HR: 17:30h
AN: OS44A-07 INVITED    [Abstracts]
TI: A Coupled GCM Intercomparison Study of the South Pacific Convergence Zone
AU: * Behera, S K
EM: behera@jamstec.go.jp
AF: Frontier Research Center for Global Change/JAMSTEC, Showamachi, Yokohama, 236001, Japan
AU: Luo, J
EM: luo@jamstec.go.jp
AF: Frontier Research Center for Global Change/JAMSTEC, Showamachi, Yokohama, 236001, Japan
AU: Takahashi, K
EM: takahasi@jamstec.go.jp
AF: Earth Simulation Center/JAMSTEC, Showamachi, Yokohama, 236001, Japan
AU: Yamagata, T
EM: yamagata@eps.s.u-tokyo.ac.jp
AF: Frontier Research Center for Global Change/JAMSTEC, Showamachi, Yokohama, 236001, Japan
AU: Yamagata, T
EM: yamagata@eps.s.u-tokyo.ac.jp
AF: Department of Earth and Planetary Science, Graduate School of Sciences, University of Tokyo, Hongo, Tokyo, 1130033, Japan
AB: The South Pacific Convergence Zone (SPCZ) is an important component of seasonal climate variations in the Southern Hemisphere. Though several associated processes are already discussed using observational data, the SPCZ is yet to be resolved properly in global general circulation models (GCMs). Particularly, the ocean- atmosphere coupled GCMs often fail to simulate the correct orientation and the zonal extent of the SPCZ. Most of these models replicate an east-west zonally oriented ITCZ similar to that in the Northern Hemisphere giving rise to the so-called double ITCZ problem. In this study simulation results from a variety of models are used to understand model biases in resolving the temporal and spatial distribution of the SPCZ. These models range from standalone atmospheric GCMs to the state of the art ocean-atmosphere coupled GCMs. It is found that the seasonal SPCZ in standalone atmospheric GCM results is better represented than that in the coupled GCM with an identical atmospheric component and a spatial resolution of about 100 km. The dry zone to the east of the SPCZ is not well-formed in coupled GCMs, particularly in austral summer when the SPCZ is pronounced. This is related to the model biases of the sea surface temperature, which is warmer in eastern Pacific in coupled GCM than the observation. The bias is not as clearly manifested in a spatially higher resolution coupled GCM in which the eastern Pacific SST is better simulated. The increase in model horizontal resolution helps in resolving the local air-sea interactions, the cross- equatorial winds and the local circulation cells. The dry zone is also improved in another experiment in which the improvement in model coupling physics improved the bias in equatorial cold tongue. It is also found that the SPCZ simulation is not improved by just increasing the vertical resolution in coupled GCM. This also imply that a proper representation of the boundary layer and the associated physics is more important. The meridional tilt, which is distinctly observed in the southern part of the SPCZ, is not well reproduced in any of the coupled GCM results. The biases in coupled GCM storm tracks seem to cause the biases in the SPCZ tilt. Interestingly, a standalone atmospheric GCM, with realistic boundary forcings, captures the tilt and the overall pattern closer to observation. The interaction of SPCZ with neighboring land processes is not very clear in that model results though. The seasonal variation in the spatial rainfall pattern of the SPCZ did not change much when the Australian continent was replaced by seasonal SSTs in a standalone atmospheric GCM experiment. On interannual time-scale, the link between the SPCZ and the El Nino/Southern Oscillation (ENSO) is not very clear. The observed data in austral summer shows a significant correlation between the rainfall index derived from the SPCZ region and the central Pacific SST anomalies. Therefore, it is possible that the newly identified El Nino Modoki (or pseudo El Nino) could influence the interannual variability of SPCZ. During an El Nino Modoki event, the warm central tropical Pacific is flanked by clod anomalies on either side. The SPCZ rainfall index is also seen to be significantly correlated with the Indian Ocean Dipole at one season lag. Apparently, the SPCZ plays an important role in the information exchanges between the two neighboring basins.
DE: 3310 Clouds and cloud feedbacks
DE: 3337 Global climate models (1626, 4928)
DE: 4200 OCEANOGRAPHY: GENERAL
DE: 4215 Climate and interannual variability (1616, 1635, 3305, 3309, 4513)
DE: 4263 Ocean predictability and prediction (3238)
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting