Optimal modes of atmospheric and oceanic motions under linear approximation have been applied to the investigations of predictability, sensitivity and stability. For example, singular vector has been applied not only in the estimation of prediction uncertainties, which is one of the central problems in predictability of weather and climate, but also in ensemble forecast and adaptive observation. In recent years, there have been considerable efforts to generalize linear optimal modes into nonlinear category, such as nonlinear singular vectors, nonlinear Lyapunov vector, and conditional nonlinear optimal perturbations. And remarkable applications have also been found in predictability, sensitivity and stability studies by demonstrating the importance of nonlinear processes in the problems studied. This session invites the contributions from scientists who are working on the theoretical and numerical studies of nonlinear optimal modes. The session also welcomes those who explore applications of nonlinear optimal modes in quantifying the predictability of different weather and climate phenomena from thunderstorms, torrential rains, tropical cyclones and typhoons to ENSO and climate change with time scales ranging from a few hours to centuries. Besides, works related to the applications of nonlinear optimal modes to the sensitivity and stability studies in geophysical fluid dynamics are also welcome.