Possible Impacts of Two Types of El Niño Events on the Western North Pacific Monsoon Trough and Tropical Cyclogenesis
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摘要: 通过对1948~2015年不同El Niño事件下西北太平洋季风槽变化和热带气旋(tropical cyclone,TC)生成进行分析,初步探讨了不同El Niño型事件对季风槽及其对TC的可能影响。分析结果表明,较东太平洋增暖(eastern Pacific warming,EPW)年,中太平洋增暖(central Pacific warming,CPW)年季风槽偏弱,位置相对偏西、偏北。在CPW年,中(西和东)太平洋海温增暖(降低)引起了从中到西太平洋热带地区的西风异常和中太平洋地区上升运动及对流活动加强,使得季风槽加强东伸,同时西太平洋副高偏弱、偏北,季风槽向北推进;而在EPW年,赤道东(西)太平洋海温增暖(降低)使得赤道地区西风异常显著加强东扩,异常Walker环流的上升支东移至东太平洋,季风活动加强,副高偏强、偏南,这使得季风槽较CPW年相比更强、更偏东。利于TC生成的大尺度环境因子随季风槽强度和位置的变化而发生改变,在CPW年,低层气旋性涡度、高层辐散、高的中层相对湿度以及低垂直风切变区随着季风槽向北移动;而在EPW年,这些因子随季风槽向南、向东偏移。这些大尺度环境因子的变化使得西北太平洋TC生成的位置在CPW年比EPW年更加偏北、偏西。Abstract: The present study analyzes the monsoon trough and tropical cyclone (TC) genesis over the western North Pacific to preliminarily investigate the anomalous monsoon trough and its possible effect on TCs during different types of El Niño events for the period of 1948-2015. It is shown that, compared with that in the eastern Pacific warming (EPW) years, the monsoon trough is weaker and its position leans toward the west and north during the central Pacific warming (CPW) years. In these years, the warmer (cooler) sea surface temperature (SST) over the central (western and eastern) Pacific induces anomalous westerly winds from the central to western Pacific in the tropical region and stronger than normal ascending motions and convective activities over the central Pacific, which can induce a strengthened and eastward extending monsoon trough. Meanwhile, the western Pacific subtropical high is weaker than normal and shifted northward, which leads to the northward displacement of monsoon trough. During EPW years, however, warmer (cooler) SSTs occur over the eastern (western) equatorial Pacific; anomalous westerly winds significantly extend eastward in the tropics; the ascending branch of the anomalous Walker circulation shifts eastward to the eastern Pacific; the monsoon activity becomes stronger; the subtropical high intensifies and leans toward the south. All the above changes are favorable for a stronger monsoon trough that extends more eastward compared to that in CPW years. Further study reveals that large-scale environmental factors that are related to TCs genesis will change with variations in the intensity and position of the monsoon trough. During CPW years, the cyclonic vorticity in the lower troposphere, the divergence in the upper-level, the higher relative humidity in the middle troposphere and the lower vertical wind shear all move toward the north with the monsoon trough. In EPW years, the above factors follow the monsoon trough to move southward and westward. These changes prompt the location of TC genesis over the western North Pacific to lean northward and eastward compared to that in EPW years.
