Eastern-and Central-Pacific ENSO And Their Possible Different Impacts On Precipitation In China

The El Nino-Southern Oscillation (ENSO) is the dominant mode of air-sea coupled interaction in the tropical Pacific, manifesting important impacts on the global climate. It has been increasingly recognized that there are two distinct types of ENSO events prevailing in the tropical Pacific:one is the Eastern-Pacific (EP) type that has sea surface temperature (SST) anomalies centered over the eastern Pacific, while the other is the Central-Pacific (CP) type with the SST anomalies situated over the central Pacific. Based on the phenomena of the two types of ENSO events, the air-sea coupled modes and the periodic variation of atmosphere and ocean signals in different ENSO category are analyzed in this thesis. The key point is the different tropic atmosphere responses and East Asian climate anomaly associated with the two types of ENSO. Also the simulations of climate system models in multi-scenarios (i.e., the pre-industrial, historical, and future climate projection, namely Representative Concentration Pathways4.5, RCP4.5) released by the Coupled Model Intercomparison Project phase5(CMIP5) are used to evaluate the simulating capability of both EP-ENSO and CP-ENSO, with an attempt to reveal the natural oscillation of EP-and CP-ENSO and their possible responses to the global climate change in future. The major conclusions are summarized as follows:(1) The EP-and CP-ENSO events show the2-7and the10-15years oscillation in the tropical SST anomalous field, respectively. The coupling between subsurface ocean temperature (SOT) and SST anomalies is featured in a zonal dipole mode in the EP-ENSO group, while in a zonal tripole mode in the CP-ENSO category. During the mature phase of CP-ENSO, the warm center of SOT anomalies is located to the west of dateline to increase the SOT in the Nino4region, causing the CP-ENSO event simultaneously. The eastern and central Pacific subsurface indices are defined by the expansion coefficients of the first and third SVD mode for SOT. Such indices are not only highly correlated with those deduced by other researchers, but they can well describe and distinguish the EP and CP-ENSO events. In addition, the SOT amplitude on the interdecadal time scale has enhanced since the late1980s, indicating the increment of frequency and intensity of CP-ENSO events. This result suggests that the impact of CP-ENSO events on the global climate has been more significant in the past decades.(2) There are differences in the responses of EP-and CP-El Nino to the zonal and vertical circulation in the tropics. Accordingly, a new eastern and central Pacific southern oscillation index (i.e., EP-and CP-SOI) is defined based on the air-sea coupled relationship in the tropics, respectively. Results suggest that EP-SOI and CP-SOI is coupled with the EP-and CP-El Nino, respectively. The EP-SOI exhibits interannual variability (2-7yr), while decadal (10-15yr) variations in the CP-SOI are more dominant. During the EP-ENSO, the Walker circulation shows a dipole structure with a signal cell over the Pacific. However, the Walker circulation shows a tripole structure with double cells over the Pacific when CP-ENSO prevails. Furthermore, the correlation of seasonal precipitation across mainland China with EP-ENSO is opposite to that with CP-ENSO during winter and spring. But the differences in relationship between rainfall and the two types of ENSO is only restricted in the lower reaches of Yangtze River and Southern China (SC) during summer and fall, respectively. Such contrary relationship associated with the EP-and CP-ENSO events is primarily reflected on the interannual time scale. For example, EP-ENSO events generally lead to more rainfall over SC in winter, while the negative rainfall anomalies are observed in SC during the CP-ENSO events. And the distinct winter rainfall anomalies over SC responses to EP-and CP-ENSO can be attributed to the difference in both strength and location of the Northwestern Pacific anticyclone circulation. Therefore, the different response of Chinese rainfall to EP-and CP-ENSO should be taken into consideration when predicting the seasonal rainfall in East Asian monsoon regions.(3) The control runs of20climate models in the pre-industrial are examined to evaluate the capability of simulating air-sea coupling modes in EP-and CP-ENSO. The results point out that the current models have higher skills in simulating the interannual variances than the interdecadal variability of SST anomalies in the tropical Pacific. And the EP-ENSO can be better simulated than the CP-ENSO by the current climate models. All the20models can simulate the observed EP-ENSO modes, but only12in them exhibits good performance in simulating the CP-ENSO modes. The composite result of the12model outputs suggests that the model-simulated EP-and CP-ENSO exhibits an interannual and multi-decadal oscillation, respectively. Because there is no external forcing in the control runs during the pre-industrial, such results support that both the decadal oscillation of CP-ENSO mode and the interannual oscillation of EP-ENSO mode are due to the air-sea interaction in natural climate system.(4) The climate system models in CMIP5can well simulate the spatial distribution of coupling modes and their periodic changes associated with EP-and CP-ENSO in the historical runs. The historical simulated EP-ENSO shows an air-sea coupling mode in zonal dipole distribution and exhibits interannual variability, while the simulated air-sea coupling mode of CP-ENSO is a zonal tripole pattern with evident decadal oscillation. The frequency and intensity of CP-E1Nino events simulated by the CMIP5models have been enhanced as that in observation. Results of four global climate system models for the21st century assessment indicate that primary characteristics of air-sea coupling modes and periodic variation associated with the two types ENSO events will be still maintained under the background of global warming, and the their strengths would increase in the future. Moreover, the major reason for changes in the long-term trend of SST over the tropical Pacific is the external forcing factors. Although the decadal variation of CP-ENSO is a natural oscillation of air-sea interaction in climate system, it is little influenced by the anthropogenic greenhouse gas forcing.