Abstract
Using a new global ocean reanalysis of the second generation Global Ocean Data Assimilation System of the Beijing Climate Center (BCC_GODAS2.0) spanning the period 1990–2009, we firstly quantify the accuracy of BCC_GODAS2.0 in representing the temperature and salinity by comparing with OISST and SODA data. The results show that the assimilation system may effectively improve the estimations of temperature and salinity by assimilating all kinds of observations, especially in the equatorial eastern Pacific. Moreover, the root mean square errors of monthly temperature and salinity are respectively reduced by 0.53°C and 0.28 psu, compared with the model control simulation results. Then, the applicability of this ocean reanalysis for sea surface temperature (SST) anomaly variability in the tropical Pacific is evaluated with the observational HadISST data. The NINO3 index of the new reanalysis shows a good agreement with that of HadISST, with a correlation of 93.6%. Variations in SST from BCC_GODAS2.0 are similar to those obtained from HadISST data along the equator, showing the major large zonal-scale features such as the strong magnitude of seasonal cycle. The amplitude of SST anomaly standard deviation in the equatorial eastern Pacific is also closer to observations (HadISST) than NCEP GODAS does. Besides, the first two leading empirical orthogonal function (EOF) modes of the monthly SST anomalies over the tropical Pacific region are explored. The EOF1 pattern of BCC_GODAS2.0 captures a traditional El Nino pattern, which improves magnitudes of the positive SST anomaly in the cold tongue of the eastern Pacific. The EOF2 pattern exhibits a El Nino Modoki pattern. Comparatively, the EOF2 pattern of BCC_GODAS2.0 extends more strongly toward the subtropics. It also overcomes the problem that negative loadings are confined in the narrow equatorial eastern Pacific. Consequently, the magnitude and spatial distribution of the leading EOF patterns of BCC_GODAS2.0 are well consistent with those of HadISST.
Highlights
Using a new global ocean reanalysis of the second generation Global Ocean Data Assimilation System of the Beijing Climate Center (BCC_GODAS2.0) spanning the period 1990–2009, we firstly quantify the accuracy of BCC_GODAS2.0 in representing the temperature and salinity by comparing with OISST and SODA data
In order to evaluate the overall performance of the data assimilation system (BCC_GODAS2.0), we present the annual mean bias for temperature and salinity at the surface (Figure 1)
The problem is overcome to a great extent in the assimilation result, in which the distribution of annual mean sea surface temperature (SST) is in good agreement with that of the OISST, especially in the equatorial eastern Pacific
Summary
The ocean model used for the BCC_GODAS2.0 is the Modular Ocean Model (MOM.v4, [19]), which is developed and supported by researchers at NOAA’s Geophysical Fluid Dynamics Laboratory. The model uses a global tripolar grid. This part of the grid has a single pole over Antarctica. In the Arctic region, the grid places a bipolar region, where the two poles are situated over land. It removes the spherical coordinate singularity present at the geographical North Pole. The topography is a montage of that developed by satellite data in the region of 72°S to 72°N (http://topex.ucsd.edu/marine_topo/mar_topo.html) [20], the NOAA (1988) 5-minute global topography ETOPO5 (http:// www.ngdc.noaa.gov/mgg/global/etopo.HTML) [21] and the International Bathymetric Chart of the Arctic Ocean (IBCAO, http://www.ngdc.noaa.gov/mgg/bathymetry/arctic/ arctic.html) [22]. The model was forced by monthly mean wind at 10-m level and air temperature at 2-m level from the NCEP–DOE reanalysis 2 [23]
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