Dominant Inverse Nonlinear Energy Cascades in Coastal and Inner Shelf Waters

As well known, almost 80% of the total oceanic kinetic energy is contained within the mesoscale eddies, but the cross-scale energy transfers linking eddy generation and dissipation remains an open question. The Earth rotation controls the generation of mesoscale eddies that transfer energy toward larger scales via an inverse cascade, but a transfer to small scale is needed for dissipation: a coexistence of transfers is indeed required. However, the turbulent energy cascade, responsible for oceanic circulation energy balance and nutrient mixing, involves such a large range of scales making extremely challenging the investigation of the full spectrum of processes.This study examines how energy and enstrophy cascade across ocean scales in the inner shelf of the Great Bay Area of the South China Sea using high resolution numerical simulations. By applying coarse-graining techniques, the results reveal a predominant inverse energy cascade scenario driven by the interplay between background stratification and rotation, particularly in areas affected by the Pearl River's freshwater influx. Geographical heterogeneity significantly influences these dynamics, with shallow inlets experiencing increased frictional drag and energy dissipation due to complex bathymetry, while open-water areas show heightened submesoscale activity. Seasonal monsoon cycles further modulate these processes, with summer winds amplifying nonlinear energy transfers through increased wind stress and river discharge, in contrast to the tidally dominated, quasi-steady flows observed in winter. Understanding how wind and tide affect ocean mixing can help to develop more accurate climate models and better strategies for protecting coastal ecosystems.众所周知，全球近80%的海洋动能（oceanic kinetic energy）都储存在中尺度涡旋（mesoscale eddies）中，但连接涡旋生成与耗散的跨尺度能量传输（cross-scale energy transfers）仍是一个悬而未决的科学问题。地球自转调控着中尺度涡旋的生成，这些涡旋通过反向级串（inverse cascade）将能量向更大尺度传输，而耗散过程则需要能量向小尺度传递，换言之，传输过程的双向共存是必需的。然而，负责海洋环流能量平衡与营养盐混合的湍流能量级串（turbulent energy cascade）涉及极宽的尺度范围，这使得全面解析相关过程的完整谱系极具挑战性。本研究依托高分辨率数值模拟，针对南海大湾区近岸陆架区域的海洋尺度间能量与涡量拟能（enstrophy）级串过程展开分析。通过应用粗粒化技术（coarse-graining techniques），研究结果揭示了一种由背景层结（stratification）与自转共同驱动的主导性反向能量级串场景，在受珠江淡水输入影响的区域尤为显著。地理异质性对该动力学过程具有显著调控作用：复杂的海底地形（bathymetry）使得浅水区的摩擦阻力与能量耗散加剧，而开阔水域则表现出更强的次中尺度（submesoscale）活动。季节性季风周期（seasonal monsoon cycles）进一步对上述过程进行调制：夏季风通过增强风应力（wind stress）与径流量（river discharge），放大了非线性能量传输；与之形成对比的是，冬季的流场以潮汐主导的准稳态流动为主。厘清风场与潮汐对海洋混合的调控机制，有助于构建更为精准的气候模型，并为海岸生态系统保护提供更优化的策略。