GFDL - Geophysical Fluid Dynamics Laboratory

Intramonthly & Intraseasonal Variability


Rossby Wave Propagation & Other Quasi-Cycles

While one earlier project (Harnack et al. 1986) dealt with long-range forecasting
using an index of intraseasonal variability, my primary interest in this area is
with regard to my Ph.D. dissertation work (Lanzante 1990). The motivation for
this study was based on some observations made in the course of real-time
monitoring of intraseasonal variability while working in the Meteorology Department
at Rutgers University. We noticed a tendency for long-wave trough retrogression
from the east to the west coast of North America over the course of about 2-3
weeks during the cold season. A discontinuous jump of the trough from the west
back to the east coast would complete the “cycle”. It seemed that during certain
years this cycle would occur repeatedly, although not always without interruption.

I designed my dissertation project with this quasi-cycle in mind, as well as
reports in the literature of high latitude, westward propagating
Rossby-wave-like phenomena. I also considered early works by Jerome Namias
on the idea of the “Index Cycle”. I used complex Principal Component Analysis
as a sophisticated tool which might be able to sort out the complicated
relationships in the data.

The results of my study (limited to the extratropics of the Northern
Hemisphere during the cold season) suggest that the 10-30 day atmospheric
variability is dominated by three types of phenomena: (1) high latitude,
low wavenumber, westward propagating disturbances, (2) somewhat higher
wavenumber progressive, midlatitude wavetrains over North America and Asia
and (3) an oscillating dipole in the North Atlantic (near Greenland). The
first type of phenomena had been documented before, however the new finding
of mine is that they seem to occur in conjunction with more regionalized
oscillating standing waves. The second type seems to be a confirmation of
our observations at Rutgers, and of some early work reported by Namias. The
third type have somewhat of the appearance of the classic Namias type
zonal index cycle with alternate periods of fast versus blocked flow in the
mid to high latitudes. While serving as a postdoctoral scientist at
NOAA’s Climate Analysis Center in 1988-89 I performed some follow-up work
(unpublished) in which I repeated my analyses for the summer season. I found
that the organized variability on this time scale is more confined to the
polar latitudes; much of it appears to be variations of size and shape of
the polar vortex.


At some time in the future I hope to continue these studies by trying to
determine some of the physical mechanisms involved (I have some theories
for these which need testing) and also to examine GCMs for these phenomena.

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