Numerical Modeling of Asymmetry of the Indian Ocean Dipole and Its Mechanism

The tropical Indian Ocean Dipole (IOD), which plays an important role in the tropical ocean-air coupled system, is one of the main characters on interannual variability of the Indian Ocean and atmospheric circulation, the IOD exhibits significant asymmetry which is similar to the ENSO phenomenon in the tropical Pacific Ocean. Forced by the observed wind stress anomaly, the seasonal cycle of the tropical Indian Ocean, the IOD mode, and asymmetry of the IOD are evaluated by an OGCM of LASG, IAP/CAS, in the study. Asymmetry of the IOD and its impact on the climatological mean state have also been simulated by the numerical experiments. The simulation results are similar to the observational ones, 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 of the IOD index, but also the spatial distribution of temperature anomalies, which is characterized by dipole with the reverse signs in the western and eastern tropical Indian Ocean (the western Tropical Indian Ocean shows positive anomaly, while the eastern tropical Indian Ocean shows negative anomaly) for both the surface and subsurface sea temperature. Therefore, for the tropical Indian Ocean, the results manifest that the IOD mode is mainly response to the asymmetry of wind stress anomaly. The analysis of correlation between the sea temperature in the tropical Indian Ocean and the Niño 3.4 index show that 2-4 months leading ENSO in the tropical Pacific Ocean the simulated IOD mode well agrees with the observed, while almost 2 months lagging ENSO in the tropical Pacific Ocean the simulated Indian Ocean basin mode disagrees with the observed, which is probably due to disconsidering the influence of net heat flux interannual anomaly in the numerical experiments. Meanwhile, the asymmetry of IOD simulated by the OGCM is similar to 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.