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. References Frederiksen, J. S., The onset of blocking and cyclogenesis in southern hemisphere synoptic flows, J. Atmos. Sci., 41, 1984, 1116--1131. doi:10.1175/1520-0469(1984)041<1116:TOOBAC>2.0.CO;2 Frederiksen, J. S., Singular vectors, finite-time normal modes and error growth during blocking, J. Atmos. Sci., 57, 2000, 312--333. doi:10.1175/1520-0469(2000)057<0312:SVFTNM>2.0.CO;2 Frederiksen, J. S., Growth and vacillation cycles of disturbances in southern hemisphere flows, J. Atmos. Sci., 38, 1981, 1360--1375. doi:10.1175/1520-0469(1981)038<1360:GAVCOD>2.0.CO;2 Frederiksen, J. S., and C. S. Frederiksen, Monsoon disturbances, intraseasonal oscillations, teleconnection patterns, blocking and†storm tracks of the global atmosphere during January 1979: Linear ${†}$theory, J. Atmos. Sci., 50, 1993, 1349--1372. doi:10.1175/1520-0469(1993)050<1349:MDIOTP>2.0.CO;2 Wei, M., and J. S. Frederiksen, Finite-time normal mode disturbances and error growth during southern hemisphere blocking, Adv. Atmos. Sci., 22, 2005, 69--89. doi:10.1007/BF02930871 Frederiksen, J. S. and C. S. Frederiksen, Interdecadal changes in southern hemisphere winter storm track modes, Tellus, 59A, 2007, 599--617. doi:10.1111/j.1600-0870.2007.00264.x Frederiksen, C. S., and J. S. Frederiksen, Northern hemisphere storm tracks and teleconnection patterns in primitive equation and quasigeostrophic models. J. Atmos. Sci., 49, 1992, 1443--1458. doi:10.1175/1520-0469(1992)049<1443:NHSTAT>2.0.CO;2 Arnoldi, W. E., The principle of minimized iterations in the solution of the matrix eigenvalue problem, Quart. Appl. Math., 9, 1951, 17--29. Frederiksen, C. S., and J. S. Frederiksen, A theoretical model of Australian Northwest cloud band disturbances and southern hemisphere storm tracks: The role of SST anomalies. J. Atmos. Sci., 53, 1996, 1410--1432. doi:10.1175/1520-0469(1996)053<1410:ATMOAN>2.0.CO;2 Frederiksen, J. S., and C. S. Frederiksen, Southern hemisphere storm tracks, blocking and low frequency anomalies in a primitive equation model, J. Atmos. Sci., 50, 1993, 3148--3168. doi:10.1175/1520-0469(1993)050<3148:SHSTBA>2.0.CO;