The Physical Relationship Between The Two Types Of El Ni(?)o And IOD And Its Impacts On The Autumn Rainfall In China

Based on the monthly Met Office Hadley Center sea surface temperature(SST) data and the monthly NCEP/NCAR Reanalysis data and SODA datasets and 160 station-observed monthly precipitation levels in China from 1951-2013,the occurrence and development status of Indian Ocean Dipole(IOD) is analyzed when the two types of El Ni(?)o occur in boreal autumn and the dynamic process is examined. Then the influence of two types of El Ni(?)o and the IOD on the autumn rainfall in China is also revealed in both observation data and AMIP mode data. The major conclusions can be summarized as follows:(1) Eastern-Pacific(EP) El Ni(?)o events exhibit a strong correlation with the IOD dependent on their magnitude in boreal autumn(r=0.96). When the IOD occurs accompany with the strong EP El Ni(?)o the SSTA composite exhibits the typical SST anomaly pattern of traditional El Ni(?)o events over the tropical Pacific, which is characterized by strong warm SST anomalies in the eastern tropical Pacific and cold SST anomalies in the western tropical Pacific. The atmospheric response occurs mainly over the tropical Pacific with strong surface westerly anomalies over the central and eastern Pacific. Simultaneously, the Walker Circulation weakens with anomalous large-scale ascending motion east of the dateline and anomalous descending motion over the Indo-Pacific region near 120°E. Associated with the anomalous sinking motion, a strong anomalous divergence is located over the Indo-Pacific region in the lower troposphere. The surface easterly anomalies near Java-Sumatra are effective in enhancing oceanic upwelling and thermocline tilting in the eastern tropical IO, which brings colder subsurface water to the surface and leads to negative SST anomalies. These cold SST anomalies can further enhance the surface easterly anomalies through the positive "Bjerknes feedback" loop, which favors the development and maintenance of the IOD. In comparison, the weak EP El Ni(?)o event composite shows a similar SST anomaly pattern over the tropical Pacific but with a much weaker intensity. Thus, the associated atmospheric response is weaker for the WEP composite. Over the Indo-Pacific region, we find much weaker sinking motion and surface easterly anomalies over the tropical IO, which are not effective in initiating the IOD.(2) In contrast, the relationship between Central-Pacific(CP) El Ni(?)o events and the IOD depends mainly on the zonal location of the sea surface temperature anomalies(r=0.93) rather than their magnitude. When the CP El Ni(?)o lies further east(ECP), the positive SSTA is confined to the central tropical Pacific, very different from the EP El Ni(?)o. And the surface westerly anomalies appear over the central equatorial Pacific. For the Walker Circulation, the center of anomalous rising air is located east of the dateline for the ECP events and the anomalous sinking air is located over the equatorial Indo-Pacific region. But when the CP El Ni(?)o lies further west(WCP), the westerly anomalies are located over the western and central equatorial Pacific. For the Walker Circulation, the center of anomalous rising air is located west of the dateline and the the anomalous sinking air is still located over the equatorial Indo-Pacific region for the WCP events. Over the southeastern equatorial IO, significant easterly anomalies occur near the Java-Sumatra coast during the ECP events while insignificant wind anomalies are found in this region during the WCP events. Away from this key upwelling region, the Bjerknes positive feedback mechanism is weak and cannot effectively produce strong negative SST anomalies over the eastern equatorial IO. Thus, the IOD is not well developed for the WCP event composite. In contrast, the ECP associated easterly anomalies are strong off Java-Sumatra, which favors the establishment of a positive IOD.(3) When EP El Ni(?)o occurs simultaneously with IOD, there is large amount of positive rainfall anomalies in the south of China and negative rainfall anomalies in the north of China. It is found that the IOD mainly plays a important role in the positive rainfall anomalies in the south of China, while the negative rainfall in the north of China is contributed by the EP El Ni(?)o. Although the CP El Ni(?)o has weak relationship with the IOD, but they also can interact with each other. Due to the sensitivity of CP El Ni(?)o to the autumn rainfall in China and the less of the sample, we cannot find much more significant signal.(4) All the 6 AMIP mode can briefly simulate the relationship between the two types of El Ni(?)o and autumn rainfall, but the simulation results of the cooperative impacts of two types and IOD on the autumn rainfall in China are different. About half of the modes can mainly simulate the results in the observation data. Such as, the GFDL-CM3 mode can better simulate the importance of IOD to the rainfall in southern China when EP El Ni(?)o occurs with IOD in autumn, although the rainfall pattern is slithtly northward, which is caused by northword of the anticyclone in WNP in this mode.Besides, both of F-GOALS-g2 and HadGEM2-A mode can simulate the importance of IOD, but they also ignore the effect of EP El Ni(?)o to the negative rainfall anomalies in the northern China.