The linkage between the extreme cold air outbreaks and wave breaking over East Asia

Abstract:

This research investigates the statistical relationship between Rossby wave breaking and extreme cold air outbreaks (CAO) in East Asia. Two typical evolution of upper-level Rossby waves during these CAOs are proposed. The extreme CAOs are selected using the quantitative CAO index, which is defined by the integral of equatorward cold air mass flux below 280K isentropic surfaces. Rossby wave breakings are identified as anticyclonic and cyclonic according to the orientation of breaking. The breaking extent is calculated as the ratio of longitudes with breaking and the total longitude range in four different regions. The composite analysis shows a significant increase of anticyclonic breaking extent upstream and cyclonic breaking extent downstream during western CAOs, and decrease of cyclonic breaking extent midstream during eastern CAOs, respectively. During the demise days of extreme CAOs, the breaking extents drop back to the level before onsets. On the other hand, the average eastern and western CAO indices are notably higher during these days which have cyclonic breakings. For the days of anticyclonic breakings, both CAOs tend to have higher indices when the breakings are upstream and lower indices when the breakings are downstream. Chi-Square test of independence reveals that the East Asia extreme CAOs and Rossby wave breaking in some regions have a significant synchronous relationship with each other. Extreme western CAOs are dependent on anticyclonic breakings in regions 1 or 4 and cyclonic breakings in regions 3 or 4, while extreme eastern CAOs dependent on cyclonic breakings in regions 3 or 4. The composited potential vorticity fields during East Asia extreme CAOs illustrates two archetypes of wave life cycles. One is the anticyclonic wave breaking upstream of the CAOs and jet core; the other is the cyclonic wave breaking downstream of the jet and CAOs.