The balance of global angular momentum in a long-term atmospheric data set

The balance of global atmospheric angular momentum is examined in a long time series of "reanalysis" data generated at the National Centers for Environmental Prediction (NCEP). A systematic negative bias of about -10 Hadleys (10(18) kg m(2) s(-2)) is obtained in the total torque in all seasons. The sum of the frictional (T-F) and mountain (T-M) torques contributes about half of this negative bias in the northern summer and autumn. The torque due to the parameterized gravity wave drag, T-G, contributes the other half of the negative bias in summer but contributes the whole of it in winter. In the annual mean the projected angular momentum imbalance due to T-G exceeds that due to T-M + T-F. Consistent with the budget analysis of the assimilated data, losses of angular momentum are also found in an extensive set of medium-range forecasts made with the NCEP reanalysis model. The average rates of loss of relative angular momentum in these forecasts, -7 Hadleys in January and -6 in July, are comparable to the bias of the total torque in the angular momentum budget. Further investigations are suggested to determine the model error, especially that associated with the parameterized gravity wave drag, related to the budget imbalance found in this study.