A High-resolution Tropical Pacific OGCM And Its Coupling To A Global AGCM

The coupled ocean-atmosphere climate model is essentail for the research on climate and climate variability. ENSO (El Nino/Southern Oscillation) was considered the strongest signal in the short-term climate variability and played an important role in the global climate change. For decades, the research of ENSO has been an active area and a number of coupled models have been established. However, problems still exist in these models, for instance, the systematic bias of the coupled model, the weakness and unrealistic positioning of simulated variability related with ENSO compared to the corresponding observation, the weak interannual variability in the coupled model based on the OGCM and the underestimated SST in the eastern equatorail Pacific, etc. For the sake of the research on ENSO mechanism and performing possible prediction in the future, a global atmospheric general circulation model was coupled to a high-resolution tropical Pacific ocean model in this paper. The atmospheric component was the 9-level global general circulation model of Institute of Atmospheric Physics (IAP). The oceanic component was a tropical Pacific ocean model with the horizontal resolution of 0.5o×0.5o, which was the development of an 2o×1o model (Zhang and Endoh 1994). The improvement on the horizontal resolution yields some new changes: for instance, the cold bias of SST in the equatorial Paciifc existing in many models was mitigated, the error of annual mean SST was decreased to -1.0oC from the previous -2.0oC, the improvement also varied with time, the bias is more serious in spring. The sea level elevation was more realistic in the new model compared to the previous simulation, accompained with improved surface currents. For instance, the NECC became stronger in the new model, the maximum reached 0.4m/s whereas in the previous one it is about 0.2m/s; the shift in the direction of surface zonal current in April to July was also represented quite clearly in the new model. Besides, the EUC was strenghtened and the maximum was 0.8m/s compared to 0.6m/s in the previous model. Some improvement was also found in the simulation of interannual variability. A monthly statistical correction method was employed to couple the two models, ie, using the simulatio of AGCM and the observation to calculate the regression coefficients and correcting the exchanged fluxs in the coupled model. The numerical experiment attested to the validity of such a method in reducing climate drift. The method could also maintain the interannual variability. 130yr experments were completed using this method. The climatology of the coupled model is realistic and the ENSO-like phenomena in the tropical Pacific were also reproduced. The main features included: the maximum interannual variability occurred in the subsurface other than at the surface, more accurately, it went along the thermocline of the the ocean, in the eastern Pacific it was at the depth of 50m along equator and at the depth of 150m on each side of the equator in the western/central Pacific. The Nino3 index was about 1.2oC, the warm/cold phase alternated with a cycle of 2-5 years and with varing magnitude. The oscillation on the equator showed the tendency of eastward propagation while on either side of the equator it propagated westward. Moreover, analysis with other diagnostic tools (EOF, power spectrum analysis, wavelet analysis, etc) also mainifested the similar results as in the observation and theory. The coupled model also produced a good representation of East Asian Monsoon (EAM) systme, incoporating the winter EAM and summer EAM. The relation between ENSO and EAM being analyzed, an out-of-phase relation could be found between summer EAM and the preceding ENSO event, for a long period the relation was unstable. From the difference of El Nino/La Nina composites, the anomaly of northwest surface wind in the region of EAM could be seen while in the region of South Asia and the western Pacific occurred the westerly anomay. Meanwhile, the simulations of the coupled model show distinct atmosphere circulation pattern between strong and weak winter EAM. Relative to the weak EAM, in the year of strong EAM the northerly wind prevailed in the central/eastern part of China in the troposphere, accompained by lower air temperature, significant strengthening of Ulaer high, anomaly cyclonic system in the west of European continent and the weak westerly belt. In China, arctic region, most parts of Mongolia and the northwest part of European contient the air temperature was lowered.