Intraseasonal Variation of Sea Surface Temperature over the Tropical Eastern Indian Ocean During Boreal Spring and Summer and Accompanied Atmospheric Response
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摘要: 基于1982—2020年OISSTv2.1数据集、NOAA Interpolated OLR数据集和NCEP/DOE Ⅱ再分析数据资料，通过功率谱分析、位相合成法等方法研究了春夏季（3—8月）热带东印度洋海温季节内振荡的特征及大气的响应。结果发现春夏季热带东印度洋海温季节内振荡的显著周期主要集中在20~50天，强海温振荡事件主要分布在4—7月，关键区内对流抑制超前暖海温异常1/8周期，滞后冷海温异常3/8周期，反之亦然。春夏季海温异常激发的对流异常呈经向偶极子分布，且有明显的北移演变特征，但北侧的对流中心较南侧弱，即对流在北移过程中迅速衰减。使用K-means聚类法提取了两种具有显著差异的环流响应类型：北移型和局地型。北移型主要分布在夏季，表现为更有组织性的对流偶极子对，南北中心的强度和影响范围相当且北移特征更为清晰。在对流偶极子北移的过程中显著影响阿拉伯海上西南季风的强弱，对印度半岛夏季风季节内变化的影响较大。局地型主要分布在春夏转换季节，对流异常呈单一中心结构，移动性较弱，主要在热带东印度洋上空发展和消亡。热带东印度洋海温季节内振荡耦合的大气环流异常主要影响赤道印度洋地区，对热带外的影响较弱。进一步探究发现，背景东风切变的差异是导致两种对流响应差异的原因。夏季，北印度洋东风切变强，垂直风切变机制导致对流明显北移，春夏转换季节，东风切变弱，垂直风切变机制不成立，较弱的正压涡度平流机制引起对流微弱的北移，但主要在局地生消。
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关键词:
- 热带东印度洋 /
- 海表温度 /
- 季节内振荡 /
- 大气响应 /
- K-means聚类法
Abstract: The sea surface temperature (SST) over the tropical eastern Indian Ocean (TEIO) during boreal spring and summer shows a significant intraseasonal oscillation (ISO). Based on the NOAA OISSTv2.1 dataset, NOAA Interpolated Outgoing Longwave Radiation dataset and NCEP-DOE Reanalysis Ⅱ data from 1982 to 2020, the characteristics of its intraseasonal variation and the associated atmospheric response were investigated by using power spectrum analysis, phase compositing and K-means clustering. The results show that the temporal variation of the SST anomalies over the TEIO was dominated by a period of 20-50-day, and the strong intraseasonal SST oscillation mainly occurred from April to July. A pronounced ocean-atmosphere interaction could be found over the TEIO at the intraseasonal timescale, with the SST warming leading local convection enhancement by 3 / 8 cycle and lagging convection suppression by 1/8 cycle. The convection anomalies driven by the intraseasonal SST showed a meridional dipole structure. It was characterized by a prominent northward propagation, with the intensity decaying rapidly during the northward movement and the convection in the north weaker than that in the south. Two significant kinds of atmospheric responses were extracted by using K-means clustering: the northwardmoving response and the local response. The northward moving response occurred mainly in summer (June-July), showing a well-organized convective dipole structure with comparable intensity and clear northward propagation. During its northward movement, it affected the strength of the southwesterly monsoon over the Arabian Sea and the intraseasonal variation of the Indian Peninsula summer monsoon significantly. While the local response occurred mainly in the transition season between spring and summer (April-June), and the sole convection center developed and disappeared over the TEIO with a weak migration. Different types of atmospheric response to the intraseasonal SST variation can be attributed to different easterly shear over the North Indian Ocean. In summer, the easterly shear was strong, and the convection propagated northward following the vertical shear mechanism. However, the background easterly shear was weak in the transition season between spring-summer and the vertical wind shear mechanism was inapplicable. The weak vorticity advection mechanism caused the convection to move northward slightly, but it mainly developed and dissipated locally. -
图 1 1982—2020年春夏季（3—8月）关键区（70~100 °E，5 °S~5 °N）区域平均的海温季节内（10~90天）信号功率谱分析的显著周期与其显著程度（a）和显著程度在不同年代的平均值（b），以及20~50天海温振荡信号解释方差的空间分布（c）
周期显著程度的值为该周期的谱密度与Markov置信上限的差值（单位：W·Hz-1），解释方差为该频段信号与全频段信号之比，以百分比显示，红框为本文定义的热带东印度洋关键区。
图 2 春夏季热带东印度洋20~50天强海温振荡事件中标准化海温（阴影）、OLR（等值线，单位：W·m-2，紫色代表正值，绿色代表负值，间隔为6 W·m-2，粗线代表绝对值为12 W·m-2）和850 hPa风场（箭头，单位：m·s-1）在1~8
位相的合成图(灰框为本文定义的热带东印度洋关键区，仅显示通过90%置信水平t检验的值)
图 5 北移（a）与局地（b）对流响应型60~100 °E纬向平均的20~50天正压涡度ζ+（阴影，单位：10-6 s-1）与OLR（等值线，单位：W·m-2，紫色代表正值，绿色代表负值）的纬度-时间演变图
OLR仅显示通过90%置信水平t检验的结果，正压涡度通过显著性检验的部分透明度为0，未通过的透明度为50%。x轴对应海温季节内振荡的时间。第0天为关键区海温异常达最暖的时刻。竖虚线分别显示两种类型事件合成中海温的第1到9位相，横虚线显示关键区的北界（5 °N）。
图 6 北移（a）与局地（b）对流响应型第4位相200~850 hPa的平均纬向风垂直切变（等值线，棕色实线与虚线分别代表正值与负值，即西风切变与东风切变，粗黑线为0线，单位：m·s-1，间隔：5 m·s-2）、其与20~50天斜压散度经向梯度的乘积（阴影，单位：10-12 s-2）和20~50天正压涡度倾向（单位：10-6 s2）
“H”与“L”分别指对流抑制与加强中心（即OLR正异常/负异常最大值所在）。透明度为50%的阴影代表未通过显著性检验的值，透明度为0的阴影代表通过显著性检验的值。打点和格子分别代表通过显著性检验的正和负正压涡度倾向。
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