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图 1 (a) 1948~2015年7~11月EMI指数与Niño3指数的时间序列;(b)CPW事件和(c)EPW事件7~11月海表温度距平(SST anomaly, SSTA)合成
Figure 1. (a) Time series of EMI index and Niño3 index during Jul-Nov of 1948-2015 and composite SST anomaly (SSTA) during Jul-Nov of (b) central Pacific warming (CPW) episodes and (c) eastern Pacific warming (EPW) episodes
图 3 (a) CPW事件和(b)EPW事件在7~11月平均850 hPa距平风场(箭头)、OLR距平场(填色)及海平面气压距平场(等值线,单位:hPa)合成;(c)CPW事件与EPW事件的差异场。红色风矢量表示通过90%信度水平
Figure 3. Composite anomalies of 850-hPa winds, OLR (shadings) and sea level pressure (contours, units: hPa) during Jul-Nov of (a) CPW episodes and (b) EPW episodes; (c) the difference between CPW years and EPW years. Wind vectors in red indicate significance above the 90% confidence level
图 4 (a) CPW事件和(b)EPW事件在7~11月0°~5°N平均的纬向—垂直环流距平场的剖面;(c)CPW事件与EPW事件的差异场(阴影区通过90%的信度检验)
Figure 4. Composite zonal-vertical circulation anomalies averaged over 0°–5°N during Jul-Nov of (a) CPW episodes and (b) EPW episodes; (c) the difference between CPW episodes and EPW episodes (shadings indicate the anomalies or differences significant above the 90% confidence level)
图 6 (a) CPW事件和EPW事件在7~11月平均500 hPa位势高度场(5870 gpm、5875 gpm、5880 gpm)合成(单位:m,红色实线、蓝色实线、黑色虚线等值线分别表示CPW年、EPW年以及气候态下位势高度场的分布)以及(b)CPW事件和EPW事件位势高度场的差异(阴影区通过90%信度检验)
Figure 6. (a) Composite 500-hPa geopotential height field (m) averaged over Jul-Nov of CPW episodes and EPW episodes (red lines, blue lines, and dotted lines denote the geopotential height fields (5870 gpm, 5875 gpm, and 5880 gpm) during CPW episodes, EPW episodes, and the climatological state, respectively); (b) the difference of 500-hPa geopotential height field between CPW episodes and EPW episodes (significant values above the 90% confidence level are shaded)
图 7 CPW事件和EPW事件在7~11月平均低层流场(流线,单位:m/s)与850 hPa涡度距平场(填色)合成(黑色点表示在7~11月生成的TC位置,红色点表示TC生成的平均位置)
Figure 7. Composite surface winds (streamlines, units: m/s) and 850-hPa relative vorticity anomalies (shadings) during Jul-Nov of CPW episodes and EPW episodes (black dots represent locations of TC genesis during Jul-Nov and red dot represents mean location of TC genesis)
图 8 (a) CPW事件和(b)EPW事件在7~11月平均SST(填色)和SSTA合成(等值线)以及(c)两者SST的差异场(单位:℃);(d)CPW事件和(e)EPW事件在7~11月500~700 hPa相对湿度(填色)及其距平(等值线)合成以及(f)两者500~700 hPa相对湿度的差异场(%);(g)CPW事件和(h)EPW事件在7~11月平均OLR场(填色)和OLR距平场(等值线)合成以及(i)两者OLR的差异场(单位:W/m2。灰色阴影区表示通过90%信度检验
Figure 8. Composite mean (a) SST (shading) and (b) SSTA (contours) during Jul-Nov of CPW episodes and EPW episodes (units: ℃); (c) the difference in SST between CPW episodes and EPW episodes. The mean 500-700 hPa relative humidity and its anomalies (%) during Jul-Nov of (d) CPW episodes and (e) EPW episodes. (f) The difference in the relative humidity between CPW episodes and EPW episodes. The mean OLR and its anomalies (units: W/m2) during (g) CPW episodes and (h) EPW episodes; (i) the difference in OLR between CPW episodes and EPW episodes. Significant values above the 90% confidence level are shaded
图 9 (a) CPW事件和(b)EPW事件在7~11月850 hPa涡度场(填色)及其距平场(等值线)合成以及(c)CPW事件与EPW事件涡度场的差异场(单位:10-6 s-1);(d)CPW事件和(e)EPW事件在7~11月200 hPa散度场(填色)及其距平场(等值线)合成以及(f)散度的差异场(单位:10-6 s-1);(g)CPW事件和(h)EPW事件在7~11月200~850 hPa垂直风切变(填色)及其距平场(等值线)合成以及(i)垂直风切变的差异场(单位:m/s)。灰色阴影区表示通过90%信度检验
Figure 9. Composite mean 850-hPa vorticity (shading) and its anomalies (contours) during Jul-Nov of (a) CPW episodes and (b) EPW episodes; (c) the difference in vorticity between CPW episodes and EPW episodes (units: 10-6 s-1). The mean 200-hPa divergence and its anomalies (units: 10-6 s-1) during Jul-Nov of (d) CPW episodes and (e) EPW episodes; (f) the difference in the divergence. The mean 200-850 hPa vertical wind shear and its anomalies (units: m/s) during (g) CPW episodes and (h) EPW episodes; (i) the difference in the vertical wind shear. Significant values above the 90% confidence level are shaded
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