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The interannual variability associated with the EI Nifio/Southern Oscillation (ENSO) cycle is investigated using a high-resolution coupled general circulation model (CGCM) of the atmosphere and ocean. The flux correction is restricted to annual means of heat and freshwater which proved sufficient to prevent the model from drifting to an unrealistic climate state. The annual as well as the seasonal climate in the CGCM is very close to that simulated in the atmospheric model forced with observed sea surface temperatures (SSTs).During a 100-year simulation of the present-day climate, the model is able to capture many features of the observed interannual SST variability in the tropical Pacific. This

The new version of the atmospheric general circulation model, ECHAM4, at the Max Planck Institute for Meteorology, Hamburg, has been coupled to the OPYC3 isopycnic global ocean general circulation and sea ice model (Oberhuber 1993) in a multi-century present-day climate simulation. Nonseasonal constant flux adjustment for heat and freshwater was employed to ensure a long-term annual mean state close to present day climatology. This paper examines the simulated upper ocean seasonal cycle and interannual variability in the tropical Pacific for the first 100 years. The coupled model's seasonal cycle of tropical Pacific SSTs is in good agreement with the observations with respect to both the warm pool variation and the Central and Eastern Pacific, with significant errors (up to-2 K) only in the cold tongue around April. The cold phase cold tongue extent and strength is as observed, and for this the heat flux adjustment does not play the decisive role; corrections beyond $40 Wm"2 are rare and only occupy small areas, such as near coasts. A well established south Pacific convergence zone is characteristic for the new AGCM version. Apart from extending the southeast trades seasonal maximum to midbasin, windstress pattern and strength are well captured. The subsurface structure is overall consistent with the observed, with a realistically sharp thermocline at about 150 m depth in the west and rising to the surface from 160°W to 100°W. The current system is clearly resolved and, on the whole, has expected shape. The equatorial undercurrent is correctly positioned but the core is only half as strong as observed. The north equatorial current and countercurrent also have reduced maximum speeds but the April minimum is captured. The south equatorial current is strong at the equator since the undercurrent has weakened influence. Like the companion publication (Roeckner et al. 1996) this study finds pronounced tropical eastern and central Pacific interannual variability. Simulated and observed NIN03 SST variability is represented by a single rather broadband maximum of power spectral density, centered on about 28 months for the simulation and four years for the observations. For simulation and observations, SST, windstress, and upper ocean heat content each exhibit a single dominant large-scale amplitude and phase pattern, suggesting the model captures the essential dynamics. The amplitude of the essentially standing oscillation in SST in the NIN03 region attains the observed strength, but is weaker at the eastern boundary. Anomalies of upper ocean heat content show off-equatorial westward and equatorial eastward propagation, the latter in turn arriving in the east of the basin in coincidence with the respective warm or cold SST anomalies. Equatorial windstress anomalies near the date line provide the appropriate forcing and clearly form a response to the anomalous SST.

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 dierent (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 from the positive linear trend of the SO, the interdecadal modulation to the El NinÄ o/SO cycle is enhanced during the GHG 2040 $ 2099. This is the result of an enhancement of the Walker circulation during that period. La NinÄ a events intensify and El NinÄ o 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.