Analysis of upper ocean response to Typhoon Doksuri in the northwest South China Sea
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摘要: 基于锚碇观测资料，本文分析了南海西北部陆坡区上层海洋对台风“杜苏芮”的动力学和热力学响应特征。在动力学响应方面，台风“杜苏芮”期间上层流速显著增强，混合层纬向流速可达1.20 m/s；“杜苏芮”经过后上层海水运动以近惯性振荡为主(流向顺时针旋转周期在36～40 h之间)。近惯性能量在垂向分布上存在两个高值中心，分别位于混合层和温跃层深度上。近惯性能量耗散过程的e折时间尺度约为3.7 d，我们认为能量的向下传播在局地近惯性能量衰减过程中起主要作用。对能量谱的分析表明，“杜苏芮”作用期间近惯性频率能量相对于其作用前增大了约29.4倍，而全日和半日频率(K1和M2)能量有所减弱。此外，能量谱显示近惯性频率存在明显的“蓝移”现象，即对于纬向和经向流速分量在400 m以浅平均的近惯性振荡频率分别为1.167 f0和1.170 f0(f0为局地惯性频率)。蓝移与近惯性内波的向下传播及正的相对涡度的输入有关。在热力学响应方面，上层海洋在台风的搅拌作用下，40～250 m深度均出现较小增温，最大增温幅度接近1°C；此外70 m以浅盐度的降低可能与台风过境时的降水相关，而Ekman抽吸引起的上升流则可能对70～100 m深度盐度的升高具有重要作用。Abstract: Based on the in-situ data from mooring deployed in the northwest South China Sea, we investigate the dynamical and thermal dynamical response of upper ocean to Typhoon Doksuri. In the aspect of dynamic response, as the Typhoon passing, the currents in upper layer enhanced strikingly, the zonal currents in the mixed layer reaches 1.20 m/s. After the passage of Typhoon Doksuri, the currents in the upper layer are dominated by near-inertial oscillation, which rotate clockwise with a period between 36–40 hours. The kinetic energy of near-inertial wave shows two high energetic cores in vertical, which locates at the mixed layer and the thermocline layer, respectively. The estimated e-folding time-scale of near-inertial energy decay is about 3.7 d, and we believe that the downward propagation of energy is the major reason for the decay. The power spectra analysis of currents reveals that power density at inertial frequency, during the period of Typhoon Doksuri, increases about 29.4 times larger than that before the Typhoon arriving. Nevertheless, power density both at diurnal (K1) and semidiurnal (M2) frequency decreases during Typhoon period. Additionally, a blue shift at inertial frequency is identified. We find that the averaged near-internal frequency in upper 400 m is 1.167 f0 for zonal near-inertial currents and 1.170 f0 for meridional near-inertial currents (where f0 is the local inertial frequency). This blue shift is connected with the downward propagation of near-inertial waves and input of positive relative vorticity. In the aspect of thermodynamic response, the temperature rises in the upper layer between 40–250 m depth, due to the stirring induced by strong wind, and the maximum increased temperature amplitude is about 1℃. In addition, the decrease of salinity above 70 m may be related to the precipitation caused by the Typhoon. While the upwelling induced by Ekman pumping may have significant contribution to the increase of salinity at the depth of 70–100 m.
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图 1 台风“杜苏芮”路径及强度
红色三角形为锚系站位，黄色虚线代表台风强风域范围
Fig. 1 The trajectory and strength of Typhoon Doksuri
The red triangle and yellow dotted lines circle denotes the mooring station and strong-wind regions, respectively
图 3 台风“杜苏芮”经过前后崖城平台风速时间序列（a），锚系站位纬向流速（b）和经向流速(c)的时间序列；台风前背景场平均流速（d）及流向剖面（e）
黑色虚线表示台风中心距离锚系站位最近的时刻
Fig. 3 Time series of wind speed in Yacheng offshore platform (a), zonal (b) and meridional (c) currents in mooring station before and after Typhoon Doksuri passing. The profiles of mean speed (d) and directions (e) of background currents
The dotted black line denotes the moment when the typhoon center is closest to the mooring station
图 4 台风“杜苏芮”作用前后观测站位流速的前进矢量图
a. 30 m; b. 50 m; c. 100 m; d. 150 m. 红色三角形代表台风中心距离站位最近的时刻
Fig. 4 The progressive vector of currents in observation station before and after Typhoon Doksuri passing
a. 30 m; b. 50 m; c. 100 m; d. 150 m. The red triangle represents the moment when the typhoon center is closest to the mooring station
图 5 台风“杜苏芮”作用前后纬向（a）和经向（b）近惯性流速的时间序列
Fig. 5 Time series of zonal (b) and meridional (c) near-inertial currents before and after Typhoon Doksuri passing
图 6 局地惯性周期平均的近惯性动能分布（a）及不同深度上的时间序列（b）
b图中每条曲线上的两个实心圆点分别代表能量最大值的位置和能量衰减至最大能量的1/e的位置
Fig. 6 Variation near-inertial kinetic energy profile with time (a), and time series of near-inertial kinetic energy at different depths (b)
The two dots on each line in b indicate the position of the maximum energy and the position where the energy decays to 1/e of the maximum energy, respectively
图 7 “杜苏芮”作用之前（a）和之后（b）纬向流速的功率谱；杜苏芮作用之前（c）和之后（d）纬向流速在50 m、100 m和150 m深度上的能谱曲线
图a的两条品红虚线为近惯性频带宽度
Fig. 7 Power spectra of the zonal currents before (a) and after (b) Typhoon Doksuri passing. Power spectra of the zonal currents before (c) and after (d) Typhoon Doksuri passing at 50 m, 100 m and 150 m depth
Two magenta dotted lines in a denoting the bandwidth of the near-inertial frequency
图 8 近惯性频率与局地惯性频率的比值的垂向剖面
Fig. 8 Vertical profiles of the ratio of near-inertial frequency to local inertial frequency
图 9 “杜苏芮”作用期间海面高度异常及地转流场分布
Fig. 9 The sea surface height abnormally and geostrophic current field during Typhoon Doksuri passing
图 10 “杜苏芮”影响期间9月14–25日海表相对涡度的时间序列
Fig. 10 Time series of sea surface relative vorticity from 14 September to 25 September during Typhoon Doksuri passing
图 11 “杜苏芮”作用前后温度（a）和盐度（b）剖面的时间序列
Fig. 11 Time series of temperature (a) and salinity (b) profile before and after Typhoon Doksuri passing
图 12 “杜苏芮”经过之前和之后5天平均的温度（a）和盐度（c）剖面，及相应的温度（b）和盐度（d）变化
Fig. 12 The time-averaged temperature (a) and salinity (c) profiles five days before and after Typhoon Doksuri passing, and the corresponding temperature (b) and salinity (d) variation
图 13 “杜苏芮”作用前后纬向流（a）及经向流（b）的垂向剪切
Fig. 13 The vertical shear of zonal (a) and meridional (b) currents before and after Typhoon Doksuri passing
表 1 不同频率上能量谱密度（单位：（m2/s2）/cpd）深度积分结果
Tab. 1 Power spectral density (unit: （m2/s2）/cpd) integrated along depth at different frequencies
f K1 M2 PSDR（f:K1:M2） 台风作用前 0.68 5.84 0.26 10:86:4 台风作用后 19.97 2.04 0.24 90:9:1 
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