Links between the mountain torque and the arctic oscillation in the LMDz general circulation model

Recent papers suggest that in the NCEP reanalysis data, there is a dynamical link between the mountain torque and the Arctic Oscillation (AO) at periodicities near and below 30--days. This link essentially occurs because the AO is associated with a redistribution of mass from the polar regions to the midlatitudes, hence giving a substantial contribution to the mass term of the Atmospheric Angular Momentum (AAM). These results are confirmed here by using a 30--year simulation done with the LMDz General Circulation Model. In particular, we verify that in the model also, the changes in mass AAM occurring during intraseasonal variations of the AO are in good part driven by the mountain torque. In this respect, the model LMDz has the great advantage to close the AAM budget near exactly, which is not the case of the NCEP reanalysis data.

As the Antarctic Oscillation (AAO) is also associated with a redistribution of mass from the polar regions to the midlatitudes, its contribution to the AAM budget is also presented. As there are much less mountains in the southern hemisphere, we show that in the model as well as in the reanalysis, the changes in mass AAM during intraseasonal variations of the AAO are in good part equilibrated by changes of opposite sign in wind AAM.

The main interest of these results is that the mountain torque drives the changes in AAM, so it can sometimes participate actively to changes of the AO. It has a predictive interest which is significant but small, around 10%-15% for periodicities near and below one month while a good fraction of the AO variability occurs at longer timescales.