Recurrent Rainfall Patterns as Important Modes of Asian Monsoon Variability
Fluctuations in the East Asian summer monsoon (EASM) often involve floods and droughts that affect large populations but remain challenging for even basic simulation in climate models. Observations show two dominant EASM rainfall spatial structures: the south flood-north drought pattern and the tripolar pattern in East Asia and its seaboard. Researchers found that these two recurrent EASM rainfall spatial patterns correspond to dynamical modes, patterns of variability, of the governing climate system. They used energy flows to connect the modes to their linked optimal forcing patterns. By employing an energetic framework and the systematic forcing-response relationships, researchers can predict the south flood-north drought pattern long-term trend in China.
2020 saw record-breaking Meiyu/Baiu rainfall over East Asia, the most since 1961. The rainfall can be thought of as a response in the modes to forcing representing heat transfer from the ocean to the atmosphere, known as ocean heat flux. This offers an alternative perspective on the source of forcing and, therefore, predictability for the major recurrent modes of EASM rainfall variability. The analysis framework extends the previous energetics thinking from a one-dimensional, idealized setting to the two-dimensional real world. The analysis also unveils the essential positive feedback roles of cloud and water vapor in amplifying EASM variability.
Previous observations of the EASM have shown coherent patterns of variability, but whether they represent the true dynamical modes of the climate system remains unknown. This study probed modeled ocean heat flux forcing from globally representative ocean locations to construct the systematic forcing-response relationship for global precipitation. This allowed researchers to extract the leading dynamical modes of variability, known as neutral modes, from the data. The results confirm the dynamical basis of the empirically determined monsoon rainfall patterns, particularly the south flood-north drought dipole over eastern China, and offer an explanation of the patterns based on the shift and/or pulsing of the global atmospheric energy flow. The dynamical mode featuring the south flood-north drought dipole is acutely responsive to an inter-hemispherically contrasting ocean heat flux. As such, the slowly evolving ocean heat uptake under increased CO2 forcing, which projects strongly onto this heat flux pattern, may contribute to a long-term south flood-north drought trend.