Similarities And Differences Of The Influence Of The Sea Surface Temperature Anomalies Over Subtropical North And South Pacific On ENSO

The El Ni?o-Southern Oscillation(ENSO) can influence Earth’s climate over most regions of the globe. The El Ni?o events are classified into two types on the basis of the spatial pattern of the sea surface temperature anomalies(SSTAs): one is the canonical Pacific(EP) type with maximum SSTAs centered in the eastern equatorial Pacific and another is central Pacific(CP) type with maximum SSTAs centered in the central Pacific. It is suggested that climatic features, such as Chinese regional precipitation patterns, show obvious difference during different El Ni?o types. Moreover, SSTAs over the subtropical North and South Pacific contribute to the formation of different types of ENSO. Therefore, a study of similarities and differences of the influence of the SSTAs over subtropical North and South Pacific on ENSO is of great importance for Chinese regional rainfall anomaly prediction in ENSO years. Firstly, based on the characteristics of North Pacific Meridional Mode(NPMM) and South Pacific Meridional Mode(SPMM), two time series which could indicate the evolution of both modes are defined and the influence of both modes on ENSO are investigated from a statistical perspective. Furthermore, factors in the atmosphere which could impact the development of both modes are found, and an Atmospheric factors–NPMM/SPMM–ENSO relationship is constructed. As an additional work, the 2014 El Ni?o which had a close relationship with SPMM were analyzed, further complement the statistical analysis work. The present study consists of two parts:1. On the basis of the observation data and the study on the NPMM, the spatial distribution pattern of SSTAs, surface wind and precipitation anomalies of both NPMM and SPMM are obtained, which confirmed the existence of SPMM.In addition, SPMM and NPMM have different seasonal variation characteristics. This study focuses on the impact of NPMM/SPMM in previous winter on the development of ENSO in following year. By two methods, regression analysis and synthesis analysis, it is confirmed that the SPMM is more conducive to the development of SSTAs in Ni?o1+2 and Ni?o3 areas, especially in Ni?o1+2 area, while NPMM is more conducive to the development of SSTAs in Ni?o3.4 area.The South Pacific mid-latitude sea level pressure in boreal winter, the zonal wind field of tropical western Pacific in boreal summer winter, and the SSTAs of central and eastern equatorial Pacific in boreal autumn are well coupled. In terms of the influence on ENSO, SPMM and NPMM may involve similar mechanisms. Similar to its North Pacific counterpart, in the South Pacific, the Pacific-South American(PSA) pattern manifests as large-scale sea-level pressure oscillations, and the variability of the semi-permanent subtropical high pressure system caused by the PSA effectively modulates the strength of the southeasterly trade winds in the tropical latitudes and causes SSTAs via changes in wind-induced latent heat flux in boreal winter and spring. The relevant meridional SST gradient can persist into boreal summer and in turn induce atmospheric circulation changes in the tropical Pacific, which involves Wind–Evaporation–SST(WES) feedback.2. An El Ni?o event was predicted to occur in the following winter of 2014. Several models even predicted a stronger event with amplitude of approximately 2°C comparable to 1997–1998 El Ni?o. However, the El Ni?o that started to develop in 2014 was hindered in the boreal summer, and only the ocean reached a weak El Ni?o condition. This outcome was largely attributed to a suppressed ocean-atmosphere interaction caused by anomalous easterly winds in the eastern equatorial Pacific. These winds were related to negative sea surface temperature anomalies(SSTAs) in the southeastern subtropical Pacific(SESP) which related to the evolution of SPMM. The anomalous easterly winds decoupled the equatorial SST warming from convection, wind and oceanic subsurface variability over the equatorial Pacific. The continuing warmer state over the western Pacific might also contribute to the suppression of the warm event development. As a result, the 2014 El Ni?o pattern was hindered during the boreal summer of 2014.Furthermore, the negative phase of the Interdecadal Pacific Oscillation(IPO) laid the foundation for the persistence of cooler SSTAs and enhanced trade winds in the SESP after the year 2000. As the recent IPO downward trend continued, the SSTAs in SESP reached an extremely low value in 2014 and imposed a serious obstacle to the evolution of a warming event. It indicates that the NPMM and the SPMM can not only promote the occurrence and development of ENSO, but also effectively prevent the occurrence and development of ENSO under certain background conditions.