The Effect Of Transient Eddy On The Meridional Displacement Of The East Asian Subtropical Westerly Jet Stream

The climatological distributions of East Asian subtropical westerly jet stream (EAJS) and transient eddy (TE) activity, as well as their spatial matching relations, are firstly investigated using NCEP and ERA-404-times daily reanalysis data for the period of1958-2002. Then the spatio-temporal characters of the interannual meridional displacement of the EAJS and the TE activities associated with the EAJS are documented with the observed data. The feedbacks of the TE activities on the EAJS are examined finally by computing the budget of the relevant terms of the seasonal-mean form of the quasi-geostrophic potential vorticity(QGPV) equation. The roles, played by TE activities in the meridional displacement of the EAJS, are revealed finally. The main conclusions are as follows:(1) Climatologically, the spatial distribution of the EAJS matches the maximum of TE activities well during boreal summer.In boreal summer (including June, July and August), the EAJS is located at around40ON in the upper levels of atmosphere, with its axis orientating in west-east direction. Meanwhile, the TE activities are prosperous within an elongated belt in the midlatitudes, and the maximum of the TE activities concentrates in a narrow area along the axis of the EAJS. This coherent spatial distribution could indicated that a close relationship might exist between the EAJS and TE activities. Hence, their relationship is further analysed with the seasonal-mean simplified zonal momentum equation, in which the TE activities may expressed in terms of TE momentum fluxes and the acceleration of time-mean zonal wind is proportional to the convergence of the horizontal momentum fluxes. It is indicated that the distribution of the meridional TE momentum fluxes oriented west-east along the climatological axis of the EAJS. Thus, the the meridional TE momentum fluxes act to accelerate the zonal component of wind.In addition, it is found that the strongest baroclinicity of atmosphere mainly lies in the levels above and under the EAJS, respectively. Among all the levels, at the level near200hPa the baroclinicity of atmosphere is weaker.(2) The interannual meridional displacement of the EAJS in boreal summer and the associated circulations.It is found that the interannual variability of the meridional displacement is the strong signal of the EAJS location variation. By introducing the approach of defining the axis of jet to depict the position of EAJS, the meridional displacement of the EASJ is defined as anomalous deviation from its climatological axis in latitude. A2-8yr band-pass filter is applied to extract its interannual variation. Performing the EOF analysis on the filtered anomalies of axes in latitude then leads to the spatial and temporal distribution of the variation of the EASJ. The results exhibit that in summertime the EASJ axis features a significant longitudinally-unanimous interannual meridional displacement, and the amplitude of the EASJ over the Pacific is larger than that over the mainland. The corresponding principal component time series is defined as the EAJS location index (PC1).Atmospheric anomalous circulation pattern associated with EASJ interannual variations can be identified by regressing the geopotential height and zonal wind against the normalized PC1. It is shown that accompanying the meridional wandering of the EASJ, the regressed geopotential height anomalies with equivalent barotropic structure are found to be in the vicinity of the climatological EASJ axis, and the zonal wind, however, exhibits a dipole pattern asymmetric about the axis. These patterns suggest that a positive geopotential height anomaly near the climatological EASJ axis can result in a westerly acceleration (deceleration) on the poleward (equatorward) side of the climatological EASJ axis, thus leading to a poleward shift of the EASJ; and vice versa. Certainly, the geopotential height and zonal wind anomalies satisfy the quasi-geostrophic relation.Also, atmospheric baroclinicity to the south (north) of the climatological axis of EAJS strengthens (weakens) accompanied by poleward (equatorward) shift of the EAJS.(3) The anomalous activities of the TE associated with the meridional displacement of the EAJS.The atmospheric movement in the midlatitudes can be described by the quasi-geotrophic potential vorticity equation (QGPV). Under the framework of the QGPV equation, the interaction between the TE activities and the time-mean flow in the manner that the time-mean flow organized the TE activities and the TE impacted on the time-mean flow by redistribution of the TE fluxes. Climatologically, the TE transports vorticity from the climatological axis to the poleward and equatorward sides of the EAJS, consequently resulting in a divergence of TE vorticity near the EAJS and a convergence of TE vorticity far away from EAJS. Meanwhile, the TE transports the TE heat from the area near the EAJS to the north side of the EAJS, which leads to a divergence of TE heat near the EAJS and a. convergence of TE heat on the northern side of the EAJS.As the EAJS shifts poleward, the most prosperous area of TE activities moves northward. On the one hand, the anomalous TE activities transport much more TE vorticity from the area near the climatological EAJS’axis to its both sides far away, hence leading to meridional dipole structure asymmetric about the climatological EASJ axis; on the other hand, the anomalous TE activities transport much more TE heat from the area near the climatological EAJS’axis to poleward of the EAJS, consequently resulting in a anomalous cooling in the vicinity of the EAJS, and a anomalous warming to the northern side of the EAJS.(4) Based on the QGPV equation, the feedback of the anomalous TE activities on the EAJS may be examined by solving the numerical solution of the QGPV equation, and the role, played by the TE in the movement of the EAJS, is revealed eventually.The effect of TE on time-mean flow may be approached by treating the convergences of TE fluxes as virtual sources or sinks of heat and vorticity and then seeking for the responses to these sources or sinks. In this study, we examine initial atmospheric responses to the TE forcing anomalies by solving QGPV equation together with appropriate boundary conditions.The geopotential height tendencies induced by TE vorticity forcing anomalies are characterized by a dipole pattern with distinct equivalent barotropic structure. Consequently, under the geostrophic balance, the zonal wind tendencies exhibit a sandwich-like pattern. These patterns indicate that, in accordance with a positive PC1(i.e., a northward displacement of the EASJ), there is a significant positive geopotential height tendency north of the climatological EASJ axis which produces a negative zonal wind tendency (decelerating zonal wind) near the climatological axis and a positive zonal wind tendency (accelerating zonal wind) far north of the axis. By comparing the anomalous circulation regressed on PC1with the initial tendency forced by TE vorticity, it can be easily found that the geopotential height and zonal wind tendencies induced by the TE vorticity forcing possess a phase that meridionally leads the geopotential height and zonal wind anomalies themselves roughly by π/2phase. This suggests that the TE vorticity forcing anomalies act to reinforce further northward progression of the EASJ as it moves northward; and vice versa. Thus, the TE vorticity forcing tends to play a positive feedback role in the interaction between the TE and the time-mean flow.The TE thermal forcing tends to give rise to a baroclinic response. There are negative geopotential tendencies on the poleward side under the axis of the EASJ, and smaller positive geopotential tendencies on the equatorward side beneath the axis of the EASJ. Resultantly, the zonal geostrophic wind tendencies are characterized by westerly acceleration beneath the axis but deceleration above the axis, especially north of the climatological EASJ axis. This suggests that the TE thermal forcing anomalies act to reduce the vertical shear of zonal wind and thus reduce the atmospheric baroclinicity, which eventually suppresses the TE activity, as the EASJ moves northward. Thus, the TE thermal forcing tends to play a negative feedback role in the interaction between the TE and the time-mean flow.