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Potential Vorticity Diagnostic Analysis on the Impact of the Easterlies Vortex on the Short-term Movement of the Subtropical Anticyclone over the Western Pacific in the Mei-yu Period

Fund Project:

This study was supported by the National Natural Science Foundation of China (Grant Nos. 41775048, 91937301, 41775050 and 91637105), the National Key R&D Program of China (Grant No. 2018YFC1507804), and the Second Tibetan Plateau Scientific Expedition and Research (STEP) program (Grant No. 2019QZKK0105)

• By employing NCEP−NCAR 1°×1° reanalysis datasets, the mechanism of the easterlies vortex (EV) affecting the short-term movement of the subtropical anticyclone over the western Pacific (WPSA) in the mei-yu period is examined using potential vorticity(PV) theory. The results show that when the EV and the westerlies vortex (WV) travel west/east to the same longitude of 120°E, the WPSA suddenly retreats. The EV and WV manifest as the downward transport of PV in the upper troposphere, and the variation of the corresponding high-value regions of PV significantly reflects the intensity changes of the EV and WV. The meridional propagation of PV causes the intensity change of the EV. The vertical movement on both sides of the EV is related to the position of the EV relative to the WPSA and the South Asian high (SAH). When the high PV in the easterlies and westerlies arrive at the same longitude in the meridional direction, the special circulation pattern will lower the position of PV isolines at the ridge line of the WPSA. Thus, the cyclonic circulation at the lower level will be strengthened, causing the abnormally eastward retreat of the WPSA. Analysis of the PV equation at the isentropic surface indicates that when the positive PV variation west of the EV intensifies, it connects with the positive PV variation east of the WV, forming a positive PV band and making the WPSA retreat abnormally. The horizontal advection of the PV has the greatest effect. The contribution of the vertical advection of PV and the vertical differential of heating is also positive, but the values are relatively small. The contribution of the residual was negative and it becomes smaller before and after the WPSA retreats.
摘要: 本文利用NCEP/NCAR 1°×1°分辨率资料，从位涡理论角度研究了梅雨期热带东风带扰动影响西太平洋副热带高压（简称西太副高）东西向异常活动的机制。结果表明：梅雨期东/西风带扰动（EV/WV）向西/东相向运动，到达同一经度120°E时，西太副高出现突然东退。在对流层高层东/西风带扰动均对应正位涡异常，在其移动过程存在高位涡下传，其正位涡值可较好地反映东/西风带扰动的强度变化；东风带扰动东西两侧的垂直运动在西太副高东退前后发生转变，与西太副高、南亚高压和东风带扰动的相对位置配置有关；西太副高东退时，东/西风扰动的高PV在经向上的不断接近，使西太副高脊线上的等1pvu等值线位置降低，强迫低层气旋式环流增强，使西太副高东退。等熵面位涡收支诊断表明，348K等熵面上东风带扰动西侧正PV局地变化开始加强，并与西风带扰动东侧的正PV局地变化在经向上连成带状时，西太副高异常东退。其中，位涡的水平平流项作用最大；位涡的垂直平流项和加热的垂直微分项贡献也为正，但数值相对较小；余差项贡献为负，在副高东退前后，余差项变小，有利于西太副高东退。
• Figure 1.  PV fields (shading; units: 10−6 m2 s−1 K kg−1) and potential height fields (red dotted line and white thick solid isoline; units: dagpm) on the 348-K (left) and 330-K (right) isentropic surfaces from 22 to 25 June 2003.

Figure 2.  Vertical profiles of PV (shading; units: 10−6 m2 s−1 K kg−1), meridional wind (solid line; units: m s−1) and potential temperature (dotted line; units: K) field on (a) 22, (b) 23, (c) 24 and (d) 25 June.

Figure 3.  Evolution of PV along 17.5°N (solid line; units: 10−6 m2 s−1 K kg−1) and meridional PV conveying ${T_{\theta,y}}$ (shading; 10−6 m s−1 K kg−1) along 25°N on the 348-K isentropic surface.

