Winter Monsoonal Surges over East and Southeast Asia¹

Abstract

Some of the characteristic features of monsoonal surges during the northern winter were investigated using wind and temperature data over China, the East and South China Seas, and the extratropical and tropical Western North Pacific during the period 16 November 1970 through 15 March 1971. Low-level monsoonal surges are most pronounced over the East China Sea, with dominant spectral peaks around 4.3-4.6 days for meridional winds, temperature, vertical velocity, and heating rate below about 700mb. For 4.3-4.6 day oscillations in the lower troposphere, the cospectrum between temperature and meridional winds is positive (northward sensible heat transport), while temperature is negatively correlated with vertical p-velocity (local kinetic energy generation). In comparison, the South China Sea is characterized by dominant spectral peaks at 6.0-6.7 days, with secondary maxima occurring around 4.3-4.6 days. The coherence between the East and South China Seas is generally higher than 0.5 in the lower troposphere. Strong monsoonal surges over the South China Sea coincide with above normal ascending motions and consequent condensation heating throughout the troposphere.
Another feature of interest in computed spectra is the existence of marked peaks around 20-30 days, which are clearly separated from short-period (4-6 day) oscillations associated with monsoonal surges. Composite 200mb maps for 20-30 day filtered geopotential data depict eastward phase propagation (-4-10m s-1) in tropical latitudes, which is most distinctly defined over the Bay of Bengal-Malaysia-South China Sea region. This is opposed to near zero phase speed (standing) at higher latitudes around 50°-60°N. It appears that wet (dry) spells near Malaysia occur in association with the passage of eastward propagating ridge (trough) systems. During wet (dry) spells, the atmospheric temperature becomes warmer (colder) than usual with above (below) normal ascending motions, as confirmed from a large negative cospectrum for 20-30 day oscillations between these two quantities. Phase changes between wet and dry spells over the South China Sea region are closely related to longperiod monsoonal fluctuations over the East China Sea and its vicinity. The coherence for T and v between these two regions is substantially large (>0.5) for 20-30 day period oscillations, with the South China Sea lagging behind the East China Sea by about 1-2 days.