ALBERT

Description: This study investigates the interannual variability of tropical cyclone (TC)- and monsoon-induced rainfall (P) in Taiwan during July–September for the period 1950–2002. To examine the relative effects of TCs and monsoons, local rainfall in Taiwan is separated into two subcomponents: TC rainfall (PTC) and seasonal monsoon rainfall (PSM). The former is induced by TC passage across Taiwan, while the later is caused by large-scale monsoon circulation. Climatologically, PTC and PSM accounts for 47.5% and 52.5% of total rainfall in Taiwan, respectively, showing a comparable contribution. On an interannual time scale, PTC and PSM anomalies tend to vary inversely. Two dominant rainfall variability types are found in Taiwan: enhanced PTC but suppressed PSM (T+S−) and suppressed PTC but enhanced PSM (T−S+). The T+S− type features a low-level anomalous cyclone and enhanced upward motion southeast of Taiwan. This favorable environmental condition leads to more TC formation in the region. TCs are further steered by mean southeasterly flows toward Taiwan to increase PTC (T+). As Taiwan is located in the western boundary of the anomalous cyclone, anomalous northeasterly water vapor fluxes hinder moisture supplies from the South China Sea into Taiwan, resulting in decreased PSM (S−). The T−S+ type concurs with an anomalous cyclone over Taiwan. Its center enhances upward motion and moisture fluxes from the South China Sea into Taiwan, yielding increased PSM (S+). Meanwhile, weak relative vorticity anomalies occur to the southeast of Taiwan, suppressing TC formation in the region. Mean southerly steering flows tend to drive more TCs toward Japan and the North Pacific, resulting in decreased TC frequency and PTC in Taiwan (T−). The present approach provides a new perspective for studying and predicting interannual rainfall variability via the separation of rainfall into TC- and monsoon-induced rainfall subcomponents, rather than looking solely at total rainfall. The result shows that there are two ways to significantly increase total rainfall in Taiwan (T+S− and T−S+), but there is only one way to decrease it (T−S−). The composites of circulation anomalies based on two rainfall indexes have more significant and coherent dynamic patterns than those sorted based on the total rainfall index.

Description: The authors investigate persistence characteristics of sea surface temperature (SST) in the South China Sea (SCS) in association with El Niño–Southern Oscillation (ENSO). It is found that a persistence barrier exists around October and November. This fall persistence barrier (FPB) is well recognized in the developing phase of strong ENSO cases, but becomes vague in weak ENSO and normal (non-ENSO) cases. During a strong El Niño developing year, salient features of the SCS SST anomaly (SSTA) associated with the FPB include a sign reversal between summer and winter and a rapid warming during fall. One possible cause of these SST changes, as well as the occurrence of the FPB, is the development and evolution of a low-level anomalous anticyclone (LAAC). The analyses show that the LAAC emerges in the northern Indian Ocean in early northern fall, moves eastward into the SCS during fall, and eventually anchors in the Philippine Sea in northern winter. This provides a new scenario for the generation of the anomalous Philippine Sea anticyclone previously studied. Its eastward movement appears to result from an east–west asymmetry, relative to the anticyclonic circulation center, of divergent flow and associated atmospheric vertical motion/moisture fields. The eastward passage of the LAAC across the SCS warms the underlying SST first via increased absorption of solar heating in October as it suppresses convective activities in situ, and next via decreased evaporative cooling in November and December as the total wind speed is weakened by the outer flows of the eastward-displacing LAAC. As such, the SCS SST changes quickly from a cold to a warm anomaly during fall, resulting in an abrupt change in anomaly patterns and the occurrence of the FPB. Analyses also suggest that the LAAC development during fall is relatively independent from the preceding Indian summer monsoon and the longitudinal propagation features of the ENSO-related Pacific SSTA. The aforementioned ocean–atmosphere anomalies contain an opposite polarity in a strong La Niña event. The low-level circulation anomaly weakens in intensity during weak ENSO cases and simply disappears during normal cases. As a result, the SCS FPB becomes indiscernible in these cases.

Description: Tropical cyclones (TCs) moving northwestward/westward across northern Taiwan are defined to have a type-2 track. This study aims to analyze heavy rainfall associated with type-2 TCs in Taiwan, focusing on the modulation processes of the intraseasonal oscillation (ISO). During 1958–2011, 21 summer type-2 TCs are separated into three rainfall types: strong, moderate, and weak. For the strong rainfall type, both 30–60-day and 10–24-day ISOs propagate northwestward across Taiwan. During landfall and the ensuing two days, both ISOs exhibit a cyclonic anomaly centering northwest of Taiwan that causes anomalous westerly flows (or enhance seasonal southwesterly flows) from the South China Sea (SCS) onto Taiwan. Persistent moisture supply and strong moisture convergence result in prolonged heavy rainfall on the windward side over western Taiwan. TCs with weak rainfall are accompanied by a northward-propagating 30–60-day ISO from the tropical western Pacific toward Japan and a westward-propagating 10–24-day ISO along 20°N latitude. During the landfall stage both ISOs have a cyclonic anomaly with its center south of Taiwan. Major anomalous westerly flows are displaced southward across the central SCS, leading to a weak moisture supply and rainfall in Taiwan. The moderate rainfall type features a 30–60-day (10–24 day) ISO resembling that of the weak (strong) rainfall type. The amount of rainfall thus ranges between the strong and weak rainfall types. Major processes regulating the rainfall of type-2 TCs relate to the intensity of the moisture supply associated with anomalous westerly flows from the SCS onto Taiwan, which is jointly modulated by 30–60-day and 10–24-day ISOs.

