Studies On The Features Of Tropical Low-frequency Oscillation And Concurrent Variation Of Upper-level Jet During Persistent Snowstorm Period

Taking the persistent snowstorm in2007/08winter over southern China as the starting point, we first explored the impact of the MJO on the snow disaster from a modeling perspective. The role of the atmosphere-ocean interaction in maintaining the MJO-precipitation relationship was also examined. Then, the concurrent variation between the East Asian subtropical jet (EASJ) and polar front jet (EAPJ), and the associated atmospheric anomalous signals were investigated during persistent snowstorm period. The impact of the MJO on the low-frequency variation of the EASJ was preliminarily analyzed. Based on the case study, we further examined the relationship between the concurrent variation of the East Asian jet stream and the winter precipitation over southern China from the climatological perspective. The main conclusions are as follows:1. The impact of the MJO on the persistent snowstormThe MJO signal was very active during the snowstorm period and the low-frequency oscillation was dominated by the EOF1and EOF2in the first two and last two precipitation episodes, respectively. The spatial distribution and intensity of the precipitation can be attributed to both the amplitude and propagation characteristics of the MJO signal. Compared with the stand-alone RegCM3, the coupled model performed better at reproducing the spatial-temporal evolution and intensity of the precipitation episodes. The power spectral analysis indicated that the coupled model successfully captured the dominant period between30and60days in the precipitation field, leading to a notable improvement in simulating the magnitude of intraseasonal precipitation variation, and further in enhancing the intensity of the simulated precipitation. These improvements were mainly due to the well-simulated low-frequency oscillation center and its eastward propagation characteristics in each MJO phase by RegCM3-POM, which improved the simulations of MJO-related low-frequency vertical motions, water vapor transport, and the deep inversion layer that can directly influence the precipitation event and that further improved the simulated MJO-precipitation relationship. Analysis of the phase relationship between convection and SST indicated that RegCM3-POM exhibits a near-quadrature relation between the simulated convection and SST anomalies, which was consistent with the observations. However, such a near-quadrature relation was not as significant when the stand-alone RegCM3was used. This difference indicated that the inherent coupled feedback process between the ocean and atmosphere in RegCM3-POM played an important part in reproducing the features of the MJO that accompanied the snow storm.2. Concurrent variation feature of the East Asian jet stream during the persistent snowstorm periodAccording to the occurrence frequency of jet cores, the East Asian subtropical jet was divided into two parts:(1) the plateau jet (EATJ), located along the southern side of the Tibetan Plateau, and (2) the ocean jet (EAOJ), situated at the southeastern Japan Island. The enhanced plateau jet was simultaneously correlated with the weakened polar front jet, while the variation of the ocean jet lagged the variation of the plateau jet (polar front jet) about5days. The concurrent variation between the plateau jet and the polar front jet acted as an important bridge that linked the snowstorm to the atmospheric anomalous signals associated with the cold and warm air activities. Due to the opposite trends of synoptic-scale transient eddy activities (STEA) variation over the southern and northern sides of the Tibetan Plateau, the plateau jet and the polar front jet exhibited a significant concurrent variation feature. The STEA anomalies over the plateau jet and polar front jet regions propagated downstream to the East Asian coast as a wave train along the southern and northern sides of the Tibetan Plateau, respectively, resulting in a5day lag variation relationship between the ocean jet and the plateau jet (polar front jet).3. Impact of the MJO on the low-frequency variation of EASJThe EASJ showed significant low-frequency variation feature during the snowstorm period. When the MJO convection was located over the central and eastern Indian Ocean, the anticyclonic circulation excited by the convection dominated the southern flank of Tibetan Plateau. The low-frequency westerlies over the north of the anomalous circulation was superimposed on the main body of the EATJ, resulting in the enhancement of the EATJ intensity. The anticyclonic circulation occupied the southern flank of Tibetan Plateau for about15days, corresponding to the period of the snowstorm. Composites analysis of the EATJ index with respect to MJO phases indicated that the intensity of EATJ notably increased (decreased) for phase3-4(7-8). On intraseasonal time scale, the low-frequency variation of EATJ was actually a direct response to the anomalous Rossby wave stimulated by the MJO convection.4. Relationship between the concurrent variation of the East Asian jet stream and the winter precipitation over southern ChinaThe simultaneous out of phase variation between the East Asian plateau jet and polar front jet was most dominant in winter. When the plateau jet was intensified and the polar jet was weakened, the precipitation was increased significantly over southern China, and vice versa. Composite analysis results showed that the variation of the plateau jet from weak to strong can reflect the promotion process of the moist warm air from south to north in reality, while that of the polar front jet can reasonably depict the invasion process of the cold air activity from north to south. In the two concurrent variation types, the thermal effect from the tropical convection of the key regions had notably impact on the variation of the plateau jet, but had little influence on that of the polar front jet. Further analysis indicated that the variation of the East Asian polar jet was mainly associated with the synoptic-scale transient eddy activities, while the variation of the plateau jet was attributed to both the thermal and eddy forcing effects.