The atmospheric dynamics of intraseasonal length-of-day fluctuations during the austral winter

The atmospheric dynamical processes associated with intraseasonal length-of-day (LOD) variability during the austral winter are examined with National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis and outgoing longwave radiation (OLR) data. The method adopted is to regress the relevant fields against the LOD tendency. All quantities in this study are bandpassed through a 30-70-day filter. The findings from an analysis of the OLR and 200-mb eddy streamfunction fields are consistent with the idea that large intraseasonal LOD fluctuations coincide with an active Madden-Julian oscillation (MJO). Further analysis suggests that the eddy response to the MJO heating drives both an anomalous meridional circulation that excites the anomalous global friction torque, and an eddy field that has the appropriate location relative to the topography for generating the anomalous global mountain torque. These results were obtained by calculating regressions of the anomalous eddy angular momentum flux convergence, mass streamfunction, surface stress, and surface pressure fields, and each term in the lowest sigma level relative angular momentum budget. The anomalous global friction and mountain torques are found to be of similar magnitude, with the former leading the latter by eight days. The largest contribution toward the anomalous global friction (mountain) torque comes from Australia and the surrounding ocean (the Andes).