全球增暖对ENSO影响的数值模拟研究

A Numerical Simulation Study of ENSO in Response to Global Warming

利用日本东京大学气候系统研究所、日本环境研究所和日本地球环境研究中心联合开发的海气耦合模式MIROC3.2,研究了全球变暖对ENSO年际变率的影响。该模式较好地模拟了ENSO循环的不同阶段表层和次表层海水温度变化,海表温度最大振幅出现在120°W以东,与观测一致,表明模式可以较好反映热带地区大气、海洋的动力、热力特征。研究还比较了控制试验和CO2浓度年增长1%的瞬时试验,结果表明,在全球变暖的大环境下ENSO事件发生频率没有显著变化,但ENSO事件强度增大,年际变率变大;热带太平洋呈现整体增暖趋势,表层温度尤其是热带中太平洋地区温度升高显著。敏感性分析表明,年际ENSO变率的振幅增大的主要贡献来自于海洋。海水增温导致热带太平洋海温垂直梯度增大,在热带西太平洋海温垂直温度梯度变化最为明显;次表层海温对单位大气风应力变化的响应大于表层海温响应。当这种响应与热带太平洋赤道地区径向温度梯度变化的共同作用导致温室效应下ENSO振幅增大。

A coupled general circulation model, MIROC3. 2, is used to investigate the impacts of global warming on the E1 Nino- Southern Oscillation （ENSO） variability. The model, with relatively fine resolution, captures the tropical atmosphere and ocean dynamics well, simulating a realistic ENSO cycle as observed. When the atmospheric greenhouse gas concentration is gradually changed by the rise of CO2, the model shows an overall warm trend in the tropical Pacific under the greenhouse warming, particularly in the central equatorial Pacific. Superimposed on the positive trend of sea surface temperature is the increase in amplitude of interannual ENSO variability. A sensitivity analysis shows that ocean, rather than the atmosphere, seems to be responsible for the strengthening. Model simulations show that the rise of greenhouse gas tends to increase the surface waters more than the subsurface waters, leading to strengthened subsurface vertical stratification in the equatorial ocean. The strengthening of the subsurfacestratification is greater in the western equatorial Pacific. The subtle differences in the mean state of tropical Pacific under the warmed climate cause the subsurface temperature anomalies in the western Pacific ocean, which is more sensitive to the change of surface wind stress force, accompanying greater in-situ subsurface temperature response in the western Pacific ocean. This as well as the change of the mean meridional temperature gradient appears to be responsible for the increase in ENSO amplitude. However, the changes in climate are far beyond our expectations. The result presented here is believed to be one of the possible ENSO responses to the global warming. How the ENSO will behave in the future need more simulations and predictions of coupled general circulation models.