SST-Forced and Internal Variability of a Winter Wave Train over the Tropical Indo-Western Pacific and East Asia

Previous studies have indicated that a high-level wave train from the tropical Indo-Western Pacific to East Asia (IWP-EA, expressed as geopotential height at 200 hPa) is triggered by dipolar convective activity anomalies over the IWP during the boreal winter. The current study highlights the relative importance of sea surface temperature forcing versus atmospheric internal variability on the IWP-EA pattern, based on an Atmospheric Model Intercomparison Project (AMIP) experiment with 30 integrations. It was found that the SST-forcing component can reproduce the observed IWP-EA pattern and the related rainfall dipole well, for both the spatial features and temporal evolutions. The internal variability of the rainfall dipole is strong in the southern and eastern Indian Ocean and region north of Australia, while the internal variability of height generally increases with latitude. The signal-to-noise ratios are just over 1 over the northernmost lobe of the IWP-EA (Japan and the region to its east), while ratios over the other centers reach values greater than 3. An inter-member EOF analysis of the rainfall dipole indicates that the variability associated with the first two modes can explain more than 70% of the total spread over most regions with large spread for both rainfall dipole and IWP-EA, including the region over Japan. Thus, some parts of internal variability of rainfall dipole and IWP-EA are connected.