Type of Document Dissertation Author Lim, Young-Kwon Author's Email Address email@example.com URN etd-06282004-155058 Title Diagnosis of the Asian Summer Monsoon Variability and the Climate Prediction of Monsoon Precipitation via Physical Decomposition Degree Doctor of Philosophy Department Meteorology, Department of Advisory Committee
Advisor Name Title Kwang-Yul Kim Committee Chair Henry E. Fuelberg Committee Member Ruby Krishnamurti Committee Member Sharon E. Nicholson Committee Member Tiruvalam N. Krishnamurti Committee Member Keywords
- Climate Prediction
- Asian Summer Monsoon
- Monsoon Variability
- Long-Range Forecast
Date of Defense 2004-05-25 Availability unrestricted AbstractThis study investigates the space-time evolution of the dominant modes that constitute the Asian summer monsoon (ASM), and, as an ultimate goal, the climate prediction of the ASM rainfall. Precipitation and other synoptic variables during the prominent life cycle of the ASM (May 21 to September 17) are used to show the detailed features of dominant modes, which are identified as the seasonal cycle, the ISO defined by the 40-50 day intraseasonal oscillation including the Madden-Julian oscillation and El Niņo mode.
The most pronounced feature of the seasonal cycle is the evolution of the sea level pressure anomaly and the ensuing evolution of other variables. The northward migration of a negative pressure anomaly and the accompanying cyclonic vortex over the Indian Ocean and the resulting moisture transport toward the Indian continent by the low-level westerly (Somali jet) depict the mean evolution of the precipitation field over India, the Bay of Bengal, and the Indochina peninsula during the early stage of the ASM. Over the western Pacific, a positive pressure anomaly cell pushes the precipitation band between 25° and 40°N northward, characterizing the onset and the early evolution of the regional monsoons in East Asia (China, Japan and Korea). This positive pressure anomaly, intruding into southern China and further west to the Bay of Bengal from mid June to mid July, accelerates the low-level wind along the eastern coast of the Asian continent, where the pressure gradient is maximum. It provides a favorable condition for moisture transport toward the East Asian countries.
Two major sources of moisture, i.e., the Indian Ocean and the western Pacific Ocean, play distinct roles during the varying phases of the monsoon. Precipitation over the Indian region is affected persistently by the Indian Ocean. Influences of the western Pacific Ocean dominate the precipitation over the Indochina peninsula in the early monsoon period followed by the influence from the Indian Ocean (mid June to mid July), and the combined contribution afterward. The East Asian monsoon regions are affected by both sources from the onset of their monsoons. Since late July, the contribution from the western Pacific Ocean is greater due to the development of a strong negative pressure anomaly pattern over the subtropical western Pacific.
The present study reveals that the ISO is the second largest component of the ASM rainfall variation. Correlation analysis indicates that ISO explains a larger fraction of the variance of the observed precipitation (without climatology) than the ENSO mode.
The dominant ISO signal faithfully explains the northward propagation of the ISO toward the Asian continent causing intraseasonal active/break periods. The interannual variation of the ISO strength suggests that the ENSO exerts some influence on the ISO. The composite convective ISO anomaly and Kelvin-Rossby wave response over the Indian Ocean shows that the ISO tends to be stronger during the early stage of the ASM than normal in El Niņo (La Niņa) years, indicating greater (smaller) possibility of ISO-related extreme rainfall over India, Bangladesh, and the Bay of Bengal.
The ENSO mode reveals that the following factors affect the evolution of the ASM system in El Niņo (La Niņa) years. (1) The anomalous sea surface temperature and sea level pressure over the Indian Ocean during the early stage of the ASM weaken (enhance) the meridional pressure gradient. (2) As a result, the westerly jet and the ensuing moisture transport toward India and the Bay of Bengal become weak (strong) and delayed (expedited), providing a less (more) favorable condition for regional monsoon onsets. (3) The Walker circulation anomaly results in an enhanced subsidence (ascent) and drought (flood) over the Maritime continent. (4) The Hadley circulation anomaly over the western Pacific drives the wetter (drier) south China monsoon, the weaker (stronger) East Asian monsoon, and the wetter (drier) late July and early August over India, the Bay of Bengal, and the Indochina peninsula. (5) The ASM system appears to exert positive feedback on the El Niņo (La Niņa) by accelerating the westerly (easterly) anomaly toward the equatorial western Pacific in August. (6) ENSO effects tend to last until the early stage of the ASM in the following year.
Based on the modal decomposition of the ASM variability, a new paradigm for climate (one month and longer) prediction is developed and is applied to the 5-day averaged ASM precipitation. The foundation of the method is to predict the amplitude of each climate signal (e.g., seasonal cycle, ISO, etc.) that constitutes the ASM system. Prediction is much facilitated by forecasting the slowly undulating amplitude time series. The present method extends the predictability of the ASM pentad precipitation event to six months in certain regions with correlation greater than 0.4. Also, ISO propagation was successfully predicted 120 days ahead of time with correlation greater than 0.4.
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