A Global Ocean Reanalysis Product in the China Ocean Reanalysis(CORA) Project

The first version of a global ocean reanalysis over multiple decades （1979-2008） has been completed by the National Marine Data and Information Service within the China Ocean Reanalysis （CORA） project. The global ocean model employed is based upon the ocean general circulation model of the Massachusetts Institute of Technology. A sequential data assimilation scheme within the framework of 3D variational （3DVar） analysis, called multi-grid 3DVar, is implemented in 3D space for retrieving multiple-scale observational information. Assimilated oceanic observations include sea level anomalies （SLAs） from multi-altimeters, sea surface temperatures （SSTs） from remote sensing satellites, and in-situ temperature/salinity profiles. Evaluation showed that compared to the model simulation, the annual mean heat content of the global reanalysis is significantly approaching that of World Ocean Atlas 2009 （WOA09） data. The quality of the global temperature climatology was found to be comparable with the product of Simple Ocean Data Assimilation （SODA）, and the major ENSO events were reconstructed. The global and Atlantic meridional overturning circulations showed some similarity as SODA, although significant differences were found to exist. The analysis of temperature and salinity in the current version has relatively larger errors at high latitudes and improvements are ongoing in an updated version. CORA was found to provide a simulation of the subsurface current in the equatorial Pacific with a correlation coefficient beyond about 0.6 compared with the Tropical Atmosphere Ocean （TAO） mooring data. The mean difference of SLAs between altimetry data and CORA was less than 0.1 m in most years.

Assessment of intraseasonal variabilities in China Ocean Reanalysis(CORA)

A regional reanalysis product-China Ocean Reanalysis（CORA）-has been developed for the China＇s seas and the adjacent areas. In this study, the intraseasonal variabilities（ISVs） in CORA are assessed by comparing with observations and two other reanalysis products（ECCO2 and SODA）. CORA shows a better performance in capturing the intraseasonal sea surface temperatures（SSTs） and the intraseasonal sea surface heights（SSHs） than ECCO2 and SODA do, probably due to its high resolution, stronger response to the intraseasonal forcing in the atmosphere（especially the Madden-Julian Oscillation）, and more available regional data for assimilation. But at the subsurface, the ISVs in CORA are likely to be weaker than reality, which is probably attributed to rare observational data for assimilation and weak diapycnal eddy diffusivity in the CORA model. According to the comparison results, CORA is a good choice for the study related to variabilities at the surface, but cares have to be taken for the study focusing on the subsurface processes.

Evaluation of the China Ocean Reanalysis (CORA) in the South China Sea

The daily regional reanalysis product of the China Ocean Reanalysis(CORA)product was released in website in 2018.Using in situ observational data including Argo profiling floats,drifters,and cruise data,the performance of CORA in the South China Sea in terms of temperature,salinity,current and mixed layer depths is evaluated based on timescale(seasonal and interannual)and spatial distribution characteristics.The CORA temperature,salinity,and mixed layer depth show certain seasonal and interannual variations.In 50-400 m depth in the SCS,the CORA temperature is colder in winter and warmer in summer and autumn.In 0-150 m in the SCS,the CORA salinity is higher in most time of the year.However,in the second half of the year,the salinity is slightly weaker in 100-150 m depth.In most years,the CORA mixed layer depths tend to be shallower,and in season,shallower in winter and deeper in summer.In spatial distribution,the closer the area is to the coast,the greater the CORA errors would be.The CORA temperature is colder in the western side and warmer in the eastern side,resulting in a weaker SCS western boundary current(SCSwbc).In most areas,the CORA mixed layer depths are shallower.In the area close to the coast,the CORA mixed layer depths change rapidly,and the deviations in the mixed layer depths are larger.In the central SCS,the CORA mixed layer depths change slowly,and the deviations in the mixed layer depths are also small.

