Interannual variability of circulation-rainfall relationship in Taiwan during the Mei-yu season

This study aims to investigate interannual variability of seasonal rainfall in Taiwan during the Mei-yu season (May-June). Its relationship with the low-level circulation anomaly and associated large-scale regulatory processes are also examined. It is found that the circulation-rainfall relationship is largely determined by the spatial distribution between Taiwan and the circulation anomaly off the southeastern coastlines of East Asia. For seasons when Taiwan lies under and near the anomalous circulation centre, strong downward (upward) motion and anomalous easterly (southwesterly) moisture flux assocaited with an anomalous high (low) provide large-scale environments favourable to decreased (increased) Mei-yu rainfall in Taiwan, resulting in a negative correlation in the circulation-rainfall relationship (anomalous high vs anomalous dry and anomalous low vs anomalous wet). For seasons when Taiwan is located over the northern boundary of the circulation anomaly, anomalous southwesterly (northeasterly) moisture flux induced by an anomalous high (low) enhances (suppresses) moisture supplies from the South China Sea into Taiwan to set up large-scale conditions suitable for increased (decreased) Mei-yu rainfall. A positive correlation in the circulation-rainfall relationship (anomalous high vs anomalous wet and anomalous low vs anomalous dry) thus occurs. The Pacific subtropical high (PSH) and monsoon trough (MT) tend to have a coherent spatial (eastward or westward) displacement in each rainfall variability type. The appearance of a low-level anomalous high (low) in the Taiwan region corresponds to a PSH expansion (retreat) and a MT retreat (expansion) towards the west (east). A strong (moderate) displacement in the PSH and MT is accompanied by a low-level circulation anomaly centering near (south of) Taiwan and a negative-correlation (positive-correlation) variability type. Analyses also suggest that large-scale processes at low latitudes play a more important role than midlatitudinal transient activity in regulating interannual variability of Mei-yu rainfall over subtropical Taiwan. Copyright (C) 2009 Royal Meteorological Society