The Time–Space Structure of the Asian–Pacific Summer Monsoon: A Fast Annual Cycle View*

Abstract

Despite the seemingly intricate and multifold time-space structure of the mean Asian-Pacific summer monsoon (APSM), its complexity can be greatly reduced once the significance of fast annual cycles has been recognized and put into perspective. The APSM climatology is characterized by a slowly evolving seasonal transition (slow annual cycle) superposed by pronounced singularities in the intraseasonal timescale, termed the ''fast annual cycle'' in this study. The fast annual cycles show nonrepetitive features from one episode to another, which are often divided by abrupt change events. The APSM fast annual cycles are composed mainly of two monsoon outbreaks, each marking a distinctive dry-wet cycle. The first cycle spans from the middle of May to early July and the second cycle from late July to early September. When the first cycle reaches its peak in mid-June, a slingshot-like convection zone, described as the grand-onset pattern, rules an area from the Arabian Sea to the Indochina Peninsula then bifurcates into a mei-yu branch and a tropical rain belt in the lower western North Pacific. After a brief recess during 20-29 July, the APSM harbors another rain surge in mid-August. This time a giant oceanic cyclone intensifies over the western North Pacific (around 20ЊN, 140ЊE); thus the rainy regime jumps 10Њ-15Њ north of the previous rain belt. This ocean monsoon gyre incubates numerous tropical cyclones. Meanwhile, the convection zone of the Indian monsoon intensifies and extends well into the subcontinent interior.From the first to second cycle the major convection center has shifted from the adjacent seas in the northern Indian Ocean to the open ocean east of the Philippine Islands. The major cloud movement also switches from a northeastward direction in the Indian Ocean to a northwestward direction over the western North Pacific.The two monsoon cycles turn out to be a global phenomenon. This can be shown by the coherent seasonal migration of upper-level subtropical ridgelines in the Northern Hemisphere. During the first cycle all the ridgelines migrate northward rapidly, a sign that the major circulation systems of boreal summer go through a developing stage. After 20-29 July, they reach a quasi steady state, a state in which all ridgelines stand still near their northern rim throughout the entire second cycle.A reconstructed fast annual cycle based on four leading empirical orthogonal function modes is capable of reproducing most fine details of the APSM climatology, suggesting that the subseasonal changes of the mean APSM possess a limited number of degrees of freedom. A monsoon calendar designed on the basis of fast annual cycles (FACs) gives a concise description of the APSM climatology and provides benchmarks for validating climate model simulations.

Characteristics of Summertime Circulation Patterns for Southern Taiwan’s Monsoon Rainfall from July to September

“…However, the rainfall of Monsoon II is less due to the suppression of convection by the western North Pacific high. Lin and Wang (2002) also find a similar pattern during their Pentad 46 -47. On the other hand, the Monsoon II-TC pattern, though less significant, yields a structure with a deeper monsoon trough located south of the subtropical ridge.…”

“…Overall, the circulation patterns of Monsoon I and I-TC resemble the large-scale Mei-Yu-like pattern as in Lin and Wang (2002) and Hsu (2005). Unlike the Monsoon I and I-TC circulation patterns, Monsoon II yields a weak trough over the South China Sea along with a convective area extending eastward to 10°N, 155°E.…”

“…Unlike the flow pattern in the Mei-Yu season, strong southeasterly wind exists from the southern flank of the subtropical anticyclone toward Japan in the second half period (Hsu 2005). Lin and Wang (2002) further introduce the concept of "fast annual cycles" which are composed of two major monsoon phases during the East Asian summer monsoon. The first phase spans mid-May to early July and the other from late July to early September.…”

Real-time multivariate indices for the boreal summer intraseasonal oscillation over the Asian summer monsoon region

“…In this study we also consider the third and fourth MV-EOF modes and argue that they capture the northward and northwestward propagating component of the BSISO, particularly during the pre-monsoon and monsoon onset period (Wang and Xie 1997;LinHo and Wang 2002). Different to the first two modes, the OLR and U850 patterns in EOF3 and EOF4 tend to be more in phase over the ISM and WNP region with a southwest-northeast tilted horizontal structure (Fig.…”

“…However, since the RMM index is designed to depict all year round MJO activity, it is not expected to fully represent the seasonality of the ISO, especially during the peak of the boreal summer when ISO activity is furthest from the equator. Figure 1 shows the variance of pentad mean OLR after removing the climatological slow annual cycle (LinHo and Wang 2002) and interannual variability, separately for boreal winter (November to April) and summer (May to October), together with the fractional variance of the anomaly explained by the two-component RMM index during the two seasons. It is noted that the RMM index is capable of capturing a large fraction of the intraseasonal OLR variance during boreal winter over the major convective regions except over the South Pacific convergence zone.…”