Normal Mode Weak Interaction: From the Diurnal to Decadal and Longer Time Scales

Monsoon systems are clearly recognized as major features of the tropical circulation. These major circulation systems in the atmosphere constitute important heat sources with regional and remote influences, with significant diurnal, synoptic, intraseasonal, decadal or multi-decadal variability. Theoretical results with simplified atmosphere/ocean models indicate the possibility of energy transfer through nonlinear terms as a major source in slow climate variability. Recent results on the role of nonlinear resonance will be discussed. These results clearly show the connection between high frequency variability (such as the diurnal scale) to the decadal and longer time scales. In particular, we show the connectivity between diurnal type scales, intraseasonal, interannual and multidecadal time scales and the coupling the stratosphere and troposphere through the interaction of vertical modes. As a consequence of the nonlinear energy transfer it is quite clear the need for what is now called “seamless prediction” i.e., models with multiple scales actually resolved or parameterized on sound physical basis.