GFDL - Geophysical Fluid Dynamics Laboratory

Publication 8301

Lanzante, J., 1983: Some singularities and irregularities in the
seasonal progression of the 700 mb height field.
J. Clim. Appl.
Meteor.
, 22, 967- 981.

Abstract:

A climatological investigation of singularities in the seasonal progression
of 5-day means of 700-mb height was undertaken for the region 30-90 o N,
from 160 o E eastward to 0 o, for the period 1947-1976. A harmonic
analysis was performed at each of the 127 grid points 10 o longitude by 10 o
latitude spacing) in order to determine the field of deviations (for each 5-day
period) between the long-term mean value and the first harmonic value (which
represents the smooth seasonal progression of heights). Student’s t-test was
applied at each grid point to test whether the long-term (population) mean is
significantly different from the first harmonic value (i.e., to test if the deviation
is zero). The significance of the field of t-statistics for each 5-day period was
estimated by using a modification of the technique presented by Livezey and
Chen (1981).

The results of the analyses described above indicate that the 700 mb height
field deviates significantly from the first harmonic during several times of
the year. A pattern characterized by height rises centered over Alaska from
late December through late January is terminated abruptly by the January thaw.
In late February, a regional singularity (manifested as rapid height rises),
perhaps related to the termination of the February minimum in Hawaiian rainfall,
is found in the eastern Pacific. The “end of winter” in the eastern North
Pacific is heralded by a very rapid northward shift of the westerlies from
late February through early March.

During the spring, the rapid rise of heights experienced in most regions is
interrupted by a southward advance and strengthening of the westerlies in
the eastern Pacific during late May and June. During summer, heights over
most of the domain rise to “excessively high” values relative to the first
harmonic; the only exception is significantly negative deviations in eastern
North America. This phenomenon results in the most deviant 5-day period
(29 July-2 August) in terms of percentage of map area (50.1%) having
significantly nonzero deviations.

An Indian summer type pattern, characterized by weak positive deviations
over most of North America and negative deviations in polar regions,
persists from late September through October This pattern exhibits the greatest
persistence, between successive 5-day periods, of all the singularities found.

Examination of the second harmonic of 5-day 700 mb heights suggests the
existence of a semiannual, east-west oscillation of height in the North
Pacific. This phenomenon seems to explain, to a large extent, most of the
singularities and irregularities described above. At two of the four extrema
are found the “January thaw” and “Indian summer”. A six-month
periodicity in (monthly) North Pacific and sea surface temperature (SST),
in phase with the atmospheric oscillation was also found. It is speculated
that the apparent association between these two phenomena is due to
advection of long-term mean SSTs by long-term mean-wind-induced surface
currents as proposed by Weare et al. (1976).