Tuesday, 11 February 2003: 5:00 PM
The coupled monsoon-ocean mode and its impacts on asian-australian monsoon
Using extended singular value decomposition analyses, we characterize the evolution of Asian-Australian monsoon (AAM) anomalies during El Niņo as two low-level, off-equatorial anticyclones--one over the South Indian Ocean (SIO), the other over the western North Pacific (WNP). The SIO anticyclone, which is responsible for unusual climate conditions over Indian Ocean, India, and east Africa, originates in boreal summer while El Niņo develops, reaches its height in fall, and decays before El Niņo matures. On the other hand, the WNP anticyclone, forms in fall, attains its maximum intensity when El Niņo matures, and persists through the next spring and summer, providing a prolonged impacts on East Asian monsoon. The A-AM anomalies associated with ENSO turnabout shows a prominent biennial tendency of variability. Although the remote El Nino influences the monsoon by suppressing convection in maritime continent, the El Niņo forcing alone cannot explain the extraordinary amplification of the SIO anticyclone in the developing stage of El Niņo nor the maintenance of the WNP anticyclone in the decaying phase of El Niņo. The atmosphere-ocean conditions in the two regions of anticyclones are similar, namely, a SST dipole with cold water to the east and warm water to the west of the anticyclone center. These conditions result from a positive feedback between the anticyclone and SST dipole, which intensifies SIO anticyclone during El Nino development and maintains the WNP anticyclone during its decay. The detailed mechanisms are elaborated The outcome of this interaction depends crucially on the background flows. The coupled mode over the SIO is often triggered by El Niņo, but also possibly by other local or remote forcing.