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While serving as a postdoctoral scientist at NOAA’s Climate Analysis Center in 1988-89 I performed some follow-up work (unpublished) in which I repeated my analyses for the summer season.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.

Plans

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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