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Restratification of the Upper Ocean after the Passage of a Tropical Cyclone: A Numerical Study

机译：热带气旋通过后对上层海洋的再定化：数值研究

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The role of baroclinic instability in the restratification of the upper ocean after the passage of a tropical cyclone (TC) is determined by means of numerical simulations. Using a regional ocean model, the Regional Ocean Modeling System (ROMS), a high-resolution three-dimensional simulation that includes the process of baroclinic instability and is initialized with moderate-amplitude eddy structures reproduces the satellite-observed decay rate of the TC-induced sea surface temperature (SST) anomaly and is also in qualitative agreement with published observations after the passage of Hurricane Fabian in 2003 that showed decaying cold and warm anomalies located in the climatological mixed layer (CML) and upper thermocline, respectively. The model ocean is restratified after approximately one month with a net heat gain in the water column due to anomalous air-sea heat fluxes. The model shows, however, that vertical heat fluxes associated with baroclinic instability dominate over air-sea heat fluxes in restoring the CML heat content during the first month. A comparison with two-dimensional simulations that exclude baroclinic adjustment further highlights the importance of baroclinic instability: it can not only input a considerable amount of heat into the CML, but also establish strong stratification there, inhibiting the downward penetration of heat contributed by diabatic heating at the surface; both effects hasten the recovery of the SST. Additional experiments were performed to examine the sensitivity of the model results to changes in Newtonian cooling rate, changes in the magnitude of the eddy structures used to initialize the simulation, and changes in poststorm wind strength; the results indicate that, although some of them may have a significant effect on the recovery time of the SST, their influence on the contribution of baroclinic instability to the recovery of the CML heat content is modest. However, the contribution of baroclinic instability exhibits pronounced positive dependence on the depth of the mixing layer relative to the CML depth and the relative size of the area with unperturbed water. Its dependence on the shape of the spatial variation of the mixing depth is relatively weak but in a more complicated manner. These dependencies are consistent with those predicted by a simple front adjustment model, whereas the latter also suggest that the contribution of baroclinic instability is independent of the prestorm stratification below the CML. Overall, the idealized simulations in this study suggest that, for a typical situation in the real ocean, baroclinic instability can account for approximately 50% of the full recovery of the CML heat content, whereas under specific conditions the contribution can be significantly smaller. Those estimates provide a limit to the maximum net warming of the water column after the initial mixing event and thus have important implications regarding estimating the long-term effect of TCs on the upper-ocean heat budget.
机译：通过数值模拟确定了热带气旋（TC）通过后，斜压不稳定在上层海洋再定殖中的作用。使用区域海洋模型，即区域海洋建模系统（ROMS），一种高分辨率的三维模拟，其中包括斜压不稳定过程，并以中振幅涡旋结构进行了初始化，从而再现了卫星观测到的TC-引起的海表温度（SST）异常，并且与2003年费边飓风通过后已发表的观测结果在质量上吻合，该观测表明分别位于气候混合层（CML）和上温跃层的冷和暖异常正在衰减。大约一个月后，由于异常的海气通量，水柱中的净热量净增加，重新模拟了模型海洋。然而，该模型显示，在恢复第一个月的CML热含量时，与斜压不稳定性相关的垂直热通量比海海热通量更为重要。与排除斜压调整的二维模拟的比较进一步强调了斜压不稳定的重要性：它不仅可以向CML输入大量热量，而且可以在CML中建立牢固的分层，从而抑制了由绝热加热导致的热量向下渗透在表面两种效果都加速了SST的恢复。进行了其他实验，以检验模型结果对牛顿冷却速率变化，用于初始化模拟的涡流结构大小的变化以及暴风后风强度变化的敏感性；结果表明，尽管它们中的一些可能会对SST的恢复时间产生重大影响，但它们对斜压不稳定对CML热含量恢复的贡献的影响是中等的。但是，斜压不稳定性的贡献表现出明显的正相关性，即相对于CML深度的混合层深度以及水不受干扰的区域的相对大小。其对混合深度的空间变化的形状的依赖性相对较弱，但是以更复杂的方式。这些相关性与简单的前部调整模型所预测的相关性一致，而后者也表明，斜压不稳定的影响与CML以下的暴风前分层无关。总体而言，该研究中的理想模拟表明，对于真实海洋中的典型情况，斜压不稳定可能占CML热含量全部恢复的约50％，而在特定条件下，贡献可能会大大减小。这些估计值限制了初始混合事件之后水柱的最大净升温，因此对于估计TC对海洋热预算的长期影响具有重要意义。

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  • 来源
    《Journal of Physical Oceanography》 |2012年第9期|p.1377-1401|共25页
  • 作者

    Wei Mei; Claudia Pasquero;

  • 作者单位

    Department of Earth System Science, University of California, Irvine, Irvine, California;

    Department of Earth System Science, University of California, Irvine, Irvine, California, and Dipartimento di Scienze Geologiche e Geotecnologiche, Universita di Milano-Bicocca, Milano, Italy;

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