WAMSI Node 1.1.1 - Downscaled hydrodynamic models

This work describes the physical oceanography at successively smaller scales, from that of the eastern Indian Ocean, to the shelf from NW Cape to Cape Leeuwin, down to the Marmion lagoon. At the largest scale, the primary tool is the global Ocean Forecasting Australia Model (OFAM), that has coarse (>2 degree) resolution distant from Australia, but has 0.1 degree (~10 km) resolution in the Australasian region. Being global, OFAM simulates the longshore pressure gradient, largely due to the Indonesian Throughflow, that drives the Leeuwin Current southward along the shelf break of the WA coast. The model provides good qualitative representation of the Leeuwin Current and its associated eddies but, at 10 km resolution, tends to underestimate the current speeds on the shelf and cross-shore fluxes. OFAM output has been corrected with observations (in a separate CSIRO project) and archived to provide nearly 2 decades of ocean conditions. As a prelude to later biological investigations, it is used to investigate the longshore connectivity of shelf waters, demonstrating that the shelf can be classified into several sections of alternating low and high retention. At this scale, openocean swells are important, with biological particles near the surface being driven north-east under their influence. At the next level, the Rutgers Ocean Modelling System (ROMS) is nested inside OFAM, covering the southern west shelf, with resolution down to 2 km near the coast. ROMS produces similar patterns of circulation to OFAM, but the Leeuwin Current and inshore wind-driven currents are narrower and stronger. The ROMS model is used primarily to drive a model of primary productivity over the shelf. ROMS produces eddies that correspond well with those observed in sea-surface temperature images, and also shows instances of intermittent upwelling along the shelf break. Upwelling and eddy transport prove to be significant mechanisms for moving nutrients and phytoplankton on and off the shelf, respectively. At the scale of the Marmion lagoon, surface waves breaking on the reefs drive currents in the lagoon. The present study incorporated a year-long measurement program to quantify the reef and lagoon circulation. At low wave heights, the lagoonal circulation is driven primarily by longshore winds. However, when the incoming waves are higher than about 1.5 m, they break on the reef, carrying water into the lagoon, and causing currents to the north and south out of the lagoon. Modelling of reef dynamics requires very high resolution, down to tens of metres, and a model that includes both waves and currents. Two model configurations have been used in the present study: ROMS together with the wave model SWAN, and Xbeach, a littoral-zone model still under development at Delft University. Xbeach accurately represents the lagoonal dynamics.

本研究依次描述了不同尺度下的物理海洋学特征，范围从东印度洋尺度，到澳大利亚西北角至卢温角的陆架尺度，再到马尔米翁泻湖尺度。

在最大尺度上，主要研究工具为全球澳大利亚海洋预报模型（Ocean Forecasting Australia Model，OFAM）；该模型在远离澳大利亚的区域分辨率较粗（>2度），但在澳大拉西亚区域的分辨率可达0.1度（约10公里）。作为全球模型，OFAM可模拟由印度尼西亚贯穿流主导的沿岸压力梯度，该梯度驱动卢温洋流沿西澳大利亚海岸陆架坡折带向南流动。该模型能较好地定性再现卢温洋流及其伴生涡旋，但受10公里分辨率限制，往往低估陆架上的流速及跨岸通量。OFAM的输出结果已通过观测数据校正（在澳大利亚联邦科学与工业研究组织（CSIRO）的独立项目中完成），并已归档，可提供近20年的海洋状况数据。作为后续生物研究的铺垫，该模型被用于探究陆架水体的沿岸连通性，结果表明陆架可划分为多个低滞留区与高滞留区交替的区段。在此尺度下，大洋涌浪至关重要，表层附近的生物颗粒会在其作用下向东北方向迁移。

在次一级尺度上，罗格斯海洋建模系统（Rutgers Ocean Modelling System，ROMS）嵌套于OFAM中，覆盖西澳南部陆架，近岸区域分辨率可达2公里。ROMS模拟的环流模式与OFAM相似，但卢温洋流及近岸风生流更窄且更强。ROMS模型主要用于驱动陆架初级生产力模型。其模拟的涡旋与海表温度图像中观测到的涡旋高度吻合，同时也再现了陆架坡折带的间歇性上升流现象。研究证实，上升流和涡旋输运分别是营养盐输入陆架和浮游植物输出陆架的重要机制。

在马尔米翁泻湖尺度上，礁石上破碎的表面波驱动泻湖内的环流。本研究纳入了为期一年的观测计划，以量化礁石与泻湖的环流特征。在低波高条件下，泻湖环流主要由沿岸风驱动；然而，当入射波高超过约1.5米时，波浪会在礁石上破碎，将水体带入泻湖，并引发向北和向南流出泻湖的环流。

礁石动力学建模需要极高的分辨率（低至数十米），以及同时包含波浪与环流的模型。本研究采用了两种模型配置：ROMS与波浪模型SWAN（SWAN）组合，以及Xbeach——代尔夫特大学仍在开发的滨海带模型。其中，Xbeach能准确再现泻湖的动力学特征。