Description: By separating total rainfall into tropical cyclone (TC) rainfall caused by TC passage and seasonal rainfall associated with moisture transport of prevailing seasonal flows, this study examines the interdecadal variability of these two rainfall components in Taiwan during fall. It is found that interdecadal variability of TC rainfall and seasonal rainfall tends to vary coherently in October. This coherent interdecadal rainfall mode features islandwide patterns in Taiwan and evident interdecadal oscillations. The associated large-scale regulating processes for the positive phase are characterized by warm sea surface temperature (SST) anomalies in the northern South China Sea and the western North Pacific, which in turn induce an anomalous anticyclonic circulation over the subtropical western Pacific to the east of Taiwan. On the western boundary of this anomalous anticyclone, anomalous southeasterly flows enhance mean moisture transport from the tropical western Pacific into Taiwan. Seasonal rainfall increases in Taiwan with stronger anomalies on the windward side over eastern Taiwan. Warm SST anomalies and accompanying large-scale convergence, ascendance, and positive relative vorticity anomalies provide favorable conditions for more TC formation in the Philippine Sea. More TCs tend to move along the western boundary of the anomalous anticyclone to cross the northern Philippines toward eastern Taiwan or toward the open oceans southwest of Taiwan. Taiwan is influenced by enhanced TC activity to have more TC rainfall with the largest anomalies being in the eastern parts.

Description: This study investigates interdecadal variability of summer (June–August) rainfall in Taiwan for the 1950–2008 period. Summer rainfall in Taiwan is partitioned into two components: tropical cyclone (TC) rainfall caused by TC passage and seasonal monsoon rainfall associated with monsoon southwesterly flows. The joint interdecadal mode of TC rainfall and seasonal monsoon rainfall is extracted by empirical orthogonal function (EOF) analysis. The first interdecadal mode features an increasing trend plus a near-20-yr oscillation. The spatial patterns of this mode are uniform in sign over the entirety of Taiwan with positive anomalies for TC rainfall and negative anomalies for seasonal monsoon rainfall. These results reveal that TC rainfall and seasonal monsoon rainfall tend to vary inversely in interdecadal variability, with a positive trend in TC rainfall and a negative trend in seasonal monsoon rainfall. Large-scale regulating processes associated with this interdecadal rainfall mode are interpreted from the correlation patterns. Significant warm sea surface temperature (SST) anomalies exist in the tropical central and eastern Pacific and the Indian Ocean. At the low levels, an anomalous large-scale divergent center occurs in the Australian regions, which in turn evokes an anomalous cyclonic circulation in the subtropical North Pacific. Taiwan is on its western edge and affected by anomalous northeasterly flows, in company with weakening in the prevailing southwesterly flows and moisture transport from the South China Sea into Taiwan. As such, negative seasonal monsoon rainfall anomalies occur in Taiwan with a decreasing trend. The subtropical anomalous cyclonic circulation also weakens vertical wind shear over the major TC genesis region, that is, the Philippine Sea. Warm SST anomalies in this region and accompanying anomalous ascending motion provide additional favorable conditions for TC genesis. More TCs are thus formed in the Philippine Sea. The appearance of an anomalous cyclonic circulation in the subtropical North Pacific reflects a weakening of the Pacific subtopical high, which tends to retreat eastward and provides southeasterly or southerly flows on its western boundary to guide TCs formed in the Philippine Sea northwestward toward Taiwan. TC frequency and TC rainfall thus increase in Taiwan with an increasing trend.

Description: Tropical cyclones (TCs) of a particular track type move northward along the open oceans to the east of Taiwan and later pass over or near northern Taiwan. Their northward movement may be associated with intensified monsoon southwesterly flows from the northern South China Sea (SCS) toward Taiwan. Prolonged heavy rainfall then occurs in western Taiwan across the landfall and postlandfall periods, leading to severe floods. Characteristics of this TC–southwesterly flow association and related large-scale regulatory processes of intraseasonal oscillations (ISOs) are studied. For summers from 1958 to 2009, 16 out of 108 TCs affecting Taiwan exhibit the aforementioned northward-moving track. Among them, four TCs (25%) concur with enhanced southwesterly flows. Intensified moisture supplies from the SCS result in strong moisture convergence and prolonged heavy rainfall in western Taiwan. Both 30–60- and 10–24-day ISOs make positive contributions to the TC–southwesterly flow association. Both ISOs exhibit the northward progress of a meridional circulation pair from the tropics toward Taiwan. During landfall and the ensuing few days, Taiwan is surrounded by a cyclonic anomaly to the north and an anticyclonic anomaly to the south of these two ISOs. The appearance of anomalous southwesterly–westerly flows acts to prolong heavy rainfall in western Taiwan after the departure of a TC. The TC–southwesterly flow association tends to occur during the minimum phase of the 30–60-day ISO featuring a cyclonic anomaly in the vicinity of Taiwan but in various phases of the 10–24-day ISO. Rainfall in western Taiwan increases when these two ISOs simultaneously exhibit a cyclonic anomaly to the north of Taiwan.