Temporal and Spatial Evolution of the Asian Summer Monsoon in the Seasonal Cycle of Synoptic Fields

Abstract

The principal mode of the seasonal variation of the Asian summer monsoon (ASM) and the temporal and spatial evolution of the corresponding synoptic fields are investigated via cyclostationary EOF analysis. This study uses the 21-year (1979-99) Xie-Arkin precipitation pentad data and National Center for Environmental Prediction daily reanalysis data focusing on the period May 21 to August 28, which covers the prominent life cycle of the ASM. The first mode, representing the seasonal cycle, explains about 20~40% of the total variability in the parameters considered in the study.The pronounced feature in the present study is that the seasonal evolution of sea level pressure anomaly contrast between the Asian continent and the surrounding oceans is the governing mechanism for the ASM evolution. The northward migration of lowpressure anomaly from the Indian Ocean and the moisture transport by low-level westerly (Somali jet) toward India describes the evolution of precipitation field over India, the Bay of Bengal and the Indochina peninsula. Over the Pacific Ocean, high-pressure anomaly to the south of precipitation band between 25°~40°N pushes it northward, characterizing the onset and evolution of regional monsoons (Meiyu, Baiu and Changma) in East Asia. It is shown that the high-pressure anomaly intruding south China and further west to the Bay of Bengal from mid-June to mid-July accelerates low-level wind along the coastal line of Asia, where the pressure gradient becomes maximum. It provides a favorable condition for moisture transport toward the East Asian monsoon region.Two major moisture sources, the Indian Ocean and the western Pacific Ocean, are found during the monsoon period. Evolutions of anomalous moisture transport patterns toward India, the Indochina peninsula and East Asia show unique characteristics from one another. While precipitation over the Indian region is persistently affected by the Indian Ocean, the Indochina peninsula is dominated by the western Pacific Ocean in the early monsoon period followed by the effect of the Indian Ocean (mid-June to mid-July) and the cooperation of the two sources afterward. The East Asian monsoon regions are influenced by the both sources from the onset of their monsoons. Since late July, the latter source is more effective to the East Asian monsoon due to the northward movement of high-pressure anomaly (late July) together with the development of low-pressure anomaly over the subtropical western Pacific region.

What controls the stable isotope composition of precipitation in the Mekong Delta? A model-based statistical approach

“…These findings for An Long agree with the general characterization of monsoonal circulation and precipitation over the Southeast Asia region, with moisture from the Indian Ocean dominating during the initial stage of monsoon evolution, and the Pacific Ocean dominating in the later stages (Lim et al, 2002;Aggarwal et al, 2004;Delgado et al, 2012). This indicates that the HYSPLIT model provides valid trajectories to be used in the MLR.…”

Shipping routes in the South China Sea and northern Indian Ocean and associated monsoonal influences

“…Climatologically, monsoon rainfall follows the emergence of southwesterly flows and first appears in the SCS in mid-May (i.e., the commencement of the East Asia summer monsoon) (e.g., Chang and Chen 1995;Chen and Chen 1995;Zhang et al 2002;Wang et al 2004). Rain-bands move northward during June and July in concurrence with the formation of the monsoonal frontal systems over East-Northeast Asia, and later shift to the tropical western North Pacific-Philippine Sea region in August (e.g., Chen 1994;Wang 1994;Lim et al 2002;Wang and LinHo 2002;Wu 2002). In South Asia, the Indian monsoon causes major rainfall over the Indian subcontinent and the NIO (e.g., Ramage 1971;Krishnamurti 1978).…”

Statistical properties of the Seasonal Fractionally Integrated Separable Spatial Autoregressive Model

“…The literature on spatial models is relatively abundant, we can also cite the Simultaneous AutoRegression model, SAR (Whittle, 1954), the Conditional AutoRegression model, CAR (Bartlett, 1971;Besag, 1974), the moving average model (Haining, 1978) or the unilateral models (Basu and Reinsel, 1993) among others. Spatial models are currently investigated in many research fields like meteorology (Lim et al, 2002), oceanography (Illig, 2006), agronomy (Whittle, 1954;Lambert et al, 2003), geology (Cressie, 1973), epidemiology (Marshall, 1991), image processing (Jain, 1981), econometrics (Anselin, 1988) and many others in which the data of interest are collected across space. This large domain of applications is due to the richness of the modelling which associates a representation with a geographical component.…”

“…The studies of spatial data have shown presence of long-range correlation structures (Lim et al, 2002). To deal with this specific feature Boissy et al (2005) had extended the long memory concept from times series to the spatial context and introduced the class of fractional spatial autoregressive model.…”