次重力波对宽刈幅高度计海表面高度观测的影响

Impact of infragravity waves on sea surface elevation observed by wide-swath altimeter

次重力波(Infragravity Wave,IGW)是一种频率较低(0.05～0.005 Hz),波长较长(约10 km)的表面重力波。由IGW引起的海表面高度变化会被宽刈幅干涉高度计SWOT(Surface Water and Ocean Topography,SWOT)卫星观测到,因此在使用SWOT观测的海表面高度来反演中尺度、次中尺度大洋环流时,IGW是一种重要的误差来源。根据数值模型模拟的全球IGW时空分布特征,本文以IGW最为活跃的东北太平洋和欧洲西北陆架附近大西洋为研究海域,估算了上述海域由IGW所引起的海表面高度变化,并将计算结果与SWOT Simulator模拟的轨道噪声(±5 cm)比较,首次定量地估算了IGW在SWOT观测海表面高度时的干扰程度。研究表明,IGW所引起的厘米量级的海表面高度变化在SWOT卫星观测海表面流场时是一种重要的,不可忽略的误差来源。在大西洋欧洲西北陆架海域,冬季IGW对海表面高度的贡献可达到SWOT卫星噪声要求水平的25%;然而,对于大陆架狭窄的美国西岸太平洋而言,由岸线产生的IGW将迅速传入深海海域,在广阔的范围内产生显著的"噪声"影响,在SWOT反演海表面流场时由IGW引起的误差将达到SWOT卫星噪声要求水平的15%。

Infragravity waves (IGWs) are surface gravity waves with low frequency (0.005-0.05 Hz) and long wavelength (about 10 km).The sea surface elevation caused by IGWs can be captured by the future wide-swath altimeter Surface Water and Ocean Topography (SWOT).Thus,IGWs will be an important source of error,when using the observed sea surface elevation from SWOT to retrieve meso- and submeso-scale ocean circulation.Based on the spatial and temporal distribution of the global IGWs,the sea surface elevation caused by IGWs is estimated in the northeastern Pacific and northwestern Europe shelf with high IGW energy.Compared with the orbit noise simulated by SWOT Simulator (±5 cm),the IGW “noise” is quantitatively analyzed for the first time.We find that the sea surface elevation of the order of 1 cm contributed by IGWs is an important source of error that can not be ignored in the surface elevation measurements of SWOT.On the northwestern Europe continental shelf,the contribution of winter IGWs to sea surface elevation has reached 25% of the noise level requirement of SWOT.While,for the US west coast where the continental shelf is narrow,IGWs are generated along shorelines and quickly spread into the deep ocean,causing significant noise effects on a broader area,about 15% of the noise level requirement of SWOT.