An Isentropic Analysis of the Temporal Evolution of East Asian Cold Air Outbreaks

Abstract

Abstract The equatorward cold airmass flux below potential temperature θT = 280 K across 45°N integrated from 90°E to 180° is used as an index to quantitatively measure cold air outbreaks (CAOs) in the East Asian winter monsoon. Intermittent CAOs over East Asia significantly contribute to the global equatorward cold airmass flux. An autocorrelation analysis indicates that CAO events persist for approximately 5 days. The geographical distributions of lagged correlations/regressions with the CAO index (CAOI) clarify the temporal evolution of synoptic conditions associated with CAOs. The developing Siberian high located northwest of Lake Baikal (65°N, 100°E) on day −4 slowly moves southeastward, reaches maximum intensity over Siberia (50°N, 110°E) on day 0, and then decays while moving rapidly southward. By contrast, the Aleutian low is almost stagnant and maintains a strong intensity. The eastward pressure gradient geostrophically induces the equatorward cold airmass flux. After day −2, the cold air mass significantly decreases over Siberia, but increases over East Asia and the western North Pacific Ocean. The cold air mass continues to migrate southward while spreading eastward, and disappears mainly over the ocean. The leading edge of the high pressure anomaly moves southward at 13 m s−1 and reaches the equator simultaneously with the equatorward wind anomaly on about day +4. An additional analysis of separating the equatorward flux into 90°–135°E and 135°E–180° suggests that CAOs are, to some extent, caused by the Siberian high and the Aleutian low acting separately.