The report presents results of the Work package 2 (WP2) of ESA Contract No. 4000139422/22/I-NS which focuses on Aeolus effects on the vertical momentum fluxes (VMFs) in the tropics. The VMFs are affected by Aeolus through changes in the horizontal winds and indirectly through vertical motions. The vertical velocity and VMFs are crucial variables to be realistically represented by high-resolution weather and climate models. In particular, the VMFs drive the stratospheric quasi-biennial oscillation (QBO) which experienced the second disruption on the record during the Aeolus operations. The results show that the assimilation of Aeolus winds leads to a systematic, albeit small, reduction of the vertical velocity in the ECMWF system. It is related to the sys- tematic reduction of the horizontal wind divergence reported in Technical Note 1. A significant impact of Aeolus in the kinetic energy spectra of vertical motions was found for the large-scale Kelvin waves with the strongest effect during the QBO disruption in early 2020. During the QBO disruption, corrections of the background zonal wind in the lower stratosphere by Aeolus were particularly large. Changes in the analyses that increased the vertical shear of the zonal wind were aligned with changes in the VMFs. Average forecast improvements by Aeolus of a few % with respect to NoAEolus increased to about 15% improvements in day 1 forecasts in the lower stratosphere early in 2020 (see the figure). In normal QBO conditions such as summer 2020, Aeolus winds improved the transition from the easterly to westerly phase of the QBO. This shows as an alignment of wave forcing (i.e. the vertical gradient of the VMFs) with the changes in the background flow. In summer 2020, the forcing was largely due to the Kelvin wave and eastward-propagating inertia-gravity waves. When measured by differences in zonally averaged fluxes within 10°S and 10°N, the effects of Aeolus winds on the VMFs are relatively small (below 1%). However, effects at individual scales can be large depending on the background flow and wave motions. In early 2020, the period of the QBO disruption, the assimilation of Aeolus winds in the ECMWF system resulted in up to about 20% change in stratospheric VMFs with respect to NoAeolus.

ESA Contract No. 4000139422/22/I-NS, 2024