of hypothesis

Most often than not, incomplete datasets or short-term recorded data in vast regions impedes reliable climate and water studies. Various methods, such as simple correlation with stations having long-term time series, are practiced to infill or extend the period of observation at stations with missing or short-term data. In the current paper and for the first time, the hypothesis on the feasibility of extending the downscaling concept to backcast local observation records using large-scale atmospheric predictors is examined. Backcasting is coined here to contrast forecasting/projection; the former is implied to reconstruct in the past, while the latter represents projection in the future. To assess our hypotheses, daily and monthly statistical downscaling models were employed to reconstruct past precipitation data and lengthen the data period. Urmia and Tabriz synoptic stations, located in northwestern Iran, constituted two case study stations. SDSM and data-mining downscaling model (DMDM) daily as well as the group method of data handling (GMDH) and model tree (Mp5) monthly downscaling models were trained with National Center for Environmental Prediction (NCEP) data. After training, reconstructed precipitation data of the past was validated against observed data. Then, the data was fully extended to the 1948 to 2009 period corresponding to available NCEP data period. The results showed that DMDM performed superior in generation of monthly average precipitation compared with the SDSM, Mp5, and GMDH models, although none of the models could preserve the monthly variance. This overall confirms practical value of the proposed approach in extension of the past historic data, particularly for long-term climatological and water budget studies.     

Atmospheric circulation epochs and climate changes

The atmospheric circulation studies allow climate changes to be diagnosed and forecasted. Variations in occurrence frequencies of the atmospheric circulation forms W, E, and C (by the Vangengeim classification) and Z, M ₁, and M ₂ (by the Girs classification), which characterize climatic conditions in most of the Northern Hemisphere, are analyzed over a period of more than 100 years. It is shown that the occurrence frequency of the forms W, C, and M ₁ continually decreased, while that of the forms E and Z increased, which indicates a significant change in atmospheric circulation in the Northern Hemisphere during the last century. The occurrence frequency of the forms C and Z demonstrates specific features at inter-decade time scales. Correlations are found between accumulated sums of anomalies of occurrence frequencies of the atmospheric circulation forms C, (W + E), Z, and (M ₁ + M ₂) and inter-decade variations of the Earth’s rotation. The causes of these relationships are discussed along with possibilities of their use for diagnosis of climatic variations in the Northern Hemisphere.     

Global radiation climate changes in Israel

A detailed study of the 26-year series of global radiation K, measurements at Bet Dagan, the Israel Meteorological Service's pyranometer station in the central coastal plain of Israel, confirmed earlier findings of a significant reduction in insolation which were based on a small sample of this data set (Stanhill and Moreshet, 1992). Between 1956 and 1987 the annual reduction averaged 45.2 ± 4.3 MJ m^–2, equivalent to –0.63% yr^–1. Relatively the reduction was greater in midwinter (–0.91% yr^–1), than midsummer (–0.56% yr^–1), and under average (–0.63% yr^–1), than cloudless (–0.48% yr^–1) sky conditions. No changes were found in the degree of cloud cover observed at Bet Dagan.The annual decrease of K at Bet Dagan was highly correlated (r = –0.78) with the increase in the number of motor vehicles using the major roads passing within 1 km of the site: each additional vehicle passing was associated with a 21.5 J m^–2 decrease in K The causal nature of this correlation was confirmed by the difference of 18% found in daily values of K measured at Bet Dagan under traffic-free and extremely congested road conditions.The reduction in the K at Bet Dagan could not, however, be attributed exclusively to the increase in motor traffic in the immediate vicinity of the site, as no significant difference was found in values measured at a relatively traffic-free site 2 km downwind of the pyranometer station. The effect of aerosol pollutants originating in Tel Aviv - the major urban and industrial connurbation upwind of Bet Dagan - was confirmed by the changes recorded in the relative size of the direct and diffuse components of K measured at this site.The importance of pollution from Tel Aviv would also explain the absence of any significant changes in the annual values of K measured at Jerusalem, a smaller and less industrialized urban center 46 km downwind of Bet Dagan, or at Qidron, an uninhabited, isolated site on the NW coast of the Dead Sea, 25 km further downwind.Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. No. 3074-E, 1990 series.     

Improved methods for estimating equilibrium climate sensitivity from transient warming simulations

Climate Dynamics - Equilibrium climate sensitivity (ECS) refers to the total global warming caused by an instantaneous doubling of atmospheric CO2 from the pre-industrial level in a climate system....