1. Introduction

[2]  One of the most important problems of modern climatology is the problem of predicting climate changes during the next few centuries. In the narrow sense, we will define climate in accordance with the definition adopted in the IPCC report [Solomon et al., 2007], where climate is defined as a statistical description with respect to the quantitative indicators (climate characteristics) over a time period of a few months to thousands or millions of years. (The IPCC is the Intergovernmental Panel on Climate Change established in 1988 under the aegis of the World Meteorological Organization (WMO)). According to the WMO definition, the classical averaging period is 30 years (1961-1990 with reference to the current state). In most cases, these quantitative indicators are surface characteristics, such as air temperature, precipitation, wind, etc. In a wider sense, climate is an ensemble of states that the climate system has passed through over a sufficiently long period of time [Dymnikov and Filatov, 1994]


[3]  According to the IPCC report [Solomon et al., 2007], the observed significant climate changes are primarily due to anthropogenic forcing mainly related to the atmospheric emissions of greenhouse gases, aerosols, and other pollutants. The only instrument providing quantitative estimates of future climate changes is a numerical model of the Earth's climate system. The understand of the system's energy balance at a base level implies that the simple models of the Budyko type [Budyko, 1980] can provide a rough quantitative estimate of some globally averaged climate characteristics. However, more accurate estimates of feedbacks and regional details can be obtained only with the aid of combined climate models [Solomon et al., 2007]. The complex character of the processes occurring in the climate system does not allow the use of extrapolations of historical trends or statistical and other purely empirical methods to obtain long-term estimates. Therefore, at present, the Coupled Model Inter-comparison Project (CMIP; http://www-pcmdi.llnl.gov/projects/cmip/index.php) is being performed to compare the climate-change predictions obtained with different climate models under the scenarios pro-posed in [Solomon et al., 2007] for possible future variations in the atmospheric concentrations of greenhouse gases, aerosols, and other pollutants. This program is a step forward as compared to a similar comparison [Covey et al., 2000] that was carried out in 2001 and whose results were reflected in the third IPCC report [Solomon et al., 2007]. The results obtained in the course of this program will be reflected in the fourth IPCC report. Now, a greater number of models take part in the comparison; each of them has been improved.

[4]  The objective of this study is to analyze the responses of the coupled atmosphere-ocean general circulation model developed at the Institute of Numerical Mathematics, Russian Academy of Sciences (INM RAS), to possible future variations in anthropogenic forcing. These responses were obtained from a series of experiments carried out in the CMIP framework under the scenarios taken from [Solomon et al., 2007]. These experiments were carried out with a new version of the coupled model as compared to that described in [Diansky and Volodin, 2002]. A detailed description of the differences between the new version of the coupled model and that described in [Diansky and Volodin, 2002] is given in [Dymnikov et al., 2005; Volodin and Diansky, 2004].

[5]  In [Volodin and Diansky, 2003], the response of the previous version of the INM coupled atmosphere-ocean model [Diansky and Volodin, 2002] to an increase in CO2 concentration in the atmosphere was analyzed and compared with the results obtained with other models. Here, the response to a prescribed increase in CO2 concentration was studied by comparing two 80-year calculations made according to the scenario proposed in the CMIP2 project (http://www-pcmdi.llnl.gov/projects/cmip/index.php). In the first (control) experiment, the atmospheric CO2 concentration was set constant and equal to its value observed in the late 20th century. In the second experiment, the concentration of CO2 was increased by 1% per year.

[6]  The experiments presented in this study were carried out according to more complicated and realistic scenarios with a new improved version of the INM RAS coupled general circulation model, whose sensitivity to external forcing was increased by more than 1.5 times (see below). In Russia, such experiments with the coupled atmosphere-ocean general circulation model were carried out for the first time. In the following sections, a short description of the new version of the INM coupled model is given, the methods of conducting such experiments are described, and the results obtained are analyzed and compared with the data given in the third IPCC report [Solomon et al., 2007]. In one of the experiments, in addition to the coupled atmosphere- ocean general circulation model, an atmospheric model coupled with a simple balance model of the heat content of a homogeneous 50-m ocean layer is used. Although the correction of heat fluxes at the ocean surface is used, this model makes it possible to promptly obtain an equilibrium response to a prescribed external forcing and also to emphasize the role of ocean circulation.


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