Global Observing System and Tropical-Extratropical Coupling

It is well established that tropical circulation influences extratropical processes on timescales from days to weeks and beyond, but the underlying mechanisms and their sensitivity to tropical observations and data assimilation remain poorly understood. Recent studies highlight the initialization of synoptic-scale disturbances in the tropics as a key factor for improving medium-range extratropical forecasts. Poleward-propagating signals interact both with the background flow and with large-scale Rossby waves propagating equatorward. Our work highlights  the role of global wind observations in disentangling tropical-extratropical coupling.   The significant impact of Aeolus wind profiles on analyses and forecasts in the tropics underscores the importance of dynamical processes in shaping tropical-extratropical coupling and highlights the need for global wind profile observations. To better understand these processes, we conducted two observing system experiments using the ECMWF data assimilation system: one assimilating observations only within the 15°S-15°N belt, and another assimilating observations only outside this latitude range. Differences between these experiments and a reference experiment using global observational coverage reveal the extent to which the current observing system (GOS) constrains the analyses in the deep tropics and extratropics against influences from unobserved regions.In the analyses produced by the experiment without tropical observations, only minor signals emerge from the tropics into the extratropics, consistent with a well-observed and well-analysed extratropical circulation in the current GOS. In contrast, we find that assimilating existing tropical observations within the GOS does not sufficiently constrain planetary- and synoptic-scale waves penetrating into the tropics from the extratropics. This echoes previously identified impacts of the Aeolus mission on large-scale tropical circulation. These uncertainties limit the reliability of large-scale tropical circulation in (re)analyses and our ability to predict tropical–extratropical coupling. In particular, uncertainty growth at large scales in medium-range forecasts is likely to dominate over the inverse cascade of effects associated with resolved small-scale processes in numerical models.