Asymmetry of the El Niño-spring rainfall relationship in Taiwan

Abstract

Spring rainfall in Taiwan can be either enhanced or suppressed by an El Niño event, revealing an asymmetric relationship. This observational study aims at examining this asymmetric relationship and associated large-scale dynamic processes. Analysis results disclose four major El Niño/Southern Oscillation (ENSO)-spring rainfall relationship types during 1950-2003: El Niño-anomalous wet (LN-w) type, La Niña-anomalous dry (LN-d) type, El Niño-anomalous dry (EN-d) type, and La Niña-anomalous wet (LN-w) type. The EN-w and LN-d (EN-d and LN-w) types exhibit a positive (negative) correlation between the spring rainfall anomaly and the ENSO-related sea surface temperature anomaly (SSTA). The overall ENSO-spring rainfall relationship is dominated by the positive correlation. The cause of the asymmetric ENSO impacts betw een the positive- and negative-correlation groups is attributed to a connection between ENSO and the Indian Ocean (IO) SSTA and associated large-scale atmospheric circulation. The positive-correlation types tend to concur with an evident ENSO-IO connection, featuring significant in-phase SSTA centers in the tropical eastern Pacific and eastern IO. During the El Niño (La Niña) event, these SST anomalies force a descending (ascending) branch over the western Pacific and help initiate and maintain a lower-level anticyclone (cyclone) anomaly in the Philippine Sea (southeast of Taiwan). Flows west of this anomalous anticyclone (cyclone) enhance (suppress) moisture transport from the South China Sea into Taiwan, resulting in increased (decreased) spring rainfall. For the negative-correlation types, the ENSO-IO connection tends to be weak or broken. The significant SSTA appears only in the tropical eastern Pacific, which induces a major vertical motion branch over the maritime continent. As a result an anomalous lower-level anticyclone (cyclone) occurs over the Asian continent (west of Taiwan) during the El Niño (La Niña) event Flows east of this anomalous anticyclone (cyclone) weaken (strengthen) moisture transport from the South China Sea into Taiwan, leading to suppressed (enhanced) spring rainfall. It is also no ted that the variability of the Pacific subtropical high (PSH) over the western Pacific is closely linked to these four relationship types. The EN-d (LN-w) type concurs with a moderate westward expansion (eastward retreat) in the western-Pacific sector, while the EN-w (LN-d) type is concurrent with a southward (great eastward) displacement. © 2008, Meteorological Society of Japan.