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 barotropic.
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.