Precursory Signals Associated with the Interannual Variability of the Asian Summer Monsoon

In this work, the interannual variability of the Asian summer monsoon is studied by analyzing outputs of the Atmospheric Model Intercomparison Project integration using the Goddard Laboratory for Atmospheres general circulation model. The main effort is devoted to exploring and understanding precursory signals associated with the interannual variability of the Asian monsoon and deciphering possible physical mechanisms responsible for the signals. It is found that strong precursory signals of highly anomalous Asian summer monsoon appear over the subtropical Asian region during the previous winter-spring seasons. Prior to a strong summer monsoon, the westerlies over subtropical Asia are weaker than normal, and vice versa. Dynamically consistent changes are found in other fields such as atmospheric temperature, geopotential height, and surface temperature. These precursory signals seem to have a barotropic structure in the troposphere. They exist over a broad region and last for two to three seasons. The robustness of the signals is important for improving seasonal prediction of the Asian summer monsoon. It is hypothesized that the above-described precursory signals of the Asian summer monsoon are linked to land-surface hydrologic processes, such as changes in snow mass and soil moisture in the Asian continent, as well as anomalous sea surface temperature forcing in the Pacific and Indian Oceans. Evidence in support of the above hypothesis can be found in the variability of a variety of parameters from both model simulation and observations.