The poleward propagation of atmospheric angular momentum with a 30-60-day time scale

The existence of the intraseasonal variation of relative atmospheric angular momentum and the poleward propagation of this physical quantity by the intraseasonal mode of atmospheric circulation have been amply demonstrated in previous studies. However, the temporal relationship between the intraseasonal variation and poleward propagation is not well understood and the possible mechanism causing the poleward propagation is unknown. Analyzing the 15-year (1979-1993) upper-air data of the National Meteorological Center (currently the National Centers for Environmental Prediction), we explored these two intriguing issues related to atmospheric angular momentum via case-study analyses. The major findings are as follows: 1. The maximum (minimum) 30-60-day globally integrated atmospheric angular momentum occurs when the positive (negative) angular momentum anomalies of a 30-60-day cycle appear in the tropics-subtropics and those of the previous 30-60-day cycle propagate to the northern and southern midlatitudes. 2. The eastward propagation of the 30-60-day global divergent circulation induces a 30-60-day flip-flop oscillation in the meridional circulation which in turn results in a similar alternation in the angular momentum tendency caused by the Coriolis torque. It was inferred from the zonal-mean angular momentum budget analysis that the 30-60-day variation of atmospheric angular momentum and its poleward propagation are results of the 30-60-day variation of the aforementioned momentum tendency.