Impact of Madden-Julian Oscillation on Persistent Heavy Rainfall over Hainan Island in October
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摘要: 本文研究了1979—2019年热带季节内振荡(MJO)对海南岛持续性强降水(PHR)的影响。结果表明，海南岛PHR的降水量和发生频次均以10月最多，且受强MJO的明显调控。当MJO对流位于热带东印度洋-西太平洋(位相3~6)时，海南岛10月持续性强降水偏多；而MJO对流位于热带西印度洋和中太平洋一带(位相7、8、1和2)时，持续性强降水较少。热带东印度洋-西太平洋的MJO对流在海南岛上空引发MJO扰动东风异常，配合其北侧有东北风异常，南侧的南海中北部存在气旋环流异常，与海南岛秋季暴雨的典型环流模态一致，使得低频水汽通量辐合增强，加湿低层大气，有利于海南岛持续性强降水的发生和维持。热带西印度洋和中太平洋一带的MJO对流引发海南岛上空的反气旋环流异常，减弱水汽辐合，不利于低层大气增湿。海洋性大陆附近偏暖的海温以及赤道中东太平洋偏低的海温有助于热带东印度洋-西太平洋MJO对流生成和发展，使得海南岛PHR增强。Abstract: This paper examined the influence of Madden-Julian Oscillation (MJO) on persistent heavy rainfall (PHR) over Hainan Island from 1979 to 2019. The results reveal that both the precipitation amount and frequency of PHR events peaked in October, exhibiting significant modulation by strong MJO activities. When MJO convection was active over the tropical Eastern Indian Ocean-Western Pacific (phases 3-6), PHR events over Hainan Island increased. Conversely, when MJO convection was located over the tropical Western Indian Ocean and the Central Pacific (phases 7, 8, 1, and 2), PHR events decreased. Active MJO convection in phases 3-6 induced anomalous easterly over Hainan Island, with northeasterly wind anomalies to the north and a cyclonic circulation anomaly over the north-central South China Sea. These conditions were consistent with the typical circulation mode of autumnal rainstorms over Hainan Island, which enhanced the low-frequency moisture flux convergence and thus moistened the lower atmosphere, favoring the occurrence and persistence of PHR over Hainan Island. In contrast, when MJO convection was active in phases 7, 8, 1, and 2, the anticyclonic circulation coupled with MJO convection over Hainan Island could limit moisture flux convergence and inhibit lower atmospheric moistening. The warmer sea surface temperatures (SST) in the tropical Eastern Indian Ocean-Western Pacific and the cooler SST in the tropical Eastern Pacific contributed to the local MJO convection in phases 3-6, which could strengthen the PHR events over Hainan Island.
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Key words:
- persistent heavy rainfall /
- Madden-Julian Oscillation /
- Hainan Island
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图 1 海南岛18个地面气象观测站的分布图(a，填色为海拔高度，单位：km)；1979—2019年海南岛各月平均降水量(b，单位：100 mm)；海南岛各月PHR事件平均个数(c)；海南岛各月PHR平均降水量，其中橙色为距离台风中心500 km内的降水(d，单位：100 mm)
图 2 1979—2019年10月份各MJO位相下10—90天OLR异常(填色，单位：W·m-2)和850 hPa纬向风场异常(等值线，单位：m·s-1，正值为实线，负值为虚线，间隔1 m·s-1)合成图(仅展示通过90%的显著性检验的区域)
(a~h)分别代表MJO位相1~8，右上数字代表用来合成的天数。
图 3 海南岛10月份MJO八个位相PHR降雨量距平所作的经验正交EOF分析：(a)为第一主分量的结果，(b)为时间序列(PC)，其中X轴为MJO位相，Y轴为振幅(单位：mm·d-1)。
图 4 海南岛10月份MJO不同位相下降雨量距平的合成图。(a)和(b)分别为位相3~6时总降水(PRE)和PHR引起的降水异常的合成，(c)和(d)分别为位相7~2时总降水(PRE)和PHR引起的降水异常的合成。
右上角的数字代表用来合成的天数，其中较大的点表示通过了90%显著性T检验，单位：mm·d-1。
图 5 (a) 10月份海南岛国家站发生PHR的总天数分布图；(b)MJO位相3~6期间PHR的天数；(c)PHR期间位相3~6发生天数占PHR总天数的百分比(%)，即(b)除以(a)；(d)MJO位相7~2期间PHR的天数。
图 6 (a) 为700~1000 hPa积分的10~90天水汽通量散度异常在MJO不同位相时的合成，单位为(m·s-1)·(g·kg-1)·10-2(其中左边是MJO位相3~6期间，右边是MJO位相7~2期间，黄色是总水汽通量散度，红色是水平平流项，蓝色是辐合-辐散项)；(b)和(c)分别为尺度分解后的辐合-辐散项对MJO位相3~6和位相7~2合成结果，单位为(m·s-1)·(g·kg-1)·10-3，其中上标“-”表示低频背景场(>90天)，“ ′ ”表示季节内分量(10~90天)；“*”表示高频扰动(< 10天)，正值为橙色，负值为蓝色。结果为各项在海南岛上(108~111 °E，18~21 °N)的区域平均
图 7 10月份MJO不同位相下10~90天850 hPa风场异常(单位：m·s-1)和比湿异常(单位：g·kg-1)合成图(仅显示通过90%的显著性检验的部分)
(a)为MJO位相3—6期间，(b)为MJO位相7—2期间。右上角的数字表示用来合成的天数。
图 8 1979—2019年海南岛18个观测站10月份的PHR降水量(a)，红色虚线为一个标准差，黑色虚线为平均值；非PHR年份、PHR年份和以及PHR活跃年的强MJO的平均天数(b)，橙色代表位相3~6，蓝色代表位相7~2，柱状图上的数字表示MJO的平均强度；(c~e)分别为2000年、2008年和2010年10月份MJO位相图
每隔1天打一个点，蓝色为上旬，红色为中旬，绿色为下旬。
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