SST and Dynamic Height (0/1000 m) from the NMC/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. Anomalous model SST averaged for March through May of 1995.

Figure 1 shows anomalous SST from the model averaged for March through May of 1995. The SST climatology is due to Reynolds (personal communication) and is for the period 1982-92. Throughout most of the basin SST values remain within 1 C of normal. Cool areas which were present during the September through November period (ACCP Notes, 4/95) have shrunk or disappeared. The cool anomaly of -1 C south of the Gulf Stream and west of 50W, which appeared in SON, has persisted but is less extensive. The cool region north of 40oN and east of 40W, which had persisted since JJA, 1994, has retreated north of 50N. There are now several warm patches with anomalies in excess of 1 C. The warm anomalies in the eastern North Atlantic and the western South Atlantic appear to have developed in place, while the anomalies in excess of 2 C, along the Gulf Stream and along the African Coast, appear to be spreading downstream with the prevailing circulation. The equatorial region remains mostly warm, although there is pe rhaps some suggestion of cooling near the eastern and western boundaries. Given the abundance of satellite SST data, the assimilation will strongly influence the model SST, masking inadequacies in the surface heat flux and model dynamics. The patterns s hown here strongly resemble Reynold's SST analyses (see e.g., Climate Diagnostics Bulletin) which are based on the same data.

Figure 2. a) Model dynamic height averaged for March through May of 1995. b) Anomalous model dynamic height averaged for March through May of 1995 and for 10 deg. x 10 deg. boxes.

Figure 2 shows dynamic height at the surface relative to 1000 m averaged for March through May of 1995. 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 mont h SON period there has been a seasonal decrease in dynamic height in the North Atlantic and a corresponding increase in the South Atlantic. However, the anomalous dynamic height is high throughout the entire Atlantic. A comparison of model output with TO PEX 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 mod el dynamic height, then the MAM anomalies, for the most part, do not exceed the uncertainty in the model with the exception of some high values in the tropics.

David W. Behringer
NOAA/NWS
Washington, D.C. 20748

REFERENCES

Behringer, D.W. (1994). Sea Surface Height Variations in the Atlantic Ocean: A Comparison of TOPEX Altimeter 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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