Temporal and spatial evolution of the Asian summer monsoon in the seasonal cycle of synoptic fields

Principal mode associated with the seasonal variation of the Asian summer monsoon and the temporal and spatial evolutions of synoptic fields are investigated using 52-year (1948-99) NCEP reanalysis daily data. In this study, cyclostationary EOF analysis is used, which can accurately describe physical modes that evolve in time with a certain physical periodicity called "nested period" in addition to longterm modulation. The nested period is 20 pentads (100 days) from May 21 to August 28, which covers the prominent Asian summer monsoon period.

The first mode of precipitation, sea level pressure, low-level (850 hPa) and upper-level (200 hPa) winds, moisture transport, and convective instability parameter describes typical northward seasonal progression of spatial features associated with Indian monsoon from late May, Meiyu (China) in June, Baiu (Japan) from mid-June to mid-July, and Changma (Korea) from late June to late July. Spatial patterns of sea level pressure anomaly such as sea-land contrast and the subtropical western Pacific high explain most of spatial distribution and evolution of precipitation. For example, northward expansion of low pressure anomaly together with increased moisture transport by low-level westerly (Somali jet) anomaly from the Indian Ocean explains the steady increase of precipitation over India and the Bay of Bengal until mid-August. In addition, lowering of sea level pressure anomaly over the subtropical western Pacific is responsible for the increase of precipitation in the region until mid-August.

Northward movement of the subtropical western Pacific high pressure anomaly to the south of zonally elongated precipitation band between 30~40°ÆN characterizes the temporal evolution of the East Asian monsoons (Meiyu, Baiu and Changma). It is shown that the subtropical western Pacific 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. Moisture transport by accelerated low-level wind from the Indian Ocean contributes to precipitation over Asia. Meiyu, Baiu and Changma, however, are shown to be dominated more by the transport from the western Pacific from June to July. The analysis also reveals that convective instability is developed over the frontal regions of the Eastern Asian monsoon due to vertically differential moist advection.

In the upper level, the Tibetan anticyclone, a large-scale clockwise circulation over the Asian continent, moves northwestward from southeast China since mid-June. The upper-level westerly jet to the north of the Tibetan anticyclone is located over Korea and Japan in accordance with the onset of Changma. This suggests that upward motion due to upper-level divergence may be induced over Korea and Japan from late June to early July because these regions are located in the downstream region from the upper level trough.