The Linkage Of Upper-level Jet Streams To East Asian Winter Monsoon And Its Seasonal Forecast

In East Asia, the climate variability in boreal winter is dominated by East Asian winter monsoon (EAWM) system. The structure of EAWM encompasses a large meridional domain and features distinct major components at different levels from lower troposphere to the tropopause. This study investigates the relationship between upper level jets and major components of EAWM and also surface temperature in East Asia, from the perspective of concurrent variations of East Asian subtropical jet (SJ) and East Asian polar front jet (PJ). The dataset used here is NECP-NCAR reanalysis data. A positive feedback between upper level zonal wind shear associated the two jets and other major EAWM components is proposed, which accounts for three-dimensional well coupled circulation anomalies. The combined influences of both external forcing and internal atmospheric dynamics play a role in driving the variations in the leading EOFs of jet streams, as well as linking them to the variations of other EAWM components. On the other hand, seasonal forecast of EAWM is investigated with dynamical and statistical modeling techniques. We have assessed the performance of the 46-year (1960-2005) retrospective forecast of winter mean temperature made by five models that participated in ENSEMBLES project. Finally, a new prediction approach utilizing a Physical-Empirical model is applied to winter mean temperature and extremely cold events prediction, based on the interpretation of the lead-lag linkages between three physical consequential predictors and the predictand. Physical-Empirical model exhibits high skill in seasonal prediction in temperate East Asia (TEA,30°-50°N,110°-140°E) where the current dynamical models exhibit limited prediction skill. The main results are as follows:1. Linkage between leading variability modes of the jets and EAWMIn the upper troposphere, both polar front jet and subtropical jet are major components of EAWM. The winter upper-level zonal wind variations over East Asian landmass (70°-120°E) are dominated by two distinct principal modes, i.e., meridional displacement of PJ and out-of-phase variation in the intensity of PJ and subtropical jet located to the south of the Tibetan Plateau (TSJ). Both modes are closely linked to EAWM variability. Meridional shift of PJ corresponds to the EAWM northern mode, while the concurrent variation of PJ and TSJ is related to the EAWM southern mode. When PJ migrates southward, the Siberian High (SH) displaces northwestward, and northern part of East Asian trough is enhanced, leading to cold winter in northern East Asia. Meanwhile the configuration of intensified TSJ and weakened PJ is linked to amplified SH, southward shift of Aleutian Low (AL), strengthened East Asian trough, and a wave like anomaly pattern extending from western Barents Sea downstream to East Asia at 500 hPa level, and vice versa.The circulation anomalies associated with the two modes feature anomalous westerly (easterly) in the mid-high latitude and anomalous easterly (westerly) to its south, corresponding to zonal wind shear anomaly in the upper level. Further analysis shows upper level zonal wind shear is an essential part of the feedback process among the three dimensional anomalous circulations. There is a positive feedback among the circulation anomaly associated with the major EAWM components:the cyclonic vorticity caused by the upper level zonal wind shear could enhance the monsoon trough at 500 hPa, which would facilitate the southward intrusion of cold air from the polar region. With the enhanced cold advection, the Siberian High could be strengthened which will modulate the upper-level zonal wind associated with the eddy-driven polar front jet. Meanwhile the enhanced cold advection will also increase the meridional temperature gradient that favors a strenghtened thermal-driven subtropical jet. This proposed positive feedback leads to synchronous changes among the EAWM components, and is involved in both leading EOFs associated with the jets. The zonal wind shear associated with the two EOFs implies this feedback process is dominant in the high latitude and mid-low latitude respectively.2. Possible reasons for the linkage between the jets’ variation modes and EAWMThe possible reasons for the linkage between the jets’ variation modes and EAWM are addressed in two aspects. On one hand, since the above positive feedback provides synchronous variations in different EAWM components, the changes in the jets may modulate other coupled EAWM components through feedback process. This feedback process could be found in the higher latitude for the jet’s meridional shift mode, and in the mid-low latitude for the second jet’s variation mode, respectively. In this occasion, both external forcing and internal atmospheric dynamics can influence the variations in the jets. For example, tropical SST anomalies associated with ENSO may induce a meridional wave train, which propagates poleward and influences the upper-level zonal wind, meanwhile the transient eddy feedback through barotropic process and stationary wave activity also modulate upper-level zonal wind. On the other hand, external forcing such as arctic sea ice extension anomaly and ENSO related sea surface temperature anomaly could exert direct impact on major components of EAWM at different levels, thus leading to the uniform variations among these major EAWM components.With diminished sea ice over the Barents Sea, and the induced warm condition, there are suppressed lower level baroclinic eddies at the poleward side and increased anomalous baroclinicity at the equatorward side, thus the main body of PJ tends to shift southward. Meanwhile, the changes in baroclinicity variability would prevent cyclones from traveling eastward and lead to fewer cyclones which results in anticyclonic anomaly over the Siberian coast. Furthermore, the rising pressure over Ural Mountain associated the winter Arctic warming can lead to downstream low pressure anomalies that deepen and westward shift the East Asian trough at 500 hPa through Rossby wave propagation. The enhanced precipitation heating over the Maritime Continent associated with La Nina could strengthen upward motion over the Maritime Continent and the associated northward divergent winds at upper level. As a result, the upper tropospheric westerly over the northern (southern) part of the subtropical (polar front) jet is accelerated due to Earth’s rotation at 300 hPa. This meridional teleconnection also features anomalous northward propagation of wave flux activity over East Asia, thus deepening the East Asian trough at 500 hPa. Results indicate the predictability of wintertime atmospheric circulation and surface temperature anomaly, with anomalous signal of the tropical SST (ENSO) and Arctic sea ice appear in the previous autumn.3. Prediction of extremely cold days and winter mean temperature in temperate East AsiaThe present work estimates the predictability of the number of extremely cold days (NECD) in temperate East Asia (TEA,30°-50°N,110°-140°E) where the current dynamical models exhibit limited prediction skill. Evaluation of seasonal forecast skill suggests that coupled model from European Center exhibits higher prediction skill than ENSEMBLES MME and other models that participate ENSEMBLES project.About 60% of the total variance of the winter NECD over the core region of East Asian winter monsoon is likely predictable which is estimated by using a Physical-Empirical (P-E) model with three consequential autumn predictors, i.e., developing El Nino/La Nina, Eurasian Arctic Ocean temperature anomalies, and snow cover anomaly over Eurasian continent in October. A bitter winter with more frequent occurrence of NECD is expected with developing La Nina, excessive snow cover over Eurasian continent in October and warm condition in Eurasian Arctic Ocean in the preceding autumn. The cross-validated correlation skill reaches 0.84 for the 41-year period of 1973-2013. This practical prediction skill provides an estimate for the lower bound of the predictability of the NECD. Therefore, there is more confidence in accurate winter mean seasonal forecasting with the established Physical-Empirical model. The sources of the NECD predictability and the physical basis for prediction of NECD are essentially the same as those for prediction of winter mean temperature over the same region. This finding suggests that forecasting seasonal mean temperature can provide useful information for prediction of extremely cold events. Seasonal prediction of NECD using P-E model provides better prediction skills than those using current seasonal forecasting models.