SST and Dynamic Height (0/1000m) from the NCEP/CMB Ocean Data Assimilation System

David W. Behringer
NOAA/NCEP
Washington, D.C. 20233

The Climate Modeling Branch (CMB) of the National Centers for Environmental Prediction (NCEP) routinely produces weekly ocean analyses using a data assimilation system based on an oceanic general circulation model. At present the system assimilates only temperature data, primarily satellite estimates of SST and vertical profiles from XBTs. Further details of the system can be found in the December 1994 issue of ACCP Notes and in the literature (Ji et. al., 1994 and Derber and Rosati, 1989). A comparison of the Atlantic system with TOPEX altimetry data has also been published (Behringer, 1994).

Figure 1. Anomalous model SST averaged for December 1966 through January of 1997. Magnitudes greater than 1° C are shaded.

Figure 1 shows anomalous SST from the model averaged for December 1996 through January of 1997. The SST climatology is due to Reynolds (personal communication) and is for the period 1982-92. Since the June 1966 through August 1966 period (ACCP Notes, Vol. III, No. 3) the western South Atlantic has warmed while the North Atlantic has generally cooled. In the southern hemisphere the largest changes have occurred south of 20° S and west of 10° W, where there has been anomalous warming of a degree or more. In the north the strongest anomalous cooling has occurred between 25° N and 50° N. The net result is a 1° - 2° C warm anomaly in the southwestern South Atlantic. In the North Atlantic, negative anomalies of 1° C, or more, extend along the model Gulf Stream. In the figure, anomalies with magnitudes greater than 1° C are shaded. Given the abundance of satellite SST data, the assimilation of these data will strongly influence the model SST, masking inadequacies in the surface heat flux and model dynamics. The patterns shown here strongly resemble Reynolds' SST analyses (see e.g., Climate Diagnostics Bulletin) which are based on the same data.

Figure 2 shows dynamic height and anomalous dynamic height at the surface relative to 1000m averaged for December 1996 through January of 1997. The anomalous dynamic height is relative to a short climatology based on the three years of data available from this analysis system. Since the 3 month JJA period there has been a systematic increase in the anomalous dynamic height in the tropics and in the northern hemisphere. Large anomalies of about 10 dyn. cm occur in the center and along the southern edge of the North Atlantic subtropical gyre. A comparison of model output with TOPEX sea surface height anomalies averaged over 10° x 10° squares (Behringer, 1994) shows that rms differences between the model and TOPEX are about 10 cm along western boundaries and about 3 cm in the tropics. If these rms differences are regarded as rough estimates of the error in model dynamic height, then only the largest DJF anomalies in the North Atlantic subtropical gyre exceed the uncertainty in the model.

Figure 2a. Model dynamic height averaged for December 1996 through January of 1997.

Figure 2b. Anomalous model dynamic height averaged for December 1996 through January of 1997 and for 10° x 10° boxes.

REFERENCES