基于区域滤波的GOCE稳态海面动力地形和地转流

GOCE mean dynamic topography and its associated geostrophic current based on the regional filtering method

基于频域法,利用最新的GOCE卫星重力场模型和卫星测高数据计算了稳态海面动力地形.结合海洋表层漂流浮标的观测结果,对稳态海面动力地形进行了最优空间滤波尺度分析,给出了区域、纬度带和全球稳态海面动力地形的最优空间滤波尺度因子.在此基础上,给出了全球和区域地转流.结果表明:在中高纬度和全球区域,可以分别获得空间尺度优于102km和127km的稳态海面动力地形信息.与海洋表层漂流浮标对比可知,在强流区域,采用稳态海面动力地形得到的地转流速可以解释观测浮标流速的70%;在中高纬度区域,由GOCE重力场得到的地转流略优于对应的GRACE结果;在近赤道区域,由GOCE重力场得到的地转流精度略低于对应的GRACE结果;在北大西洋和阿古拉斯强流区域,由GOCE得到的地转流场明显优于对应的GRACE结果,其精度分别提高了16%和24%.

The Gravity field and steady-state ocean circulation explorer（GOCE）satellite mission which measures the Earth′s gravity field with an unprecedented accuracy at short spatial scales promises to significantly advance the research of geodetic ocean mean dynamic topography（MDT）.To fully exploit the GOCE′s advantages and precisely determine the MDT and its associated geostrophic currents globally and regionally,we must quantify the spatial resolution of GOCE-derived MDT and the geostrophic currents′retrieving ability of GOCE.Global MDT is firstly retrieved from the GOCE earth gravity field model（GO-CONS-GCF-2-TIM5）and the altimetry sea surface height model（CNESCLS2011MSS）by the spectral-wise approach which can effectively suppress the omission errors.Then the Gaussian filter method is used to suppress the noise of raw MDT results.To acquire the optimal spatial filter radius of the Gaussian filter,we calculate the RMS difference between the buoy-derived geostrophic currentsand those calculated from the geodetic MDT with different filter radii.Those filter radii which make the above RMS difference acquired to be minimum are the best choice of the Gaussian filter radius.Based on this filter radius determining strategy,the optimal filter radii of MDT are determined in regional,zonal and global areas.The above optimal MDTs are then used to determine the corresponding geostrophic current fields.Finally,the characteristics of GOCEderived geostrophic currents are studied carefully by three statistics factors,i.e.the RMS differences,correlation coefficients,and the speed proportion coefficients,between geodetic and buoy-derived geostrophic currents data.（1）The optimal spatial filter radii of GOCE MDT are 102 km,131km,154 km and 127 km in the regions of south and north latitudes greater than 40°,between 20°and 40°,less than 20°and the global range,respectively,which are 24 km,27km,21 km and 27 km better than that of GRACE.（2）The comparison between geostrophic currents acquired from MDT and buoy data shows that 1）in strong current regions,the speed（amplitude of velocity）of buoy derived geostrophic current can be explained 70% by geostrophic current acquired from MDT;and 2）the geostrophic current speeds derived from GOCE and altimeter data are closer to the buoy data compared with that of GRACE and altimeter data.（3）The correlation coefficients of geostrophic speed derived from two different geodetic MDTs（GOCE and GRACE）and buoy-derived current speed have obvious spatial characteristics.The correlation coefficients based on GOCE results are higher than that of GRACE in the Antarctic circumpolar current region,north Atlantic region and Agulhas region,but vice versa in the equator region.（4）The RMS differences of geostrophic current velocity calculated from GOCE MDT and buoy-derived current velocity are generally smaller than that of GRACE in strong geostrophic currents regions（except the equator region）.For example,the above RMS differences in GOCE results are 16% and 24%smaller than that of GRACE results in the North Atlantic and Agulhas region,respectively.Firstly,the regional optimized filter radii of MDT are somewhat distinct in different regions.The filter radius is shorter in strong current regions than the low current speed regions at the same latitude,which decreases with the increasing latitude on average.Secondly,the GOCE geoid has good signal to noise ratio at short wavelength than that of GRACE geoid,which enables the use of shorter optimal filter radius of corresponding GOCE based MDT than that of the earlier GRACE based MDT.Furthermore,shorter optimal filter radius of GOCE based MDT ensures the GOCE based MDT and its associated geostrophic currents retain more information on small spatial scales.Lastly,the GOCE-based geostrophic currents are better than that of GRACEbased results in middle and high latitude regions.