Linkage between the Extreme Cold Air Outbreaks and Rossby-Wave Breaking over Asia

The statistical relationship between Rossby-wave breaking and extreme cold air outbreaks (CAO) is investigated for East Asia. Two typical life cycles of upper-level Rossby waves during these CAOs are identified. The extreme CAOs are selected using a quantitative CAO index, which is defined by the integral of equatorward cold air mass flux below 280K isentropic surfaces. Rossby-wave breaking events are identified as anticyclonic and cyclonic according to the orientation of breaking. The breaking extent is calculated as the ratio of longitudes with breaking to the whole longitude range in each of four different regions. The composite analysis shows a significant increase in anticyclonic breaking extent upstream and cyclonic breaking extent downstream during western CAOs, and a decrease in cyclonic breaking extent midstream during eastern CAOs, respectively. During the demise days of extreme CAOs, the breaking extent returns to the level before onset. On the other hand, the average eastern and western CAO indices are notably higher during the days of cyclonic breaking. For the days of anticyclonic breaking, both CAOs tend to have higher indices when the breaking is upstream and lower indices when the breaking is downstream. Chi-Square tests of independence reveal that extreme western and eastern CAOs have prominent dependent relation with Rossby-wave breaking. The former synchronizes well with anticyclonic breaking upstream and cyclonic breaking downstream, while the latter lags the cyclonic breaking in the midstream for about three days. 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.