Asymmetry Of Temperature Anomaly In The Tropical Pacific Ocean And Tropical Indian Ocean In An OGCM

The first part of the study used LASG/IAP(State Key Laboratory of Numerical Modeling for Atmospheric Science and Geophysical Fluid Dynamics/Institute of Atmospheric Physics)Climate System Ocean Model(Version 2.0,abbreviated as"LICOM2.0") to make a series of numerical experiments forced by ovserved wind stress, and discussed physical mechanism of ENSO's asymmetry and its impact on climatological Mean State, also the potential relationship between them.One control run and two sensitive runs were designed to estimate the response of the OGCM to different wind stress in this study. The control run is forced by the climatological mean wind stress,and it has been integrated for 44 model years.The two sensitive experiments are as same as the control run expect that monthly mean wind stress anomaly from 1958 to 2001 are superimposed to the climatologically mean wind stress between 30°S and 30°N in the tropical Pacific Ocean. The difference between these two sensitive experiments is that the sign of wind stress anomaly is reversed. .Both the observed and simulatied results pointed out that the asymmetry of ENSO not only existing in the surface but also in the subsurface. And the main result is that the skewness of SST is positive in the eastern Pacific Ocean while negative in the western Pacific Ocean. It speaks for the amplitude of warm anomaly is stronger than the amplitude of cold anomaly in the eastern Pacific Ocean but reverse signs in the western Pacific Ocean. The temperature skewness of subsurface is positive in the equatorial eastern Pacific while negative in the equatorial western Pacific. It also illustrates that the amplitude of warm anomaly in the equatorial eastern Pacific is larger than the amplitude of cold anomaly and the reverse character showing in the equatorial western Pacific. They meet the character of ENSO's warm phase, which is, SST presents warm anomaly in the eastern Pacific Ocean while temperature shows cold anomaly in the subsurface of equatorial western Pacific. That is, El Nino is stronger than La Nina, ENSO's warm phase and cold phase is asymmetry.We also pay attention to rectification of ENSO on the Mean State by model experiments. The important finding is that temperature is increasing in the equatorial eastern Pacific but decreasing in the equatorial western Pacific. Nonlinear dynamical heating and mean vertical advection probably lead to temperature increasing while vertical upwelling and meridional convergence excited by Rossby wave bring on temperature decreasing. Meanwhile, consider the method of analyzing eddy momentum and eddy heat which was used in some atmospheric papers to illustrate upwelling and downwelling, we also try to use eddy momentum and eddy heat to interrupt upwelling in the tropical Pacific Ocean.The same method designing numerical experiments used in Tropical Pacific Ocean is also used in Tropical Indian Ocean in the latter part. Forced by the observed wind stress anomaly, the seasonal cycle of Tropical Indian Ocean,IOD mode and asymmetry of IOD are all evaluated by an OGCM of LASG in the study. Asymmetry of IOD and its impact on the climatological Mean State have also been simulated by the numerical experiments. The simulated results are similar with the observed, reproducing seasonal change of SST character forced by the monsoon. On interannual timescales, the model can successfully reproduce not only the trends of the time series index of IOD, but also the spatial distribution of temperature anomalies, which is characterized by dipole with the reverse signs in the western and the eastern (the western Tropical Indian Ocean shows positive anomaly, while the eastern Tropical Indian Ocean shows negative anomaly) both in the surface and the subsurface. Therefore, to the tropical Indian Ocean, the results manifest that the IOD mode mainly responses from the asymmetry of wind stress anomaly. The analysis of correlation between temperature and NINO3.4 index show that 2～4 months leading ENSO of Tropical Pacific Ocean the simulated IOD mode well agrees with the observed, while almost 2 months lagging ENSO of Tropical Pacific Ocean the simulated IOBM mode disagrees with the observed, which is probably due to no considering the influence of net heat flux interannual anomaly in the numerical experiments. Meanwhile, the asymmetry of IOD simulated by the OGCM is similar with the observed. The experimental results forced by the wind stress anomaly illustrate that the asymmetry of wind stress anomaly slightly contributes to the asymmetry of the IOD index, and the asymmetry of temperature anomaly in the subsurface and deep ocean mainly result from the internal nonlinear dynamics in the tropical Indian Ocean. From the numerical experiments it is obviously found that the asymmetry of temperature impacts the climatological Mean State, which makes temperature stratification of the upper ocean system more stable in the Tropical Indian Ocean.