Numerical simulation on Bay of Bengal's response to cyclones using the Princeton ocean model

ABSTRACT This study used the Princeton ocean model (POM) which includes second-order turbulent closure scheme to investigate the fluid dynamics of the Bay of Bengal (BoB) in the upper ocean's response to a cyclone. The model uses an orthogonal curvilinear grid and 26 sigma levels in conformity with realistic bottom topography. The model is forced with wind and heat plus salinity fluxes as surface forcing to simulate the BoB's response during a cyclone. In order to provide the realistic cyclonic vortex the model as input, the synthetic cyclonic vortex is generated and superimposed on the QSCAT/NCEP blended ocean wind fields. Analyses of results show significant sea surface temperature (SST) cooling on both sides of the storm track. This cooling could be attributed to the strong cyclonic winds, surface divergence and upwelling. However, less commonly observed features such as a leftward bias in SST cooling due to the relatively slower motion of TC and southward moving coastal boundary currents are also reported in this study. Model SST is compared with the observed Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) filled up SST for the evaluation of the model's performance. Moreover, not only sea surface cooling but subsurface warming due to intense downwelling and coastal jet parallel to the coast were also observed in the model's simulation. The mixed layer depth (MLD) variation is revealed by the model. MLD deepening due to the convergence of near surface flow at the periphery of the cyclone is observed; however, beneath the cyclone centre, in the direction of the track the upsloping of isotherms due to the surface divergence and upwelling causes the shoaling of the MLD. Modeled surface currents are compared with 5-day interval OSCAR (Ocean Surface Current Analyses - Real time) surface currents, which are not very coherent, though some of the important features like higher values of boundary layer currents are captured. However, strong near surface, asymmetrical responses such as divergent currents in the open oceanic region are reflected by the model but when the cyclone approaches the coast the current patterns do not show the right bias due to interaction with the coast.

摘要
本研究采用包含二阶湍流闭合方案的普林斯顿海洋模型（Princeton Ocean Model, POM），探究孟加拉湾（Bay of Bengal, BoB）上层海洋对气旋的流体动力学响应。该模型采用正交曲线网格与26层西格玛垂直坐标，贴合真实海底地形设置；以风通量、热通量及盐通量作为表面强迫场，模拟孟加拉湾在气旋过境期间的响应。为向模型提供真实可信的气旋涡旋输入，本研究生成合成气旋涡旋，并将其叠加至QSCAT/NCEP混合海洋风场中。

结果分析显示，风暴路径两侧均出现显著的海表温度（Sea Surface Temperature, SST）降温现象，该降温可归因于强劲的气旋性风力、表层辐散与上升流作用。此外，本研究还观测到较为罕见的特征：受热带气旋（Tropical Cyclone, TC）移动速度相对较慢以及南向沿岸边界流的影响，海表温度降温存在左侧偏差。

为评估模型性能，本研究将模拟得到的海表温度与经热带降雨测量任务（Tropical Rainfall Measuring Mission, TRMM）微波成像仪（Microwave Imager, TMI）反演补全的观测海表温度进行对比。除海表降温外，模型模拟结果还揭示了由强下沉流与沿岸平行急流引发的次表层增温特征。

研究揭示了混合层深度（Mixed Layer Depth, MLD）的变化规律：在气旋外围，近表层流辐合导致混合层加深；而在气旋中心下方沿移动路径方向，表层辐散与上升流引发等温线抬升，使得混合层深度变浅。

本研究将模拟表层流与每5天更新一次的实时海洋表层流分析（Ocean Surface Current Analyses - Real time, OSCAR）观测流场进行对比，二者整体一致性欠佳，但模型成功捕捉到了边界层流流速偏高等部分关键特征。此外，模型还反映出远洋区域存在近表层非对称响应（如辐散流）；不过当气旋逼近海岸时，模拟流场未呈现出因海岸相互作用导致的右侧偏差特征。