Remarks on Northern Hemisphere Forecast Error Sensitivity from 1996 to 2000

Abstract

The sensitivity of 2-day Northern Hemisphere extratropical forecast errors to changes in initial conditions, computed daily over a 4-yr period, is documented. The sensitivity is computed using the (dry) adjoint of the Navy Operational Global Atmospheric Prediction System. This diagnostic enables an assessment of where initial errors have had the largest impact on forecast errors and allows for an examination of interannual variations in predictability. Both the forecast error and sensitivity exhibit a large seasonal cycle, with distinct maxima in winter. The monthly mean sensitivity maxima are correlated with regions of baroclinic instability and occur upstream from the forecast error maxima. Interannual variability is also apparent, and results show that the El Niño winter of 1997/98 was anomalously predictable. In contrast, the recent La Niña winters have been relatively unpredictable, especially during January 2000. These sensitivity calculations highlight the significant impact of middle?lower-tropospheric initial condition errors on forecast errors, particularly over the North Pacific.