Numerical Simulations Of The Influence Of Global Warming On Tropical Atmospheric Intraseasonal Oscillation

Both the observed and simulated tropical Intraseasonal Oscillation (ISO) is analyzed and compared in this study. Firstly, the temporal and spatial structures of tropical ISO are characterized by the use of daily mean ECMWF reanalysis data;then the characteristics of ISO simulated by 6 coupled GCMs from IPCC AR4 are compared in order to evaluate the ability of coupled GCMs to reproduce the tropical ISO and explore the possible effect of the global warming on tropical ISO. Various diagnosis methods including power spectra analysis, Lanczos filter, space-time spectra analysis, EOF, composite analysis etc. are applied in this study, and we also compared the simulated ISO with the observed one. Thus, the comprehensive syntheses about tropical ISO are performed in this study. The main conclusions are described as follows.(1)Through power spectra analysis, it is found that the global warming resulted from the increase in concentration of CO₂ enhances ISO activity for both the high and low frequency components.(2)ln order to describe the intensity of ISO quantificationally, the variance of 200hPa zonal wind with 20-100 day filter in the tropical area is defined as the ISO index. The results show that The intensity and the amplitude of ISO index are increased under the global warming background, which indicates that tropical ISO is more active than before, and with more obvious seasonal cycle, e.g., stronger in winter and spring, but weaker in summer and autumn. The interannual variability of ISO is related to the sea surface temperature (SST) anomalies averaged in the Nino3 region of central and east equatorial Pacific. When positive SST anomalies occur, ISO activity is weak.(3)Analyzing spatial patterns of percent of daily variance explained zonal wind at 200 hPa with 20-100 day filter and EOF1, it is found that the spatial distribution of ISO hardly changed during the last 20 years, but the ISO in Indian Ocean and the Bay of Bengal is more active, which indicates that ISO in the Indian Ocean and the Bay of Bengal becomes more important to the general atmosphere motion under the global warming background.(4) The space-time spectra of 200 hPa zonal wind indicate that ISO is mainly eastward propagation waves, in which the energy of ISO with wave number 1-2 is the largest. There are also has westward propagation waves with wave number 2-4 mostly, but the energy of westward propagating ISO are much weaker. The energy of ISO mainly concentrates in the wave number 1-3, and the frequency of ISO tends to increase.(5)Through the intercomparison of climatological mean state by 6 coupled GCMs, it is shown that all the coupled models have accurate description of the characteristics of the space structure of 200hPa zonal wind, which is a prerequisite for producing a realistic intraseasonal oscillation. Analyzing the period, the intensity, propagation and the spatial structure of tropical ISO, it is found that the coupled models captures the salient features of ISO in the tropics, especially in the period, the seasonal cycle and the spatial structure of ISO. However, there are also placesome systematic errors needed to be improved. For example, all the models underestimates the strength of ISO, and the simulated relationship between interannual variability of ISO and El Nino is not remarkable such as observed.