Quantification of the intensity of mixing hotspots to clarify the dynamics of the global meridional overturning circulation

使用无系留自由落体微观结构剖面仪“TurboMAP-D”在太平洋内部共收集了53条速度微观结构剖面。在20°~ 30°N处，内潮扩散系数与局地有效能量密度的对数基本成正比，在伊豆-小笠原海岭附近达到10^<-4> m^2s^<-1>以上。相反，无论局地可用的半日内潮能有多少，在30°N极向和20°N赤道向，深周期扩散率保持在~ 10^<-5> m^2s^<-1>数量级。我们已经证实，这些观测到的特征与Hibiya等人（2006年）预测的主温跃层的内旋扩散率全球图是一致的。利用理想化的海洋环流模式，我们接下来检验了Hibiya等人（2006）预测的“混合热点”对太平洋经向翻转环流的影响。我们发现混合热点的上升流估计不足以平衡深水产量。此外，发现穿越赤道的深水向北输送比以前的海流计系泊和水文测量所估计的要少得多。对这些结果的一个似是而非的解释是太平洋经向翻转环流的强度被观测高估了。我们对以前的海洋环流模式的有效性提出了质疑，在这些模式中，为了获得从海流计系泊和水文测量中得到的海流强度，专门指定了海流扩散系数。

The un-tethered free-fall microstructure profiler “TurboMAP-D" was used to collect a total of 53 velocity microstructure profiles in the interior of the Pacific Ocean. We found the diapycnal diffusivity at 20°-30°N was roughly in proportional to the logarithm of the locally available energy density of the semidiurnal internal tide and reached more than 10^<-4> m^2s^<-1> near the Izu-Ogasawara Ridge. In contrast, diapycnal diffusivity was found to remain of the order of〜10^<-5> m^2s^<-1> poleward of 30°N and equatorward of 20°N, irrespective of the amount of locally available semidiurnal internal tide energy. We have confirmed that these observed features are consistent with the global map of diapycnal diffusivity in the main thermocline predicted by Hibiya et al. (2006).Using an idealized ocean general circulation model, we next examined the effect of“mixing hotspots", predicted by Hibiya et al. (2006), on the meridional overturning circulation of the Pacific Ocean. We have found that the estimated upwelling in the mixing hotspots is not sufficient to balance deep-water production. In addition, the northward transport of the deep water crossing the equator was found to be much less than estimated from previous current meter moorings and hydrographic surveys. One plausible explanation for these results is that the magnitude of the meridional overturning circulation of the Pacific Ocean has been overestimated from observations. We raise doubts about the validity of previous ocean general circulation models where diapycnal diffusivity is assigned ad hoc to attain the current magnitude suggested from current meter moorings and hydrographic surveys.