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Abstract In this study, we investigated the impact of global warming on the variabilities of large-scale inter- annual and interdecadal climate modes and ...
Climate Dynamics (2001) 17: 361±374

Ó Springer-Verlag 2001

Z.-Z. Hu á L. Bengtsson á E. Roeckner M. Christoph á A. Bacher á J. M. Oberhuber

Impact of global warming on the interannual and interdecadal climate modes in a coupled GCM

Received: 5 March 1999 / Accepted: 13 September 2000

Abstract In this study, we investigated the impact of global warming on the variabilities of large-scale interannual and interdecadal climate modes and teleconnection patterns with two long-term integrations of the coupled general circulation model of ECHAM4/OPYC3 at the Max-Planck-Institute for Meteorology, Hamburg. One is the control (CTRL) run with ®xed present-day concentrations of greenhouse gases. The other experiment is a simulation of transient greenhouse warming, named GHG run. In the GHG run the averaged geopotential height at 500 hPa is increased signi®cantly, and a negative phase of the Paci®c/North American (PNA) teleconnection-like distribution pattern is intensi®ed. The standard deviation over the tropics (high latitudes) is enhanced (reduced) on the interdecadal time scales and reduced (enhanced) on the interannual time scales in the GHG run. Except for an interdecadal mode related to the Southern Oscillation (SO) in the GHG run, the spatial variation patterns are similar for di€erent (interannual + interdecadal, interannual, and interdecadal) time scales in the GHG and CTRL runs. Spatial distributions of the teleconnection patterns on the interannual and interdecadal time scales in the GHG run are also similar to those in the CTRL run. But some teleconnection patterns show linear trends and changes of variances and frequencies in the GHG run. Apart Z.-Z. Hu á L. Bengtsson á E. Roeckner M. Christoph á A. Bacher Max-Planck-Institut fuÈr Meteorologie, Bundesstrasse 55, D-20146 Hamburg, Germany Z.-Z. Hu (&) Center for Ocean-Land-Atmosphere Studies, 4041 Powder Mill Road, Calverton, MD 20705, USA J. M. Oberhuber Deutsches Klimarechenzentrum GmbH, Bundesstrasse 55, D-20146 Hamburg, Germany Current aliation: M. Christoph Institut fuÈr Geophysik und Meteorologie der UniversitaÈt zu KoÈln, D-50923 KoÈln, Germany

from the positive linear trend of the SO, the interdecadal modulation to the El NinÄo/SO cycle is enhanced during the GHG 2040  2099. This is the result of an enhancement of the Walker circulation during that period. La NinÄa events intensify and El NinÄo events relatively weaken during the GHG 2070  2090. It is interesting to note that with increasing greenhouse gas concentrations the relation between the SO and the PNA pattern is reversed signi®cantly from a negative to a positive correlation on the interdecadal time scales and weakened on the interannual time scales. This suggests that the increase of the greenhouse gas concentrations will trigger the nonstationary correlation between the SO and the PNA pattern both on the interdecadal and interannual time scales.

1 Introduction Variability and prediction of the climate system on the interannual and interdecadal time scales and the impact of the atmospheric trace gas concentrations on the climate system are one of the major scienti®c issues in the research program of Climate Variability and Predictability (CLIVAR) (WCRP 1997, 1998). The large-scale interannual and interdecadal variabilities of the atmosphere and ocean interaction system have signi®cant impact on the world-wide climate variations. The most dominant interannual variation in the global ocean and atmosphere system is associated with the El NinÄo/ Southern Oscillation (ENSO) phenomenon (Philander 1990). Some ENSO related climate variations can already be predicted and simulated to a large extent by atmospheric general circulation models (AGCM) (Bengtsson et al. 1996a; Barnett et al. 1997). The interannual climate variations have been extensively studied both in model simulations and in observational data by some investigators (Trenberth 1992; Peixoto and Oort 1992; WCRP 1998, and references therein). There are also some investigations on the decadal and interdecadal variations of the climate system, especially in the regions

