Toward a global ocean data assimilation system based on ensemble optimum interpolation: altimetry data assimilation experiment |
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| Authors: | Weiwei Fu Jiang Zhu Changxiang Yan Hailong Liu |
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| Institution: | (1) Nansen-Zhu International Research Center (NZC), Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences, 100029 Beijing, China;(2) International Centre for Climate and Environmental Sciences (ICCES), Institute of Atmospheric Physics, Chinese Academy of Sciences, 100029 Beijing, China;(3) State Key Laboratory of Numerical Modeling for Atmospheric Science and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, 100029 Beijing, China |
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| Abstract: | A global ocean data assimilation system based on the ensemble optimum interpolation (EnOI) has been under development as the
Chinese contribution to the Global Ocean Data Assimilation Experiment. The system uses a global ocean general circulation
model, which is eddy permitting, developed by the Institute of Atmospheric Physics of the Chinese Academy of Sciences. In
this paper, the implementation of the system is described in detail. We describe the sampling strategy to generate the stationary
ensembles for EnOI. In addition, technical methods are introduced to deal with the requirement of massive memory space to
hold the stationary ensembles of the global ocean. The system can assimilate observations such as satellite altimetry, sea
surface temperature (SST), in situ temperature and salinity from Argo, XBT, Tropical Atmosphere Ocean (TAO), and other sources
in a straightforward way. As a first step, an assimilation experiment from 1997 to 2001 is carried out by assimilating the
sea level anomaly (SLA) data from TOPEX/Poseidon. We evaluate the performance of the system by comparing the results with
various types of observations. We find that SLA assimilation shows very positive impact on the modeled fields. The SST and
sea surface height fields are clearly improved in terms of both the standard deviation and the root mean square difference.
In addition, the assimilation produces some improvements in regions where mesoscale processes cannot be resolved with the
horizontal resolution of this model. Comparisons with TAO profiles in the Pacific show that the temperature and salinity fields
have been improved to varying degrees in the upper ocean. The biases with respect to the independent TAO profiles are reduced
with a maximum magnitude of about 0.25°C and 0.1 psu for the time-averaged temperature and salinity. The improvements on temperature
and salinity also lead to positive impact on the subsurface currents. The equatorial under current is enhanced in the Pacific
although it is still underestimated after the assimilation. |
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| Keywords: | Data assimilation EnOI GODAE Satellite altimetric data Global ocean |
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