Role of dynamical modes in changing southern hemisphere climate

We examine the ability of leading dynamical eigenmodes to capture important properties of the changing southern hemisphere weather systems during the 20th century. A global atmospheric model uses three dimensional July basic states taken from observations for the periods 1949-1968, 1975-1994 and 1997-2006. Iterative eigensolvers using the Arnoldi method of Krylov subspace reduction are contrasted with direct methods of matrix construction and solution. The results are related to the large changes in winter weather systems, rainfall and circulation that have occurred during the last sixty years in the Australian region. We find that storm track and onset-of-blocking modes growing on the subtropical jet have significantly reduced growth rates since the 1970s. On the other hand, there has been significant increase in the growth rates of northwest cloud-band modes and intra- seasonal oscillations that cross Australia, and some smaller increase in the intensity of African easterly waves.