SST and Dynamic Height (0/1000m) from the NCEP/CMB Data Assimilation System
D. W. Behringer
NOAA/NCEP
Washington, D. C.
Figure 1 shows anomalous SST from the model averaged for December 1997 though February 1998. The SST climatology is due to Reynolds (personal communication) and is for the period 1982-92. Since the June through August 1997 period (ACCP Notes, Vol. IV, No. 3) the pattern of generally warm anomalies north of 20°N and cool anomalies in the southern hemisphere has changed sign. Now in DJF, the anomalies north of 20°N are almost exclusively negative. In the southern hemisphere, positive anomalies larger than 1°C cover much of the area, while anomalies greater than 2°C can be found near the coasts of Africa and South America. 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 1. Anomalous model SST averaged for December 1997 through February 1998. Magnitudes greater than 1° are shaded.
Figure 2 shows dynamic height and anomalous dynamic height at the surface relative to 1000m averaged for December 1997 though February 1998. The anomalous dynamic height is relative to a short climatology based on three years, 1993-5, of data from this analysis system. Large anomalies of about 10 dyn.cm. occur along the southern edge of the North Atlantic subtropical gyre. Along the equator anomalies in the west approach 4 dyn.cm, while in the east they are as large as 8-9 dyn.cm. A comparison of model output with TOPEX sea surface height anomalies averaged over 10O x 10O 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 the largest DJF anomalies in the North Atlantic subtropical gyre and in the tropics appear to exceed the uncertainty in the model.
Figure 2. (a) Model dynamic height averaged for December 1997 through February 1998.
(b) Anomalous model dynamic height averaged for December 1997 through February 1998 and for 10 ° x 10 ° boxes.