Figure 4.  Vertical cross section of vorticity (shading; units: 10−5 s−1), PV (isolines; units: 10−6 m2 s−1 K kg−1) and vertical circulation (vectors; units: 10−2 Pa s−1 along 17.5°N on (a) 23, (b) 24 and (c) 25 June. Meridional cross section of PV (shading), isentropic temperature (white isolines; units: K) and the zonal anomalous height field (black isolines; units: gpm) along the center of EV at (d) 0000 UTC 23 June and (e) 0600 UTC 24 June.

Figure 5.  (a) Vertical cross section of PV (shading; units: 10−6 m2 s−1 K kg−1), meridional wind (isolines; units: m s−1), isentropic temperature (dashed lines; units: K) and vertical circulation (vectors; units: 10−2 Pa s−1 along the center of the WV on 23 June. (b) Evolution of PV along the center of the WV. (c) Divergence (shading; units: 10−5 s−1), full wind speed (contours; units m s−1) and wind field at 200 hPa. (d) Meridional cross section of PV (shading), potential temperature (white isolines; units: K) and the zonal anomalous height field (black isolines; units: gpm) along the center of the WV at 0000 UTC 23 June.

Figure 6.  Vertical cross section of PV (isolines; values in the shaded region are greater than 0.5 PVU; units: 10−6 m2 s−1 K kg−1) along the line between the centers of the EV and WV from 22 to 25 June. The dotted line represents the zero line of zonal wind.

Figure 7.  Distribution of PV (shaded area is the region with PV larger than 1 PVU; units: 10−6 m2 s−1 K kg−1) and the local variation of PV $\partial P/\partial t$ (units: 10−6 m2 s−2 K kg−1) on the 348-K isentropic surface from 22 to 25 June.

Figure 8.  Distribution of PV (shading; units: 10−6 m2 s−1 K kg−1) and the terms (a) B, (b) C, (c) D and (d) E+F in Eq. (1) (solid and dotted isolines; units: 10−6 m2 s−2 K kg−1) on the 348-K isentropic surface on 23 June.

Figure 9.  As in Fig. 8 but on 24 June.

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Manuscript History

Manuscript revised: 14 June 2020
Manuscript accepted: 16 June 2020
通讯作者: 陈斌, bchen63@163.com
• 1.

沈阳化工大学材料科学与工程学院 沈阳 110142

Potential Vorticity Diagnostic Analysis on the Impact of the Easterlies Vortex on the Short-term Movement of the Subtropical Anticyclone over the Western Pacific in the Mei-yu Period

Corresponding author: Xiuping YAO, yaoxp@cma.gov.cn;
• 1. China Meteorological Administration Training Centre, Beijing 100081, China
• 2. Zibo Meteorological Bureau of Shandong Province, Zibo 255048, China
• 3. State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing 100081, China

Abstract: By employing NCEP−NCAR 1°×1° reanalysis datasets, the mechanism of the easterlies vortex (EV) affecting the short-term movement of the subtropical anticyclone over the western Pacific (WPSA) in the mei-yu period is examined using potential vorticity(PV) theory. The results show that when the EV and the westerlies vortex (WV) travel west/east to the same longitude of 120°E, the WPSA suddenly retreats. The EV and WV manifest as the downward transport of PV in the upper troposphere, and the variation of the corresponding high-value regions of PV significantly reflects the intensity changes of the EV and WV. The meridional propagation of PV causes the intensity change of the EV. The vertical movement on both sides of the EV is related to the position of the EV relative to the WPSA and the South Asian high (SAH). When the high PV in the easterlies and westerlies arrive at the same longitude in the meridional direction, the special circulation pattern will lower the position of PV isolines at the ridge line of the WPSA. Thus, the cyclonic circulation at the lower level will be strengthened, causing the abnormally eastward retreat of the WPSA. Analysis of the PV equation at the isentropic surface indicates that when the positive PV variation west of the EV intensifies, it connects with the positive PV variation east of the WV, forming a positive PV band and making the WPSA retreat abnormally. The horizontal advection of the PV has the greatest effect. The contribution of the vertical advection of PV and the vertical differential of heating is also positive, but the values are relatively small. The contribution of the residual was negative and it becomes smaller before and after the WPSA retreats.

Reference

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