Properties of forecast errors of tropical circulation are more difficult to establish than in the extra-tropics, where the geopotential height often provides sufficient information about the large-scale circulation. This is a consequence of a strong coupling between the geopotential height and winds in the extra-tropics, known as quasi-geostrophic balance. In the tropics, such balance is often weak and, therefore, the flow has to be decomposed into both balanced and unbalanced components. Such a "modal decomposition" is provided by MODES and it has been done on the ECMWF 10-day deterministic forecasts to provide a deeper understanding of the circulation in the tropics, and of the sources of deficiencies in analyses and forecasts. The full article is available from the ECMWF webpage http://www.ecmwf.int/en/about/news-centre/media-resources