深层斜压环流基本规律的实验研究

Planetary β has been considered important for determining distribution of potential vorticity (PV) in the world’s oceans, and is essential for the theoretical understanding of the wind-driven circulation. However recently, observational analysis shows that in the deep North Atlantic basin, instead of being zonally oriented as ruled by the β effect, PV contours are aligned with the western boundary, especially in extra-tropical regions. Meanwhile, the resulting abyssal circulation in our previous laboratory experiments on rotating horizontal convection, which did not consider β effect at all, demonstrates surprisingly similar features of PV distribution to the real ocean. This seemingly weird similarity between lab results and ocean reality suggests that the potential significance of another factor, the one we called “baroclinic” PV and relates to elevation of density surfaces, to PV distribution and hence abyssal circulation. We hypothesize that for thermohaline circulation, which is certainly strongly baroclinic, the interface elevation may make a bigger contribution than planetary β effect, and dominates PV. To verify this hypothesis, we propose here an experimental study, which, based on the classical one-layer source-sink driven circulation in a “pie-shaped” rotating tank performed by Stommel and his colleagues more than half a century ago, investigates the “baroclinic” PV gradient, the β effect, and its relative relation to the source-sink driven flow in a two-layer fluid.

一直以来行星β被认为是海洋位势涡度（PV）分布的重要特征，在风生环流理论中占重要位置。然而最近对观测资料的分析表明北大西洋深层中高纬度地区的等PV线并不是依照β效应沿纬向分布，反而在西边界附近沿经向延伸。更令人意外的是，我们前期利用旋转水平对流实验模拟的热盐环流虽然不具备β效应，却具有极为相似的PV分布特征。大洋实际和实验结果显示的这种看似“矛盾”的相似性启发我们思考之前研究中一直被忽略的“斜压”PV梯度（即由密度面起伏造成的PV分布）与β效应的相对关系及其对热盐环流的影响。我们猜测热盐环流由于具有明显的斜压特征，所以与之相关的“斜压”PV梯度在很多区域强于β效应，主导PV分布进而控制环流。为了验证这一猜想，我们以Stommel经典一层源汇实验为雏形，研究两层源汇驱动的流动中“斜压”PV与β效应及环流三者的关系，并以此为切入点研究“旋转”与“层结”影响下三维热盐环流的建立和变化机理。