INTERDECADAL CHANGE IN THE RELATIONSHIP BETWEEN SUMMER TROPICAL CYCLONE GENESIS FREQUENCY OVER THE WESTERN NORTH PACIFIC AND TROPICAL SEA SURFACE TEMPERATURE
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摘要: 探讨了夏季(6—8月)西北太平洋(Western North Pacific，WNP)热带气旋生成频次(Tropical Cyclone Genesis Frequency，TCGF)与热带海温关系的年代际变化，发现影响WNP TCGF的热带海温型在1991/1992年发生了年代际变化。在1990年代初之前，TCGF正异常对应的热带海温异常(Sea Surface Temperature Anomaly，SSTA)呈现东部型La Niña衰减位相，前冬至春季WNP局地暖SSTA在其西北侧激发气旋异常，夏季时由热带印度洋冷SSTA继续维持。在1990年代初之后，TCGF正异常对应的热带SSTA呈现东部型La Niña向中部型El Niño快速转换的位相，夏季中太平洋暖SSTA在其西北侧激发气旋异常，同时热带东印度洋至海洋性大陆以及热带大西洋的冷SSTA通过垂直环流圈加强中太平洋的辐合上升运动，进一步维持其西北侧气旋异常。由于激发气旋异常的暖SSTA在第二个年代相较第一个年代明显偏南偏东，气旋异常和TCGF正异常在第二个年代也整体偏南且向东扩展至更远的区域。WNP TCGF与热带海温关系的年代际变化与1990年代初之后厄尔尼诺-南方涛动演变速率加快有关。Abstract: The interdecadal change in the relationship between summer (June—August) tropical cyclone genesis frequency (TCGF) over the western North Pacific (WNP) and tropical sea surface temperature anomaly (SSTA) is investigated in this paper. Results show that the tropical SSTA pattern affecting the WNP TCGF has experienced an interdecadal change in 1991 / 92. Before the early 1990s, the positive TCGF anomaly corresponds to the eastern Pacific La Niña decaying phase. From the preceding winter to spring, the WNP local warming triggers an anomalous cyclone to its northwest, which is maintained by the cold SSTA over the tropical Indian Ocean in summer. After the early 1990s, the positive TCGF anomaly is related to the transition phase from eastern Pacific La Niña to central Pacific El Niño. In summer, an anomalous cyclone is induced by the warm SSTA over the central Pacific to its northwest and further intensified by the cold SSTA over the tropical eastern Indian Ocean, Maritime Continent and the tropical Atlantic, which could strengthen the convergent and upward motions over the central Pacific through vertical circulations. Since the warm SSTA is located further southward and eastward in the later period, the cyclonic anomaly and positive TCGF anomaly are both located more southward and extend further eastward in the later period. The interdecadal change in the relationship between the WNP TCGF and tropical SSTA is related to the accelerated evolution of El Niño-Southern Oscillation after the early 1990s.
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图 2 1949—2018年夏季5 °S~15 °N纬带平均的SSTA与TCGF的13年滑动相关系数分布(a) (等值线，间隔0.15)以及1979—1991年(b)、1992—2006年(c)标准化TCGF序列回归的夏季SSTA(单位：℃)
a中填色代表达到不同置信度的相关系数，*、**、***分别代表通过了0.10、0.05、0.01显著性检验。
图 3 1979—1991年(a)、1992—2006年(b)标准化TCGF序列回归的TCGF异常空间分布以及(a、b)中异常场对100 ºE~180 º经向平均值的纬向分布(c)、对0 °~30°N纬向平均值的经向分布(d)，c中折线为柱状的5点滑动平均，橙色(蓝色)柱子和红实线(蓝虚线)为第一(二)个年代。
图 4 1979—1991年（a、c、e）和1992—2006年（b、d、f）标准化TCGF序列回归的200 hPa(a、b)、500 hPa(c、d)、850 hPa(e、f)位势高度异常(等值线，单位：gpm；填色代表达到90%置信度)和风场异常(矢量，单位: m/s；只给出通过0.1显著性检验分布)
a~b、c~d、e~f中的等值线间隔分别为1 gpm、1 gpm、2 gpm。
图 5 1979—1991年（a、c、e）和1992—2006年（b、d、f）标准化TCGF序列回归的850 hPa风场(矢量，单位：m/s)及相对涡度(填色，单位: 10-6 s-1)异常(a、b)；700 hPa相对湿度(填色，单位：%)和500 hPa垂直速度(等值线，单位: 10-2 Pa/s，间隔1，绿色实线代表异常上升，红色实线代表异常下沉) 异常(c、d)；200 hPa与850 hPa之间的垂直风切变异常(填色，单位单位: m/s) (e、f)
其中风场及垂直速度仅画出通过0.1显著性检验的值。
图 6 1979—1991年(a、c、e、g)和1992—2006年(b、d、f、h)标准化TCGF序列回归的前冬(a、b)，春季(c、d)，夏季(e、f)，秋季(g、h)SSTA场分布(填色，单位：℃)、降水异常(等值线，单位：mm/d，间隔0.5，绿色实线代表降水正异常，橙色实线代表降水负异常)和850 hPa风场异常(矢量，单位：m/s)，(f)中的蓝色实线(20 °S, 83 °E)~(155 °E, 40 °N)用于图 7a的垂直环流分析
图中所示数值均通过0.1显著性检验。
图 7 1992—2006年标准化TCGF序列回归的垂直环流圈异常
a为沿着图 6f蓝斜线的垂直环流，b为5 °S~10 °N纬带平均的纬向垂直环流。矢量为风场异常，水平风单位为m/s，垂直风单位：-10-2Pa/s；填色表示风场通过0.1显著性检验；等值线为通过0.1显著性检验的垂直速度异常，单位：10-2Pa/s，间隔为0.3。
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