Based on the statistics of all surface drifting buoys of 1978-2011 and Lagrangian tracers simulated from high quality ocean reanalysis currents,the impact times and strength of Fukushima nuclear pollution to the east coast of China and the west coast of America have been estimated.Under the circumstances of the radioactive pollutants drifting in the ocean surface,preliminary research results show that while the tracers took about 4 years to reach the west coast of USA,there are two types of tracers to carry out Fukushima nuclear pollutants to reach the east coast of China,corresponding to 1.5-year recirculation gyre transport and 3.5-year subtropical circulation transport.The distributions of the impact strength at these time scales are given according to the variation of relative number concentration with time combined with the decaying rate of radioactive matter.For example,starting from 1% at 1.5-year,of the initial level at the originating area of Fukushima nuclear pollution,the impact strength of Cesium-137 in the South China Sea continuously increases up to 3% by 4 years,while the impact strength of Cesium-137 in the west coast of America is as high as 4% due to the role of strong Kuroshio-extension currents as a major transport mechanism of nuclear pollutants for that area.
HAN GuiJunLI WeiFU HongLiZHANG XueFengWANG XiDongWU XinRongZHANG LianXin
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.
At the interface between the lower atmosphere and sea surface,sea spray might significantly influence air-sea heat fluxes and subsequently,modulate upper ocean temperature during a typhoon passage. The effects of sea spray were introduced into the parameterization of sea surface roughness in a 1-D turbulent model,to investigate the effects of sea spray on upper ocean temperature in the Kuroshio Extension area,for the cases of two real typhoons from 2006,Yagi and Soulik. Model output was compared with data from the Kuroshio Extension Observatory(KEO),and Reynolds and AMSRE satellite remote sensing sea surface temperatures. The results indicate drag coefficients that include the spray effect are closer to observations than those without,and that sea spray can enhance the heat fluxes(especially latent heat flux) considerably during a typhoon passage. Consequently,the model results with heat fluxes enhanced by sea spray simulate better the cooling process of the SST and upper-layer temperature profiles. Additionally,results from the simulation of the passage of typhoon Soulik(that passed KEO quickly),which included the sea spray effect,were better than for the simulated passage of typhoon Yagi(that crossed KEO slowly). These promising 1-D results could provide insight into the application of sea spray in general circulation models for typhoon studies.
Upper ocean heat content is a factor critical to the intensity change of tropical cyclones(TCs). Because of the inhomogeneity of in situ observations in the North Indian Ocean,gridded temperature/salinity(T/S) profiles were derived from satellite data for 1993–2012 using a linear regression method. The satellite derived T/S dataset covered the region of 10°S–32°N,25°–100°E with daily temporal resolution,0.25°×0.25° spatial resolution,and 26 vertical layers from the sea surface to a depth of 1 000 m at standard layers. Independent Global Temperature Salinity Profile Project data were used to validate the satellite derived T/S fields. The analysis confirmed that the satellite derived temperature field represented the characteristics and vertical structure of the temperature field well. The results demonstrated that the vertically averaged root mean square error of the temperature was 0.83 in the upper 1 000 m and the corresponding correlation coefficient was 0.87,which accounted for 76% of the observed variance. After verification of the satellite derived T/S dataset,the TC heat potential(TCHP) was verified. The results show that the satellite derived values were coherent with observed TCHP data with a correlation coefficient of 0.86 and statistical significance at the 99% confidence level. The intensity change of TC Gonu during a period of rapid intensification was studied using satellite derived TCHP data. A delayed effect of the TCHP was found in relation to the intensity change of Gonu,suggesting a lag feature in the response of the inner core of the TC to the ocean.
To further explore enthalpy-based sea-ice assimilation, a one-dimensional （1D） enthalpy sea-ice model is implemented into a simple pycnocline prediction model. The 1D enthalpy sea-ice model includes the physical processes such as brine expulsion, flushing, and salt diffusion. After being coupled with the atmosphere and ocean components, the enthalpy sea-ice model can be integrated stably and serves as an important modulator of model variability. Results from a twin experiment show that the sea-ice data assimilation in the enthalpy space can produce smaller root-mean-square errors of model variables than the traditional scheme that assimilates the observations of ice concentration, especially for slow-varying states. This study provides some insights into the improvement of sea-ice data assimilation in a coupled general circulation model.
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.
Based on the statistics of surface drifter data of 1979-2011 and the simulation of nuclear pollutant particulate move- merits simulated using high quality ocean reanalysis surface current dataset, the transport pathways and impact strength of Fuku-shima nuclear pollutants in the North Pacific have been estimated. The particulates are used to increase the sampling size and en- hance the representativeness of statistical results. The trajectories of the drifters and particulates are first examined to identify typical drifting pathways. The results show that there are three types of transport paths for nuclear pollutants at the surface： 1） most pollutant particles move eastward and are carried by the Kuroshio and Kuroshio-extension currents and reach the east side of the North Pacific after about 3.2-3.9 years; 2） some particles travel with the subtropical circulation branch and reach the east coast of China after about 1.6 years according to one drifter trajectory and about 3.6 years according to particulate trajectories; 3） a little of them travel with local, small scale circulations and reach the east coast of China after about 1.3-1.8 years. Based on the par-tieulates, the impact strength of nuclear pollutants at these time scales can be estimated according to the temporal variations of relative concentration combined with the radioactive decay rate. For example, Cesium-137, carried by the strong North Pacific current, mainly accumulates in the eastern North Pacific and its impact strength is 4% of the initial level at the originating Fuku- shima area after 4 years. Due to local eddies, Cesium-137 in the western North Pacific is 1% of the initial pollutant level after 1.5 years and continuously increases to 3% after 4 years. The vertical movement of radioactive pollutants is not taken into account in the present study, and the estimation accuracy would be improved by considering three-dimensional flows.
FU HongliLI WeiZHANG XuefengHAN GuijunWANG XidongWU XinrongZHANG Lianxin
