It has been a great challenge to estimate changes in hydrological cycle under global warming, because current observational data does not indicate a clear trend. Since El Niņo events also exhibit similar warming effects which impact the hydrological cycle, some of its mechanism may explain that of global warming. Analysis of the 1997-98 El Niņo event shows asymmetric changes of tropical precipitation and water vapour with respect to the equator. In response to winter El Niņo forcing, precipitation and water vapour increase over Southern Hemisphere due to a warmer environment with stronger convection. On the other hand, a colder environment with weaker convection over Northern Hemisphere reduces precipitation and water vapour. Under global warming similar changes occur in hydrological cycle, particularly precipitation. As temperature rises, the Northern Hemisphere precipitation increases more significantly than that of the Southern Hemisphere. This asymmetry of precipitation between the two hemispheres is therefore intensified. Two satellite observations show a consistent trend of an enhanced asymmetry of precipitation between Northern Hemisphere and Southern Hemisphere during 1984-1997.