Recent studies have found a connection between Indian Ocean Basin Warming and the anomalous Northwest Pacific Anticy- clone （ANPWA） during El Nifio decaying year. This study focuses on the necessary condition for this connection by using ob- servation and numerical simulation. The seasonal transition of the Indian Ocean sea surface wind is critical to the climatic ef- fect of Indian Ocean Basin Warming. When the South Asian Summer Monsoon reaches its peak, the background wind be- comes desirable for basin warming, which then affects the climate in the Northwest Pacific. Via the Kelvin waves and Ekman divergence, the wind anomalies exist in the lower atmosphere east of the Indian Ocean warm Sea Surface Temperature （SST） anomalies, and intensify and sustain the ANWPA throughout the E1 Nifio decaying summer. This impact plays an important role in the inter-annual variability of the East Asian Summer Monsoon.
Both the tropical Indian and tropical Pacific Oceans are active atmosphere-ocean interactive regions with robust interannual variability, which also constitutes a linkage between the two basins in the mode of variability. Using a global atmosphere- ocean coupled model, we conducted two experiments （CTRL and PC） to explore the contributions of Indian Ocean interannual sea surface temperature （SST） modes to the occurrence of E1 Nino events. The results show that interannual variability of the SST in the Indian Ocean induces a rapid growth of E1 Nino events during the boreal autumn in an E1 Nino developing year. However, it weakens E1 Nino events or even promotes cold phase conversions in an E1 Nino decaying year. Therefore, the en- tire period of the E1 Nino is shortened by the interannual variations of the Indian Ocean SST. Specifically, during the E1 Nino developing years, the positive Indian Ocean Dipole （IOD） events force an anomalous Walker circulation, which then enhances the existing westerly wind anomalies over the west Pacific. This will cause a warmer E1 Nino event, with some modulations by ocean advection and oceanic Rossby and Kelvin waves. However, with the onset of the South Asian monsoon, the Indian Ocean Basin （IOB） warming SST anomalies excite low level easterly wind anomalies over the west tropical Pacific during the El Nino decaying years. As a result, the E1 Nino event is prompted to change from a warm phase to a cold phase. At the same time, an associated atmospheric anticyclone anomaly appears and leads to a decreasing precipitation anomaly over the northwest Pacific. In summary, with remote forcing in the atmospheric circulation, the IOD mode usually affects the E1 Nino during the developing years, whereas the IOB mode affects the E1 Nino during the decaying years.
HONG XiaoYuanHU HaiBoYANG XiuQunZHANG YuanLIU GuoQiangLIU Wei
