Study on the vertical migration characteristics of the acoustic scattering layer under the influence of internal waves in the northern South China Sea
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摘要: 利用深海潜标搭载的声学多普勒流速剖面仪得到的后向散射强度,研究了南海北部声散射层的昼夜垂直迁移和内孤立波对其的影响。观测结果表明:受浮游动物影响,声散射层主要表现为“昼沉夜浮”,日出前约1 h至日出期间向下移动,日落至日落后约1 h期间向上移动,平均迁移速度为4.7 cm/s(上移)和5.8 cm/s(下移)。此外,经过观测海域的内孤立波引起一对先向下后向上的垂向流,最大垂向流速超过50 cm/s,导致声散射层发生数十至上百米的起伏,海洋上层的声学后向散射强度在内孤立波波谷处达到极大值。进一步的研究显示日间出现内孤立波垂向流速与深度平均后向散射强度变化幅度相关性比夜间的内孤立波高。在两种类型波动引起的垂向流速相当时,日间的深度平均后向散射强度变化幅度通常大于夜间。
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关键词:
- 声学多普勒流速剖面仪 /
- 内孤立波 /
- 声散射层 /
- 昼夜垂直迁移 /
- 南海
Abstract: Based on the Acoustic Doppler Current Profiler carried by deep-sea moorings, we investigated the diel vertical migration of the acoustic scattering layer and the impact of internal solitary waves on it in the northern South China Sea. The observational results reveal that the acoustic scattering layer, influenced by zooplankton, ascends to shallower depth within approximately an hour after sunset, remains there throughout the night, then migrates to the deeper depth within about an hour before sunrise. The average migration velocities are 4.7 cm/s (upward) and 5.8 cm/s (downward). Additionally, internal solitary waves observed in the region induce a pair of downward and upward currents, with maximum vertical velocities exceeding 50 cm/s, leading to fluctuations of tens to hundreds of meters in the acoustic scattering layer. The acoustic backscattering strength of the ocean’s upper layer reaches its maximum value at the troughs of internal solitary waves. Further research indicates that the daytime internal solitary waves exhibit a stronger correlation between the vertical velocity and the depth-averaged backscattering strength variation compared to the nighttime internal solitary waves. When the vertical velocities induced by both types of waves are equal, the depth-averaged backscattering strength variations during the day are typically greater than those at night. -
图 2 观测期间的平均昼夜垂直迁移模式
a. S1站位后向散射强度,b. S2站位后向散射强度,c. S1站位垂向流速,d. S2站位垂向流速。空白区域为两台ADCP的盲区,蓝色(红色)虚线代表观测期间的平均日出(日落)时刻
Fig. 2 The average diel vertical migration patterns during the observation period
a. MVBS of Station S1, b. MVBS of Station S2, c. vertical velocities of Stations S1, d. vertical velocities of Stations S2. The blank areas represent the blank distance of the two ADCPs. The blue (red) dashed lines indicate the average sunrise (sunset) times during the observation period
图 4 S1站位昼夜平均后向散射强度剖面(a),S2站位昼夜平均后向散射强度剖面(b)(虚线部分为ADCP盲区);海洋上层50~270 m深度平均后向散射强度(c),海洋下层450~650 m深度平均后向散射强度(d)
Fig. 4 Daytime and nighttime average MVBS profiles at Station S1 (a); daytime and nighttime average MVBS profiles at Station S2 (b) (the dashed sections represent the blank distance of two ADCPs); depth-averaged MVBS of the ocean’s upper layer from 50 m to 270 m depth (c); depth-averaged MVBS of the ocean’s lower layer from 450 m to 650 m depth (d)
图 5 S1站位垂向流速在4月1日(a)、4月2日(b)、4月3日(c)、4月4日(d)的分布和东西向流速在4月1日(e)、4月2日(f)、4月3日(g)、4月4日(h)的分布(虚线框内标示了内孤立波)
Fig. 5 Vertical velocities of Station S1 on April 1 (a), April 2 (b), April 3 (c), and April 4 (d), and the east-west velocities on April 1 (e), April 2 (f), April 3 (g), and April 4 (h) (dashed boxes mark the presence of internal solitary waves)
图 7 S1站位4月4日日间的内孤立波垂向流速(a),东西向流速(b),后向散射强度(c),海洋上层(50~270 m)的深度平均后向散射强度(d)
空白区域为两台ADCP的盲区,棕色线表示13℃等温线,白色实线(虚线)为垂向流速15 cm/s(−15 cm/s)等值线
Fig. 7 Internal solitary waves at Station S1 during daytime on April 4 vertical velocity (a), east-west velocityv (b), MVBS (c), depth-averaged MVBS of the ocean’s upper layer (50 m to 270 m) (d)
Blank areas indicate the blank distance of the two ADCPs. The brown line represents the 13°C isotherm. The white solid line (dashed line) represents contour lines of vertical velocity at 15 cm/s (−15 cm/s)
图 8 S1站位4月4日夜间的内孤立波垂向流速(a),东西向流速(b),后向散射强度(c),海洋上层(50~270 m)的深度平均后向散射强度(d)
空白区域为两台ADCP的盲区,棕色线表示13℃等温线,白色实线(虚线)为垂向流速15 cm/s(−15 cm/s)等值线
Fig. 8 Internal solitary waves at Station S1 during nighttime on April 4 vertical velocity (a), east-west velocity (b), MVBS (c), depth-averaged MVBS of the ocean’s upper layer (50 m to 270 m) (d)
Blank areas indicate the blank distance of the two ADCPs. The brown line represents the 13°C isotherm. The white solid line (dashed line) represents contour lines of vertical velocity at 15 cm/s (−15 cm/s)
图 9 4月4日内孤立波期间后向散射强度变化幅度深度剖面
蓝色为日间的内孤立波,黄色为夜间的内孤立波,虚线部分为ADCP盲区
Fig. 9 Depth profile of MVBS variation during internalsolitary waves on April 4
Blue indicates daytime internal solitary waves and yellow indicatesnighttime internal solitary waves. The dashed sections representthe blank distance of two ADCPs
图 10 内孤立波导致的深度平均后向散射强度变化幅度与最大垂向流速的关系
绿色点为日间出现的内孤立波,红色点为夜间出现的内孤立波
Fig. 10 Relationship between the depth-averaged MVBS variation induced by internal solitary waves andthe maximum vertical velocity
Green dots represent internal solitary waves occurring during the daytime, while red dots represent those occurring during the nighttime
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