Changing relationship between La Niña and tropical cyclone landfalling activity in South China (La Niña and TC landfalling activity in South China)

This study investigates the effect of El Nino–Southern Oscillation and Indo-Pacific warm pool sea-surface temperature (SST) on tropical cyclone (TC) landfalling activity along the South China coast and develops a scheme for predicting this activity. In general, landfalling activity tends to be suppressed in El Nino years but a large uncertainty is found for La Nina years. Landfalling activity is generally enhanced in La Nina years before 1997 but suppressed in those after 1997, which may be related to the changes in TC frequency and track pattern over the western North Pacific (WNP) as well as the northward shift in genesis locations. The Indo-Pacific warm pool SST is found to be related to the TC activity over the WNP and an SST index, defined as the average SST anomalies over the North Indian Ocean and the sea near the Philippines, is used to represent this anomaly. A significant warming of the Indo-Pacific warm pool is found in the recent decade. The temperature change of this warm pool appears to modulate the TC activity over the WNP and TC landfalling activity in South China in a La Nina year, with an enhancement (a suppression) of TC activity if a cooling (warming) is found. This difference is related to the changes in the strength of monsoon trough, vertical wind shear and steering flow pattern. Based on these results, a schematic prediction scheme of landfalling activity in South China is proposed. Landfalling activity is likely to be normal or below normal if an El Nino event is expected to develop during the TC season. If a La Nina event is expected, and the predicted state of the Indo-Pacific warm pool (obtained using the persistence forecast) shows a warming (cooling), landfalling activity is likely to be normal or below normal (above normal).