GFDL - Geophysical Fluid Dynamics Laboratory

Publication 9001

Lanzante, J., 1990: The leading modes of 10-30 day variability in the
extratropics of the Northern Hemisphere during the cold season.

J. Atmos. Sci., 47, 2115-2140.


The 10-30 day variability of extratropical, cold season (November – March)
700 mb geopotential height data of the Northern Hemisphere was studied
through the use of rotated complex principal component (RCPC) analysis. The
intramonthly modes (IMM’s) which result from RCPC analysis of 36 years of
10-30 day bandpass filtered data were examined. In order to assess seasonality
separate analyses were done for three subseasons within the cold season.

Tests of sensitivity (to the number of eigenvectors rotated) and robustness
(to the deletion of part of the sample) were conducted to insure the
stability of the RCPC analysis. A Monte Carlo procedure was used to
objectively identify episodes of occurrence. Based on the episodes a
sequence of ten composite maps was constructed for each mode to depict the
evolution over a typical lifecycle. The significance of these lifecycle
composite maps was tested using a Monte Carlo procedure.

The average periods of the IMM’s are 16-18 days; almost all occurrences fall
in the range 13-22 days. For objectively defined episodes (which occur about
10-30% of the time) each IMM explains about 30-45% of the 10-30 day bandpass
variance averaged over those gridpoints deemed significant at the 1% level
(which cover about 20-40% of the grid area). Since a number of distinct
IMM’s were found their collective impact is considerable for intramonthly
time scales.

Initial interpretation of RCPC loading maps was difficult due to the
superposition of phenomena and non- idealized evolution. Composite maps
which depict the evolution of each IMM over a typical lifecycle were found
to be invaluable for interpretation.

Three classes of IMM’s were found. The first class contains several modes
which each involve a high latitude transient disturbance (zonal wavenumber
1 or 2) and an oscillating standing wave. The distinction between the modes
is mostly in the location of the standing wave pattern. The wave number
1 transient is probably the “16-day wave” or (1,3) Rossby normal mode which
has been identified in the atmosphere by others; however, the strong
association of these transients with regional standing waves has not
previously been documented. It is speculated that regional index cycle
like variations in the winds associated with oscillation of the standing
wave may excite the transient component. The second class consists of only
one mode, which is to a first approximation an oscillating di-pole
concentrated in the Atlantic sector; it is speculated to be the result of a
regional, baroclinic zonal index cycle. The third class contains several
distinct regional mid-latitude wave trains; the two most important are
located over Eurasia and North America. In a broad sense IMM’s represent
favored “modes of evolution” or “paths through phase space” which the
atmosphere follows on the 10-30 day time scale.

Vertical structures of the IMM’s were found to be generally consistent with
those of other studies of large-scale atmospheric motions. Vertical tilt
was found to be large over the continents (especially central and eastern
portions) while over the oceans the IMM’s were found to be nearly equivalent

About a third of the IMM’s were found to be associated with a particular
low frequency state. These states are configured in the form of two well
known atmospheric teleconnections patterns. They are such that weaker
than normal mid-latitude westerlies occur during episodes of some IMM’s.