珠江口峡谷对南海北部陆坡区海盆-陆架水交换调制机制研究

The exchange flow between the basin and shelf currents in the northern South China Sea (NSCS) merely occured along its steep continental slope. This exchange flow is the major source of the external forces for shelf and coastal currents in the NSCS. It greatly influenced not only the shelf current, but also the associated regional thermohaline and biogeochemical processes. Historic studies have revealed the underlying mechanisms that govern the propagation of intrusive waters from the basin over the continental shelf in the NSCS. However, our understanding on the dynamics that modulate the exchange flow, which trigers these invasions, is still very limited, although there are limited studies recoganized that the along-slope meandering of the slope topography, e.g. Pearl River Canyon, played critical roles in determing the exchange flow between the basin and shelf in the NSCS. This applicant comprehensively revisited the historic studies and synthesized the historic field observations as well as numerical simulations and propose this hypothesis: when the three-layered slope current in the NSCS flows over the Pearl River Canyon, it responses to this concave canyon topography. This process will then horizonally redistribute the pressure field, and generate along-slope pressure gradient force that geostrophically alter the exchange flow and the interaction between the basin and shelf currents. This project intends to deeply mine the data from observations and coarse-resolution simulations, and progressively develope both an idealized simulation and a mulit-layer nested downscaling numerical modeling system to investigate the characteristics of slope current in the NSCS, as well as its response to the shelf-ward incising Pearl River Canyon. The mechanism that governs the exchange flow between the basin and shelf currents can then be revealed.

南海北部陆坡是海盆-陆架水交换发生的主要区域，跨陆坡水交换既是陆架环流的主要外海驱动力，又对陆架环流、水文性质和生物地球化学过程具有重要影响。历史研究对入侵的海盆水体在南海北部陆架上的运动规律已经有了深入认识，但对调控跨陆坡水交换的动力机制理解还非常不充分。申请人分析历史研究，现场观测和数值模拟资料后，认识到珠江口峡谷，这一楔入陆坡的陡峭峡谷的地形，可以增强海盆-陆坡水交换，并对其动力机制提出以下科学假说：陆坡流对珠江口峡谷地形的响应会导致压强场空间再分配和沿陆坡的压强梯度力变化，调控海盆-陆架水交换。本项目拟在前期工作基础上，对现场调查和数值模拟数据进行深度挖掘，循序渐进搭建高分辨率理想地形、真实地形数值模拟系统，重构南海北部陆坡流细节，观测和模式互相印证，揭示珠江口峡谷中，海盆-陆坡水交换的演化特征，阐明其调控机理。

本项目主题聚焦于南海北部陆坡，珠江口峡谷附近的海盆-陆架水交换的动力机制，将陆架-海盆之间沿等深线西南向传播的低频波动的传播特征与南海三层环流结构结合，基于现场观测和长时间现场调查数据研究复杂地形、陆坡-陆架流联合调控下的波流相互作用导致的跨陆坡水交换过程和动力机制。本研究将前期研究工作基于准定常状态诊断分析获取的跨陆坡-陆架水交换动力机制的研究推进到对建立过程的剖析。本研究涉及到三个主要方向的内容，南海陆坡流的能量和涡度动力调控机制、南海陆坡低频波动的主要成分、建立过程和演化特征、南海陆坡-陆架流捕获低频波动的动力学解释。在本项目支持下，结合气候态再分析数据和数值模式，本研究发现不但在珠江口峡谷附近，整个南海北部陆坡都存在以涡度和能量热点为核心的水交换通道，这些水交换通道在斯托克斯定理框架下定义了南海海盆的基本环流结构。珠江口峡谷附近，上游吕宋海峡交换流设定的陆坡流对峡谷地形响应建立的底压力扭矩是决定海盆-陆架水交换通道建立的主要维持因素。本研究借助底流观测分析了南海北部陆架、陆坡流的季节内振荡现象，对南海深层地形罗斯贝波的时空特征和潜在能量来源进行全面分析，发现南海北部季节内变化尺度的运动主要依靠上层中尺度过程通过扭曲等压面做功向下输入能量，而深层环流通过斜压不稳定过程释放的有效位能以及通过环流输运的能量通量在能量平衡中处于次要地位。黑潮在吕宋海峡向南海的入侵以及相关涡旋活动可能是南海北部深层流动季节内振荡的重要能量来源。本研究分析了低频波动被背景流捕获的涡度动力机制，发现除了波流相互作用外，底摩擦也是将低频波动捕获形成平均意义下跨陆架-陆坡水交换的主要调控机理之一，当水深较浅时，底应力旋度、相对涡度平流过程是决定跨等深线水交换的主要因素。陆坡上，水体斜压性对地形扰动响应在水体里建立的地转过程是调控跨陆架-陆坡水交换的主要动力因素。