Wednesday, 17 January 2007: 10:45 AM
An Observational and Modeling Study of the Winter-to-Spring Transition in East Asia-Onset of the South China Spring Rain
214C (Henry B. Gonzalez Convention Center)
Analysis of observations from 1979 to 2002 shows that the seasonal transition from winter to spring in East Asia is marked with a distinctive abrupt change--the onset of the South China spring rain (SCSR). In late February, the reduced thermal contrast between ocean and land leads to weakening of the Asian winter monsoon as well as the Siberian High and the Aleutian Low. Meanwhile, convection over Australia and the western Pacific maritime continent is suppressed due to passage of the dry phase of a MJO. Within two to three weeks, the decreased ascending motion over that region can no longer sustain an inter-hemispheric local Hadley cell in the East Asia-Australia sector. The eventual collapse of this Hadley-type circulation is accompanied by a series of adjustments in both the tropics and middle latitudes. These changes include the attenuation of the planetary stationary wave, the disappearance of westerly jetstream over much of the central Pacific adjacent to Japan, and the reduction of baroclinicity near the East Asian trough. The weakened baroclinicity results in confinement of frontal activities to the coast regions of East Asia, as well as rapid expansion of the subtropical Pacific High from the eastern Pacific to the western Pacific. The extended Pacific High and the nearby convective zones soon reach a new springtime equilibrium, which can last for about two months until the beginning of the pre-meiyu onset. A parallel analysis using output from an experiment with a GFDL coupled GCM shows that the above sequence of circulation changes are well simulated in that model.