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Hu et al.: Impact of global warming on the interannual and interdecadal climate modes in a coupled GCM

of the North Atlantic and the North Paci®c (Nitta and Yamada 1989; Trenberth 1990; Latif and Barnett 1994; Lau 1997; WCRP 1998, and references therein). There are a number of papers studying the impact of global warming on the variability of the climate system. For example, with the Hadley Centre high-resolution coupled GCM (CGCM), Murphy and Mitchell (1995) indicated the similarity of the patterns of change between the transient response of the climate system to a gradual increase of CO2 at the time of doubling and the equilibrium response. Tokioka et al. (1995) found that there are some similarities in the interannual and interdecadal variations between observations and a transient response of the climate system to the gradual increase of CO2 using a CGCM. Bengtsson et al. (1996b) have demonstrated that the number of storms is signi®cantly reduced, particularly in the Southern Hemisphere, when CO2 concentrations have doubled. Recently, Hu and Bengtsson (2000) investigated the relationship between the increase of the greenhouse gas concentrations and stratospheric circulation change in a long term integration of transient greenhouse warming with the ECHAM4/OPYC3 CGCM. The impact of global warming on the physical processes involved was studied through analyses of energy for various wavenumbers, vertical propagation of planetary waves and feedback of transient eddy activity on the zonal mean circulation. With this long term integration, Hu et al. (2000) also demonstrated the signi®cant weakening of the Asian winter monsoon in the global warming scenario. This is associated with the northward and eastward shift and weakening of the East Asian trough which results from the warming being more pronounced in the Eurasian continent than in the Paci®c Ocean in the global warming scenario. There are also a series of research works about the impact of global warming on the tropical climate and ENSO cycle. For example, Knutson and Manabe (1995) found that a quadrupling of CO2 in their model produced a 1 °C reduction in the east-west sea surface temperature (SST) contrast in the tropical Paci®c as compared with a much larger (4  5 °C) overall warming of the region. Knutson et al. (1997) indicated that the amplitude of the model's ENSO decreases slightly in two 1000-year CO2 sensitivity experiments relative to the control run in response to either a doubling or quadrupling of CO2. They also pointed out that in contrast to the weaker overall amplitude, the multidecadal amplitude modulations become more prominent with increased CO2, and the frequency of ENSO in the model does not appear to be strongly in¯uenced by increased CO2. Tett (1995) indicated that there is no signi®cant change in the interannual variance of SST in the eastern Paci®c, and the size of SST anomalies during the warm or cold events in the greenhouse world may not be signi®cantly di€erent from those of today. Meehl et al. (1993) showed that the interannual variability of SST superimposed on mean SSTs in the tropical eastern Paci®c is higher by about 1 °C with doubled CO2 than

with present amounts of CO2. With the ECHAM4/ OPYC3 CGCM, Timmermann et al. (1999) found that when the model is forced by a realistic future scenario of increasing greenhouse-gas concentrations, more frequent El NinÄo-like conditions and stronger cold events in the tropical Paci®c Ocean result. At present, most investigations of model simulations of the global warming scenario are based on linear trends of the climate system, and a few studies deal with the impact of global warming on the variabilities of the large-scale interannual and interdecadal climate modes and the teleconnection patterns. The results concerning the impact, for example on the ENSO cycle, are still model dependent (Meehl et al. 1993; Tett 1995; Knutson et al. 1997; Timmermann et al. 1999). Therefore, although many researchers have considered the interannual and interdecadal variations of the climate system and the underlying possible physics, it is still a challenge to study climate variations in the global warming scenario. This investigation is focused on the potential impact of global warming on the variabilities of largescale interannual and interdecadal modes and the teleconnection patterns based on two long-term integrations with and without a time dependent greenhouse gas increase in a coupled ocean-atmosphere-sea ice-land surface climate model. In Sect. 2, we ®rstly describe the coupled model, data, and methods. Trends and standard deviations in the control (CTRL) run and in the greenhouse gases (GHG) run are analyzed in Sect. 3. Spatial and temporal variations of H500 on di€erent time scales in the CTRL and GHG runs are revealed and compared in Sect. 4. The similarities and di€erences of the variabilities of the teleconnection patterns in the CTRL and GHG runs are investigated in Sect. 5. Discussion is given in Sect. 6.

2 Model, data and methods 2.1 Model Data used are from the global coupled ocean-atmosphere-sea iceland surface climate model, ECHAM4/OPYC3 CGCM. The atmospheric component of the CGCM is the ECHAM4 model, which is the fourth-generation of the Hamburg version of the European Centre for Medium-Range Weather Forecasts atmospheric model modi®ed for climate studies. It is a spectral model with triangular truncation of total horizontal wave-number 42 (T42). The nonlinear terms are calculated on a horizontal resolution of about 2.8° ´ 2.8° in latitude and longitude. A hybrid r-pressure coordinate system is used (Simmons and StruÈ®ng 1981) with 19 irregularly spaced levels and with the highest resolution in the atmospheric boundary layer. The vertical domain extends up to a pressure level of 10 hPa. The ECHAM4 includes a comprehensive set of physical parametrizations, such as radiation, vertical di€usion, gravity wave drag, cumulus convection, shallow convection, stratiform clouds, and a simple model for soil processes. The soil model comprises the budgets of heat and water in the soil, the snow pack over land and the heat budget of land ice. Prognostic variables are vorticity, divergence, temperature, speci®c humidity, the logarithm of surface pressure and the mixing ratio of total cloud water. Both seasonal and diurnal cycles of solar forcing are simulated. A detailed description of the ECHAM4 model is given in Roeckner et al. (1996a, b, and references therein).

