The long-term circulation driven by density currents in a two-layer stratified basin
The long-term circulation driven by density currents in a two-layer stratified basin
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DOI：
10.1017/s0022112006003478
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发表时间：
2007-01
影响因子：
3.7
通讯作者：
M. Wells;J. Wettlaufer
中科院分区：
文献类型：
--
作者：
M. Wells;J. Wettlaufer
实验和理论用于研究流入两层分层盆地的二维密度流的长期动力学。当表征背景分层与密度流浮力通量之比的初始理查森数 $\hbox{\it Ri}_{\rho}^{\hbox{\scriptsize\it in}}$ 小于 $\hbox{\it Ri}_{\rho}^{*} 的临界值时 \,{=}\,21-27$，发现密度电流穿透分层界面。对于 30° 到 90° 之间的角度，该结果表面上与斜率无关。如果电流最初没有穿透界面，那么它会缓慢增加顶层的密度，直到界面密度差充分减小以驱动穿透。发生这种情况的时间尺度 $t_{p} \,{=}\, (\hbox{\it Ri}^{\hbox{\scriptsize\it in}}_{\rho} - \hbox{\it Ri}_{\rho}^{*}) L/B^{1/3}$ 为 明确地是浮力通量 B 和盆地长度 L 的函数。初始理查森数 $\hbox{\it Ri}^{\hbox{\scriptsize\it in}}_{\rho}$ 是深度、界面初始减小重力和坡角的弱函数。在 75° 和 90° 的非常陡的斜坡没有初始渗透的情况下，我们观察到界面处的渗透对流会导致显着的局部夹带。结果，顶层变厚，界面夹带率随着界面弗劳德数的五次方而增加。相比之下，在 30°、45° 和 60° 的较低斜率上，在可比较的界面弗劳德数下没有观察到这样的过程，从而证明了冲击角对穿透对流的重要作用。我们将陡密度流增加的界面夹带归因于密度流侵入点处从波状孔过渡到湍流水力跳跃的结果。我们讨论了观测到的环流对北极盐跃层稳定性的适用性，我们发现在一系列当代海洋条件下，$0.56\,{\lesssim}\, t_{p} \,{\lesssim}\,1.2$ 年。
Experimentation and theory are used to study the long-term dynamics of a two-dimensional density current flowing into a two-layer stratified basin. When the initial Richardson number, $\hbox{\it Ri}_{\rho}^{\hbox{\scriptsize\it in}}$, characterizing the ratio of the background stratification to the buoyancy flux of the density current, is less than the critical value of $\hbox{\it Ri}_{\rho}^{*} \,{=}\, 21-27$, it is found that the density current penetrates the stratified interface. This result is ostensibly independent of slope for angles between 30° and 90°. If the current does not initially penetrate the interface, then it slowly increases the density of the top layer until the interfacial density difference is reduced sufficiently to drive penetration. The time scale for this to occur, $t_{p} \,{=}\, (\hbox{\it Ri}^{\hbox{\scriptsize\it in}}_{\rho} - \hbox{\it Ri}_{\rho}^{*}) L/B^{1/3}$, is explicitly a function of the buoyancy flux B and the length of the basin L. The initial Richardson number, $\hbox{\it Ri}^{\hbox{\scriptsize\it in}}_{\rho}$, is a function of depth, the initial reduced gravity of the interface and a weak function of slope angle. In the absence of initial penetration for very steep slopes of 75° and 90°, we observe that penetrative convection at the interface leads to significant local entrainment. In consequence, the top layer thickens and the interfacial entrainment rate increases as the fifth power of the interfacial Froude number. In contrast, such a process is not observed at comparable interfacial Froude numbers on lower slopes of 30°, 45° and 60°, thereby demonstrating the important role of impact angle on penetrative convection. We attribute the increased interfacial entrainment by the steep density currents as the result of the transition from an undular bore to a turbulent hydraulic jump at the point where the density current intrudes. We discuss the applicability of the observed circulation to the stability of the Arctic halocline where we find $0.56\,{\lesssim}\, t_{p} \,{\lesssim}\,1.2$ years for a range of contemporary oceanographic conditions.