Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093.
Environmental Studies Program and Physics Department, Brandeis University, Waltham, MA 02453.
Proc Natl Acad Sci U S A. 2023 Jun 27;120(26):e2301664120. doi: 10.1073/pnas.2301664120. Epub 2023 Jun 20.
Turbulence-enhanced mixing of upper ocean heat allows interaction between the tropical atmosphere and cold water masses that impact climate at higher latitudes thereby regulating air-sea coupling and poleward heat transport. Tropical cyclones (TCs) can drastically enhance upper ocean mixing and generate powerful near-inertial internal waves (NIWs) that propagate down into the deep ocean. Globally, downward mixing of heat during TC passage causes warming in the seasonal thermocline and pumps 0.15 to 0.6 PW of heat into the unventilated ocean. The final distribution of excess heat contributed by TCs is needed to understand subsequent consequences for climate; however, it is not well constrained by current observations. Notably, whether or not excess heat supplied by TCs penetrates deep enough to be kept in the ocean beyond the winter season is a matter of debate. Here, we show that NIWs generated by TCs drive thermocline mixing weeks after TC passage and thus greatly deepen the extent of downward heat transfer induced by TCs. Microstructure measurements of the turbulent diffusivity ([Formula: see text]) and turbulent heat flux ([Formula: see text]) in the Western Pacific before and after the passage of three TCs indicate that mean thermocline values of [Formula: see text] and [Formula: see text] increased by factors of 2 to 7 and 2 to 4 (95% confidence level), respectively, after TC passage. Excess mixing is shown to be associated with the vertical shear of NIWs, demonstrating that studies of TC-climate interactions ought to represent NIWs and their mixing to accurately capture TC effects on background ocean stratification and climate.
海洋上层的混合加强了，热带大气与冷水团相互作用，影响了高纬度的气候，从而调节了海气耦合和向极热输送。热带气旋（TCs）可以极大地增强海洋上层的混合，并产生强大的近惯性内波（NIWs），这些内波向下传播到深海。在全球范围内，TC 通过时向下混合热量会导致季节性温跃层变暖，并将 0.15 到 0.6 PW 的热量泵入未通风的海洋。为了了解 TC 对气候的后续影响，需要了解 TC 贡献的多余热量的最终分布；然而，目前的观测并不能很好地约束这一点。值得注意的是，TC 提供的多余热量是否足够深地渗透到冬季之外的海洋中并被保留下来，这是一个有争议的问题。在这里，我们表明，TC 产生的 NIWs 在 TC 通过后数周内驱动温跃层混合，从而大大加深了 TC 诱导的向下热传递的范围。在三个 TC 通过之前和之后，我们对西太平洋的微结构测量了湍流通量（[Formula: see text]）和湍流传热率（[Formula: see text]），结果表明，在 TC 通过之后，湍流通量和湍流传热率的平均温跃层值分别增加了 2 到 7 倍和 2 到 4 倍（95%置信水平）。多余的混合与 NIWs 的垂直剪切有关，这表明 TC-气候相互作用的研究应该代表 NIWs 及其混合，以准确捕捉 TC 对背景海洋分层和气候的影响。
