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Please use this identifier to cite or link to this item: http://hdl.handle.net/2031/4630

Title: Variability and predictability of South China Sea summer monsoon : from interdecadal to intraseasonal
Other Titles: Nan hai xia ji feng de bian hua ji ke yu bao xing : cong nian dai ji zhi ji jie nei
南海夏季風的變化及可預報性 : 從年代際至季節內
Authors: Zhou, Wen (周文)
Department: Dept. of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2004
Publisher: City University of Hong Kong
Subjects: Monsoons -- China, Southeast
Monsoons -- Forecasting
Notes: CityU Call Number: QC939.M7 Z46 2004
Includes bibliographical references (leaves 152-168)
Thesis (Ph.D.)--City University of Hong Kong, 2004
xxii, 168 leaves : ill. (some col.) ; 30 cm.
Type: Thesis
Abstract: The objective of this research is to explore the variability and predictability of the South China Sea summer monsoon (SCSSM) on timescales ranging from interdecadal to intraseasonal. First of all, an observational study of monsoons from the general circulation point of view is a prerequisite for studying its predictability, which aims to answer the questions of to what extent and under what conditions can the occurrence and strength of SCSSM be predicted on various timescales. Interdecadal variations in the monsoon rainfall over South China (SCMR) are examined by analyzing their relationships with ENSO (El Niño/Southern Oscillation) and PDO (Pacific Decadal Oscillation). An interdecadal variation in the SCMR can be identified, with more dry monsoon years in two periods (1925-1946 and 1997-1995), but more wet years during the 1947-1976 period. These three epochs accord with shif ts in the PDO, with wet (dry) epochs coinciding with low (high) PDO index. Wavelet analyses show the interdecadal timescale of both PDO and ENSO correlating well with that of SCMR. These results suggest that the PDO (a decadal signal) could couple with the ENSO (an interannual signal) to affect the SCMR rhythm on decadal timescales. Indeed, it is found that when the ENSO and PDO are in phase, i.e. high PDO phase/El Niño events, or low PDO phase/La Niña events, the SCMR tends to be below or above normal respectively more often, and hence the SCMR is more predictable. But an out-of-phase ENSO-PDO relationship gives a lower SCMR predictability. A plausible mechanism is as follows. In an in-phase scenario, the sea-surface temperature anomalies (SSTA) associated with the PDO and the ENSO over the North Pacific are of the same sign. The SSTA then modifies the intensity of the subtropical high east of the Philippine Sea. An enhanced subtropical high would then extend further westward into the South China Sea while a weakened one would not, which then causes variations in the SCMR. On the other hand, for an out-of-phase scenario, whether the SSTA is positive or negative depends on whether the PDO or the ENSO dominates. Furthermore, in such a scenario, anomalies of the subtropical high associated with the mature phase of the ENSO event will be opposite in sign to those associated with the PDO. The two circulation anomalies therefore tend to cancel each other, which results in a low predictability of the strength of the subtropical high, and hence the intensity of the SCMR. On an interannual timescale, the relationship between the onset date of SCSSM and ENSO is examined. The ENSO signal is represented by the ocean heat content (OHC), which is proportional to the depth of the 20 oC isotherm. It is found that associated with a warm (cold) ENSO event or the year after, the monsoon tends to have a late (an early) onset and the intensity of the SCSSM also tends to be weaker (stronger). During a two-year period prior to the onset, anomalies of OHC have an obvious eastward propagation. The 850-hPa flow east of the Philippines, specifically the strength of the subtropical high, is also found to be critical in determining the onset date. The link between these two results appears to be the propagation of cold (warm) sub-surface water into the western North Pacif ic, which strengthens (weakens) the subtropical high, and hence a late (an early) SCSSM onset. The intraseasonal variability of the SCSSM has two preferred time scales (30- 60 days and 10-20 days). The 30-60-day mode in the intraseasonal oscillation (ISO), which originates from convection over equatorial Indian Ocean, exhibits an initial eastward and then northward propagations, while the 10-20-day mode exhibits a northwestward propagating associated with the retreat of the western Pacific subtropical high. It is shown that both the 10-20-day and 30-60-day modes are less active during winter to spring, and tends to be more active prior to the monsoon onset, and then most active after onset. The coupling of the two modes prior to the monsoon onset might thus lead to the monsoon outbreak, and further modulate the monsoon active/break cycle. Another important feature is that the ISO prior to the monsoon onset seems more active in cold ENSO years, which could be a reason why SCSSM onsets earlier in a La Niña year. A possible hypothesis is then proposed that the evolution of SCSSM results from a combination of oscillations at different timescales, with the PDO setting a long-term background, while the rhythm of ENSO tunes its annual magnitude, and finally the ISO amplitude modulates not only the monsoon onset/end cycle but also the active/break cycle. Based on this assumption, the SCSSM is potentially predictable in terms of the different timescales.
Online Catalog Link: http://lib.cityu.edu.hk/record=b1871451
Appears in Collections:AP - Doctor of Philosophy

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