Hu et al.: Impact of global warming on the interannual and interdecadal climate modes in a coupled GCM The OPYC3 oceanic general circulation model was developed by Oberhuber (1993a, b). The OPYC3 uses isopycnals as its Lagrangian vertical coordinate system. The OPYC3 consists of three sub-models, which are the interior ocean, the surface mixed layer and the sea ice. The OPYC3 includes a realistic equation of state, employs primitive equations and has a surface mixed layer coupled to a snow and sea-ice model and to the interior ocean. Poleward of 36°, the horizontal resolution is identical to the Gaussian grid of the atmospheric model (2.8° for the T42 model). At low latitudes the meridional spacing is gradually decreased to 0.5° at the equator. In the vertical, 10 interior layers below the surface mixed layer are used. The sea-ice model calculates the thickness and concentration of ice and its momentum. A detailed description of the OPYC3 model can be found in Oberhuber (1993a, b). The ECHAM4 and OPYC3 are coupled through a mutual exchange of ¯uxes. Fluxes of momentum are unconstrained, while ¯uxes of heat and freshwater are ¯ux adjusted, but only as annual averages (Bacher 1998). The ECHAM4 is forced by SSTs simulated by the OPYC3, and the OPYC3 by the momentum, heat, and freshwater ¯uxes simulated by the ECHAM4. In general, the ECHAM4/OPYC3 CGCM appears to be able to simulate the present climate and its variability with considerable skill (Bacher 1998). The ECHAM4/OPYC3 CGCM has successfully hindcasted the onset of El NinÄo in 1997 (Oberhuber et al. 1998). The detailed coupling strategy and technology of the ECHAM4 and OPYC3 are given in Bengtsson (1996), Bacher (1998) and Bacher et al. (1998).

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comparison with EOF, the REOF has superior characteristics; for example, its spatial patterns are more analogous to teleconnection patterns, are less dependent on its boundary values, and provide more physical meaning (Richman 1986). Power spectral analysis (Press et al. 1989) and composite analysis are also used.

3 Linear trends and standard deviations of H500 In this section, we ®rstly analyze the linear trends of H500 in the CTRL and GHG runs, then compare the standard deviations (STDV) on the interannual and interdecadal time scales in the CTRL 300  399 and GHG 2000  2099. 3.1 Linear trends Variability of the averaged H500 is without obvious linear and nonlinear trends in the CTRL run (Fig. 1a). In the CTRL 300  399, minor positive trends are

2.2 Data and methods Two experiments are performed with the ECHAM4/OPYC3 CGCM. The ®rst experiment is a 240 year long simulation of transient greenhouse warming (GHG). In the GHG run, from 1860 to 1990, the annual concentrations of the greenhouse gases are prescribed as observed and, from 1990 onward, according to IPCC scenario IS92a (IPCC 1992). The transient integration starts from the year 1860 and ends in the year 2099 (denoted by GHG 1860  2099). The second experiment is a 300 year integration (CTRL) with ®xed present-day concentrations of greenhouse gases (IPCC 1994). More details on the GHG and CTRL experiments can be found in Roeckner et al. (1999). In order to have comparable data length with the GHG run and reduce the impact of spin-up, the last 240 year integration of the CTRL run (denoted by CTRL 160  399) is used in this analysis. Present analyses mainly concentrate on the contrast between the GHG 2000  2099 and the CTRL 300  399. The CTRL 300  399, as a 100 year period chosen casually in the CTRL run, represents the nature of the CTRL run without global warming, and the GHG 2000  2099 represents the character of the GHG run in a period with signi®cant warming. H500 and sea level pressure (SLP) in December, January and February (DJF) are used in this analysis. The analysis focuses on the tropics and Northern Hemisphere (NH), 30°S±90°N. Through linear regression and a low pass ®lter, on the basis of fast Fourier transformation, H500 and SLP variabilities of all time scales are divided into three parts: linear trend, interdecadal (³10 years), and interannual (