Near-inertial mixing: Modulation of shear, strain and microstructure at low latitude
Near-inertial mixing: Modulation of shear, strain and microstructure at low latitude
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DOI：
10.1029/2000jc000370
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发表时间：
2001-08-15
影响因子：
3.6
通讯作者：
Gregg, MC
中科院分区：
文献类型：
--
作者：
Alford, MH;Gregg, MC
我们报告了1998年10月在6.5度，128度的班达海，一个单一的，有能量的，向下传播的近惯性波的三个周期的混合的直接定量测量。波浪主导剪切，占总方差的70%。同时深度/时间序列的剪切、应变、弗劳德数（Fr）和微观结构允许直接计算它们的相干性和相位，从50-120米，持续14天。在这个深度范围内，72%的横旋扩散率（68%的耗散率）发生在三个不同的脉冲中，间隔为4.4天的惯性周期。它们与横向剪切、应变和Fr的最大值同时存在。在95%的置信水平上，惯性带对数横向扩散系数log(10) Kp与剪切和弗劳德数的两个分量一致。在该数据集中，应变对Fr的调制比剪切更重要。由于纬度低，惯性频率（f(o)=1/4.4 cycles / day）远小于日频率和潮汐频率。因此，通过时域滤波，可以将近惯性运动与潮汐和其他运动分开研究。观测频率ω (o) = 1.02f(o)和垂直尺度为100 m的半经验WKB平面波解分别解释了66%和42%的惯性带剪切和应变变化。根据观察到的剪切和应变之间的相位关系，波正向赤道方向传播，向295度方向传播。剪切与应变之比以及平行与横向剪切之比表明，波的固有频率ω(1)近似于1.18f（eff）。这表明背景涡度降低了有效科里奥利频率f(eff) = f(o) + zeta /2，相对于它的行星值f(o) [Kunze, 1985]。光线追踪显示，该波在6.9度附近产生，130.6度，与巡航前20天相似，与东南季风相关的大风结束一致。采用国家环境预测中心（NCEP）地表风模型的平板混合层模型[Pollard和Millard， 1970]证实，此时从风到海洋的通量足以产生海浪。一个非常简单的模型表明，季风产生的惯性波的混合可能会给某些地区的能量收支增加一个重要的、强烈依赖时间的方面。
We report direct, quantitative measurements of mixing associated with three cycles of a single, energetic, downward-propagating near-inertial wave in the Banda Sea at 6.5 degreesS, 128 degreesE during October 1998. The wave dominates the shear, containing 70% of the total variance. Simultaneous depth/time series of shear, strain, Froude number (Fr), and microstructure allow direct computation of their coherence and phase from 50-120 m, for 14 days. In this depth range, 72% of diapycnal diffusivity (68% of dissipation) occurs in three distinct pulses, spaced at the inertial period of 4.4 days. These are collocated with maxima of transverse shear, strain and Fr. Inertial-band log diapycnal diffusivity, log(10) Kp, is coherent at the 95% confidence level with both components of shear and Froude number. In this data set, strain is more important than shear in modulating Fr. Owing to the low latitude, the inertial frequency (f(o)=1/4.4 cycles per day) is much smaller than the diurnal and tidal frequencies. Consequently, near-inertial motions may be studied separately from tides and other motions via time-domain filtering. Semiempirical WKB plane-wave solutions with observed frequency omega (o) = 1.02f(o) and vertical scale 100 m explain 66% and 42% of inertial-band shear and strain variance, respectively. On the basis of the observed phase relationship between shear and strain, the wave is propagating equatorward, toward 295 degrees true. Ratios of shear to strain and of parallel to transverse shear suggest that the wave's intrinsic frequency omega (1) approximate to1.18f(eff). This indicates that background vorticity has lowered the effective Coriolis frequency, f(eff) = f(o) + zeta /2, relative to its planetary value, f(o) [Kunze, 1985]. Ray tracing suggests that the wave was generated near 6.9 degreesS, 130.6 degreesE, similar to 20 days prior to the cruise, coincident with the end of high winds associated with the SE monsoon. A slab mixed layer model [Pollard and Millard, 1970], forced with National Center for Environmental Prediction (NCEP) model surface winds, confirms that fluxes from the wind to the ocean at this time were sufficient to generate the wave. A very simple model shows that mixing by monsoon-generated inertial waves may add an important and strongly time-dependent aspect to some regions' energy budgets.