Climatic Effect Of The Subtropical Westerly Jet Anomaly And Its Dynamical Diagnosis Northern Hemisphere Winter

Based on the NCEP/NCAR reanalysis dataset, the observation data of160-stations in China, CMAP and ERSST dataset, this paper focuses on the theme of "climatic effects of the subtropical westerly jet anomalies and its causes" by using the theoretical analysis and diagnostic methods of statistics and dynamics. The starting point for analyses is the definition and calculation of indices of the two subtropical westerly jets. The climatic and anomalous features of these indices are analyzed and found that there are obvious seasonal and interannual variability, and they are closely related to the regional climate anomalies in Northern Hemisphere. On this basis, the relationship between the subtropical westerly jet stream and angular momentum transport, tropical SST, the mean meridional circulation are discussed. The cause of the jet stream anomaly is revealed. The systematic and in-depth understanding of subtropical westerly jet stream in Northern hemisphere winter is obtained. The main conclusions are as follows:1) The two large-scale circulation systems, northern hemisphere subtropical westerly jet stream (J,0～360°) and Asian subtropical westerly jet stream (JA,70～145°E), are defined by zonal average distributions of [u] in two different longitude range. The64-year seasonal and monthly series of the circulation indices (intensity P, central position C (φc, pc)) of J and JA in winter are established. The jets’positions described by them are two-dimensional, including the meridional and vertical position of the jet stream center.2) From the description of J and JA by intensity index and position index, it can be found that J and JA exist throughout the winter with their center near30°N,200hPa. J (JA) is the weakest in December and up to the strongest in February (January). During the winter, the centers constantly move southward and oscillate between high and low level of the vertical direction. The interannual variations of the seasonal and monthly intensity anomalies of J and JA are consistent. J has the interannual scale oscillation mainly. Besides this, JA has the10-20a-period interdecadel oscillation. The central positions of J and JA are higher and southerly before the early of1970’s, lower and northerly after that with the period of≤10a. J’s interdecadel oscillation is more significant than JA.3) J and JA are significantly associated with the climate anomalies of the Northern Hemisphere. The significant correlation region between J (JA) intensity and temperature (T), precipitation (R) is mainly in the Western Hemisphere (Eastern Hemisphere). The correlation between J (JA) intensity and T, R is obviously stronger (weaker) than its central position. Their correlation with T is distinctly bigger than R. In particular, JA has the significant correlation with China climate anomalies. So in the research of winter climate anomalies analyses and the short-term climate prediction in China, the index of JA is more reasonable. In the stronger JA years, most of China except the southwest has the lower temperature (especially in the area to the south of the Yangtze River) and the northern, central and eastern China has the less precipitation (especially in northern China). In the years of JA central location anomalies, the significant correlation area appears analogous in a certain geographical area.4) From December to February, westerly angular momentum transport meridionally from tropical easterlies to extratropical westerlies in northern winter, which intensity in winter is stronger than summer. The seasonal difference in the northern hemisphere is greater than the southern hemisphere. The strong meridional transportation of global u angular momentum is at upper troposphere of the middle and low latitudes and the high latitude stratosphere in winter hemisphere. J is in the transportation convergence zone to the high latitude side of strong angular momentum poleward transportation center. During the northern hemisphere winter, J is maintained by the average meridional circulation transportation and its convergence mainly, and then is preserved by the equivalent steady wave, transient eddy transportation and its convergence. In addition, Hadley circulation transports the global Ω angular momentum at the ground of the tropical easterlies upward and poleward to the subtropical latitudes. This is the main reason that the easterlies and westerlies in the two hemispheres and J maintain. The transportation of Ω angular momentum by Hadley circulation and the accompanying transformation of u angular momentum contribute to equatorward inclining the interface of the easterlies and westerlies in the tropical troposphere with the increase of height and the asymmetry about the equator in the tropopause.5) The meridional and vertical transportation characteristics of local westerly angular momentum are similar to the global situation with the difference that the local westerly angular momentum is related with the zonal transportation at the west and east boundary. During the maintenance of JA, the meridional transportation of local u angular momentum and its convergence has the positive contribution, but the zonal and vertical transportation has the negative contribution. Their balance is the circulation reason of the maintenance of JA. JA has the zonal net export in most area. The main compent is the average zonal circulation and its interaction with the steady wave. Otherwise, local Hadley circulation plays a key role on the interface structure formation of the easterlies and westerlies in Asian regional tropical troposphere. The interface is quasi-symmetric about the equator.6) In the jet stream search area of northern hemisphere winter, the abnormal increase of u angular momentum transportation convergence enhances J and weakens JA, accompanying the northward and upward movement of the J’s central position and the southward movement of JA. The u angular momentum anomaly transportation of J is caused by the transportation components of the steady wave and transient eddy, while J is caused by the interaction between the average zonal circulation and the steady wave, and the component of the zonal steady wave. The SST anomalies in the tropical ocean are significantly related with the anomaly of J and JA. In the years of strong El Nino (La Nina), Hadley circulation in northern hemisphere is stronger (weaker) and its central position is southward (northward), but Hadley circulation in Asia is weaker (stronger) and Ferrel circulation is stronger (weaker). This situation can lead to the stronger (weaker) J, its central position moves southward (northward) and JA moves southward and higher (northward and lower).