The fundamental physics of the nonlinear behavior of fluids is briefly recalled with a discussion on particular examples frequently observed in the atmosphere and the ocean. Cases of nonlinear systems at different levels of complexity, leading from simple to coupled and disordered systems are exemplified as essential elements of the climate system. The nonlinear behavior of such systems encumbers us with the ‘predictability’ problem, often a major obstacle in predicting the future states of these systems. Loss of predictability is more significant in nonlinear coupled systems of high degrees of freedom, affecting a wide range of scales extending from short-term to climatic. It is well known that sea straits constitute complex, high-energy physical environments with rapid currents, hydraulic transitions, stratification and turbulence, and are controlled strongly by geometric constraints, often creating multi-scale complex interactions influencing the states of the interconnected basins. The nearest well-known examples are the Bosphorus and Dardanelles Straits. Models addressing different types and levels of complexity are often implied and have successfully been used.