Roles of Regional Ocean-Atmosphere Coupling and Remote Forcing in Northwest Pacific Monsoon Variability

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Summer is the rainy season for East Asia. The Meiyu-Baiu rain band, extending from the eastern flank of the Tibetan Plateau northeastward through Korea and Japan, is the single most important climate phenomenon of the region. The termination of this summer rain band in late July has been tied to the abrupt onset of tropical convection over the Northwest Pacific east of the Philippines, through a meridional dipole pattern of summer rainfall variability. The abrupt onset of Northwest Pacific convection remains unsolved with local sea surface temperature (SST) rising above 28 degrees C as early as in June. Effort under this project will investigate the hypothesis that the gradual moistening of the lower troposphere via shallow convection leads to a threshold behavior of abrupt convective onset over the tropical Northwest Pacific. During the satellite era starting 1979, interannual variability in tropical Northwest Pacific convection is well correlated with El Nino/Southern Oscillation (ENSO). Specifically, an anomalous anticyclonic circulation (AAC) forms and rainfall decreases over the tropical Northwest Pacific in post-El Nino summers. Through earlier studies a positive feedback has been identified due to interactions between the AAC and Indo-Northwest Pacific SST during summer when the southwest monsoonal winds prevail. We will investigate the mechanisms and predictability of this regional coupled mode. The correlation between the AAC and ENSO breaks down during half of the 20th century from the 1920s and 1970s. Weak ENSO amplitudes are considered to have caused the low correlation. This aspect as well as the role of random noise due to atmospheric internal variability will be explored critically. Locally, Northwest Pacific convection affects the U.S. affiliated Northern Mariana Islands and Guam. Remotely, it affects the South and East Asian monsoon regions, home to three billion people. This research will help improve seasonal forecast and climate change projection in the regions, thereby contributing to several ongoing international research programs. The project trains the next generation of climate scientists by including graduate students and postdocs in the research team, and by exposing them to international collaborations.