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

The Climate Modeling Branch (CMB) of the National Meteorological Center (NMC) 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 shows anomalous SST from the model averaged for September through November of 1994. The SST climatology is due to Reynolds (personal communication) and is for the period 1982-92. Throughout most of the basin SST has returned to near normal value as compared to the earlier June through August period (ACCP Notes, 12/94). Previous cool anomalies of -1oC in the regions between 10oN and 20oN and within 10o of the equator have disappeared and the entire equator is slightly warm. The large positi ve anomaly in excess of 2oC which in JJA, occupied the area south of the Gulf Stream and west of 50oW has also disappeared, replaced by a region of cool surface water. The cool region north of 40oN and east of 40oW, which appeared in JJA, has persisted t hrough the September through November analysis. Given the abundance of satellite data, the assimilation will strongly influence the model SST, masking inadequacies in the surface heat flux and model dynamics. The patterns shown here strongly resemble Re ynolds’ SST analyses (see e.g., Climate Diagnostics Bulletin) which are based on the same data.

Figure 1. Anomalous model SST averaged for September through November 1994.

Figure 2. a) Model dynamic height averaged for September through November 1994. b) Anomalous model dynamic height averaged for September through November 1994 and for 10o x 10o boxes.

Figure 2 shows dynamic height and anomalous dynamic height at the surface relative to 1000 m averaged for September through November 1994. The anomalous dynamic height is relative to a short climatology based on the three years of data available from thi s analysis system. The largest change since the previous 3 month period is in the eastern equatorial region where the dynamic height has increased by about 10 cm. Most of this change is seasonal, but the anomalous dynamic height in the region has also i ncreased by 3-4 cm. A comparison of model output with TOPEX sea surface height anomalies (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 the anomalies in Figure 2, for the most part, do not exceed the uncertainty in the model.

David W. Behringer
NOAA/NWS
Washington, D.C.

REFERENCES

Behringer, D.W. (1994). Sea Surface height variations in the Atlantic Ocean: A comparison of TOPEX atimeter data with results from an ocean data assimilation system JGR, 99: C12, 24, 685-24,690.

Derber, J. and A. Rosati. (1989). A global oceanic assimilation system, J. Phys. Oceanography 19: 1,333-1,347.

Ji, M., A. Leetmaa and J. Derver. (1994). An Ocean Analysis System for Seasonal to Interannual Climate Studies Mon. Wea. Rev., 123: 460-481.

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