Potential Vorticity Perspective on the Stratospheric Origin of Cold Surge in East Asia

Traditionally, the coupling between the troposphere and the stratosphere has been considered as one-way influence of the troposphere on the stratosphere, with the troposphere being regarded as a source of Rossby waves and gravity waves, which propagate upward into the stratosphere. However, observational and modelling evidence is now strong that the stratosphere may play an active role in the tropospheric circulation leading to changes in the troposphere. Jeong et al. (2006) found a precursory signal in the stratosphere prior to the cold surge occurrence in East Asia. Over northern Eurasia, about one week before the cold surge occurrence, strong stratospheric negative potential vorticity and rising of geopotential height are observed. From this, we hypothesized the upper-level signal is really linked to the amplification of Siberian High through the deep penetrating circulation anomalies of stratospheric PV. To prove this, we separated entire PV anomaly into two pieces which are above and below the tropopause and examined the balanced circulation of each piece. Piecewise PV inversion technique introduced by Davis (1994) has been used for the calculation. From the inversion results, we could successfully verify that the Siberian High amplification largely depends on the circulation anomaly originating from stratospheric PV. East Asian cold surge is accompanied by the upper tropospheric short-wave trough over Lake Baikal that grows as propagating toward East Asia. However, the processes causing the upstream development of the upper-level trough are not clear. Given the reliable stratospheric influence, therefore, it is worth inquiring the subsequent cold surge event after the amplification of Siberian High. For this purpose, I constructed a simple model which has an embedded life cycle of artificial Siberian High. The model is designed to be relaxed toward radiative equilibrium temperature of northern Hemisphere Winter (perpetual winter run). Several diabatic heating anomalies are also prescribed over Tropical ocean to obtain realistic model climatology. The repeated life cycle of Siberian High was simply controlled by periodic diabatic cooling over Siberia. From the result, the organized baroclinic eddies started to appear downstream of Siberia when the Siberia is sufficiently cold due to the local baroclinic instability. The increase of Eady growth rate and downstream sequence of wave activity vector supports this fact. The links revealed in this study may also offer scope for improving the skill of extended-range tropospheric forecasts.