China Ocean ReAnalysis(CORA)version 1.0 products and validation for 2009-18

China Ocean ReAnalysis(CORA) version 1.0 products for the period 2009-18 have been developed and validated.The model configuration and assimilation algorithm have both been updated compared to those of the 51-year(1958-2008) products.The assimilated observations include temperature and salinity field data,satellite remote sensing sea surface temperature,and merged sea surface height(SSH) anomaly data.The validation includes the following three aspects:(1) Temperature,salinity,and SSH anomaly root-mean-square errors(RMSEs) are computed as a primary evaluation of the reanalysis quality.The 0-2000 m domain-averaged RMSEs of temperature and salinity are 0.61℃ and 0.08 psu,respectively.The SSH anomaly RMSE is less than 0.2 m in most regions.(2) The 35°N temperature section is used to evaluate the ability to reproduce the thermocline,mixing layer,and Yellow Sea cold water mass.In summer,the thermocline is reinforced,with the gradient changing from 3℃ in May to 10℃ in August.The mixing-layer depth reproduced by CORA is consistent with that computed from the observed climatology.The Yellow Sea cold water mass forms at a depth of 50 m.(3) The reanalysis current is examined against the tracks of some drifting buoys.The results show that the reanalysis current can capture the mesoscale eddies near the Kuroshio,which are similar to those described by the drifting buoys.Overall,the 2009-18 CORA reanalysis products are capable of reproducing major oceanic phenomena and processes in the coastal waters of China and adjacent seas.

A New Version of Regional Ocean Reanalysis for Coastal Waters of China and Adjacent Seas

A new regional ocean reanalysis over multiple decades （1958 2008） for the coastal waters of China and adjacent seas has been completed by the National Marine Data and Information Service （NMD[S） under the CORA （China Ocean ReAnalysis） project. Evaluations were performed on three aspects： （1） the improvement of general reanalysis quality; （2） eddy structures; and （3） decadal variability of sea surface height anomalies （SSHAs）. Results showed that the quality of the new reanalysis has been enhanced beyond ~40% （39% for temperature, 44% for salinity） in terms of the reduction of root mean squared errors （RMSEs） for which the reanalysis values were compared to observed values in the observational space. Compared to the trial version released to public in 2009, the new reanalysis is able to reproduce more detailed eddy structures as seen in satellite and in situ observations. EOF analysis of the reanalysis SSHAs showed that the new reanalysis reconstructs the leading modes of SSHAs much better than the old version. These evaluations suggest that the new CORA regional reanalysis represents a much more useful dataset for the community of the coastal waters of China and adjacent seas.

veKdv方程的若干系数在南海北部的地理分布特征及其季节变化分析

应用中国近海及邻近海域海洋再分析资料(简称CORA)研究南海北部第一模态内波场运动学参数的地理分布特征及其季节变化。首先分析了Brunt-Visl频率的统计特征;其次,基于弱非线性变系数扩展Kortewed-de Vries(veKdv)方程模型,计算了它的输入系数,即线性长波相速度,平方和立方非线性系数和频散系数,这些参数可用于定性评估内孤立波传播可能的极性,内孤立波的形态,幅度限制以及传播速度等。分析结果表明,南海北部季节性密度跃层从2月开始出现,最大浮力频率约在20m。它在6—7月达到最强,自8月开始减弱,在10月消退。另一密度跃层出现在8—11月,最大浮力频率约在80m,冬季大致在120m。季节性密度跃层在4—9月十分明显,而8—10月双跃层现象显著,冬季仅出现较弱的第二密度跃层。在1—3月和10—12月海盆深水区最大Brunt-Visl频率值要大于陆架浅水区;而在5—9月情况则相反。Brunt-Visl频率最大值所在深度随季节变化显著,冬季最深,6—7月则最浅。计算的线性内波相速度、频散系数和幅度放大因子的空间特征主要取决于地形变化;平方(立方)非线性系数与地形关系较小,随季节变化明显,它们主要取决于局地海洋环境特征。通过分析veKdv方程的系数特征,解释了为何在夏季南海北部最容易观测到大振幅内孤立波和在吕宋海峡以东海域难以观测到孤立波的原因。