Multidecadal variations in the modulation of Alaska wintertime air temperature by the Madden-Julian Oscillation (Oliver, Theoretical and Applied Climatology, 2015)
Abstract: The Madden–Julian Oscillation (MJO), the dominant mode of intraseasonal variability in the tropics, is known to influence extratropical air temperature in the Northern Hemisphere. In particular, it has been shown that intraseasonal variations in wintertime Alaska surface air temperature (SAT) is linked with variations in cross-shore surface wind and that this mechanism is driven by a train of Rossby waves originating in the tropics due to MJO forcing. Oliver, Theoretical and Applied Climatology, 2014 show, using long station records of Alaska SAT and an independent reconstruction of the MJO index over the twentieth century, that the MJO–SAT connection in Alaska has undergone significant multidecadal variability over the last century. The Pacific Decadal Oscillation appears to explain some of the observed multidecadal variability but fails to capture a large proportion of it. We identify four distinct periods between the years 1910 and 2000 that exhibit either a weak, moderate or strong MJO–SAT connection. The nature of our method ensures that the detected multidecadal variability is due to changes in the teleconnection mechanism and not due to changes in the strength of the MJO index. Finally, we speculate on the mechanism which may bring about such multidecadal variations in the teleconnection mechanism.
Multidecadal variations in the modulation of station SAT by the MJO:
Figure 1: Connection between MJO and Alaska station SAT over the twentieth century. The MJO response of wintertime SAT, calculated over running 15-year blocks, at (a,b) Fairbanks and (c,d) Nome due to an MJO cycle of amplitude 1.5 is shown for (a,c) reconstructed index and (b,d) Wheeler and Hendon (2004) index. Note that all panels share the same colour scale.
This was found to be caused by changes in the teleconnection mechanism (cross-shore temperature advection driven by propagating Rossby waves) over multidecadal time scales:
Figure 2: Changes in the teleconnection pattern linking the MJO and Alaska SAT over the twentieth century. The magnitude of the MJO response of wintertime sea level pressure due to an MJO cycle of amplitude 1.5 is shown for four time periods: (a) 1910–1919, (b) 1920–1945, (c) 1946–1966 and (d) 1970–2000. The solid lines indicate the MJO phase of maximum response (not plotted for an amplitude less than 35 Pa). The circle and square indicate the location of Fairbanks and Nome, respectively.