A Study Of Key Region Of Ocean-Atmosphere Interaction For ENSO And The Response Of ENSO To External Forcing

El Nino and Southern Oscillation (ENSO) is a phenomenon that exhibits interannual variability in tropical Pacific Ocean and it plays an important role in global climate change. On the basis of former studies about ENSO mechanism and how external forcing affects ENSO, we brought forward two important questions as follows: which region is the key region of ocean-atmosphere interaction in the tropical Pacific Ocean and how does the interseasonal oscillation (MJO) and monsoon influence ENSO cycle? This paper studied these two important questions by using the Cane-Zebiak model.By using observation data, we test the simulation ability of the ocean model and the atmosphere model of ZC model respectively. The result shows that ZC model can mostly simulate the primary characteristics of recent ENSO variability. The ocean component can simulate the ENSO events better, especially in the tropical central/eastern Pacific Ocean(160°W—90°W) and the atmosphere component can simulate the ENSO events better, specially in the tropical central Pacific Ocean(160°E — 160°W). We found that in ZC model the simulation effect of ocean model's response to atmospheric forcing is better than that of atmosphere model's response to SST anomaly.To study role of wind stress anomaly and SSTA in different regions in ENSO, we made the sensitivity experiments by using ZC model, determined the key region of ocean-atmosphere interaction. The experiment results show that the most important region for wind stress anomaly to affect the ocean is the equatorial central Pacific (168.75°E — 135°W) , the reason is that the longitudinal Sverdrup transportation formed by wind stress anomaly in tropical central Pacific Ocean is the dynamic mechanism of recharge-discharge process in ENSO cycle. If there is large error in surface wind stress anomaly in central Pacific Ocean, the longitudinal Sverdruptransportation is interrupted (reduced) and the recharge-discharge process in ENSO cycle is interrupted (reduced), so the ENSO cycle is damped. Also the tropical central Pacific Ocean is the region at the eastern boundary of warm pool in which the temperature gradient reaches maximum, the surface wind stress anomaly in this region can directly influence the transport of warm/cold advection which is one of the negative processes of ENSO cycle. The most important region for SSTA to affect the atmosphere is the equatorial eastern Pacific (146.25°W-73.125°W), this related to the fact that in equatorial eastern Pacific the thermocline is the lowest and the amplitude of SSTA is the largest. The forcing of SSTA to the atmosphere in the equatorial eastern Pacific is the necessary condition to sustain the positive feedback process in the ENSO cycle, while the influence for surface wind stress anomaly in central Pacific Ocean to affect the ocean is the requirement for ENSO cycle to sustain its negative feedback process.Numerical experiments give reasonable explanation for the inconsistency of the conclusion about how MJO affect ENSO. The experiment shows that the influence of MJO in west Pacific to ENSO is different with the different intensity of MJO. When the intensity of MJO is weak, it may excite strong ENSO events, but when the intensity of MJO is intermediate, it can hardly affect ENSO; and when the intensity of surface wind stress anomaly formed by MJO is virtually equal to the surface wind stress anomaly in western Pacific Ocean when there is no external forcing, the nonlinear "frequency entrainment" formed by the high frequency fluctuation caused by MJO and low frequency variability of ENSO leads to the decrease of the amplitude of ENSO and the appearance of new oscillation with different periods.The numerical experiments also confirmed the former conclusion about how monsoon affect ENSO: when the intensity of monsoon forcing in west Pacific is weak, the amplitude of ENSO is amplified; when intensity of monsoon forcing is strong, the amplitude of ENSO is decreased, the period is shortened and the phase-locking changes from wintertime to fall time.In conclusion, we studied the key region of ocean-atmosphere interaction for and the response of ENSO to external forcing (especially MJO and Monsoon) by using theZC model, and determining the key region of ocean-atmosphere interaction is important for the distribution of the ocean observation net. In addition we found that the influence of MJO to ENSO exhibits different forms along with the different intensity of MJO and further confirmed the former conclusion about how monsoon affects ENSO.