Spatiotemporal variability of zonal wind spectra in the lower and middle atmosphere based on the ERA5 reanalysis data

Abstract

A study was conducted on the spatio-temporal variability of the
wave nature of the zonal wind field in the troposphere, stratosphere, and lower
mesosphere of the Northern (NH) and Southern (SH) Hemispheres (in the 1000-
0.01 hPa layer, 51 isobaric surfaces) for two five-month time intervals (November-
March and May-September, 151 days each) for the period 1978-2025 using hourly
ERA5 reanalysis data. The study is based on the Hayashi spectral approach, which
allows us to express the power density of the wave field in terms of frequency and
zonal wave number k. This approach was used to identify the stationary (S) and
eastward (E) and westward (W) wave components of the zonal wind. In the region
of high frequencies, tidal components are statistically reasonably distinguished:
diurnal (k = 1), semidiurnal (k = 2) and terdiurnal (k = 3) periods of oscillations
with the highest amplitude values for waves W. The amplitude of the diurnal tide of
waves W reaches maximum values of more than 320 m2/s2 in the polar mesosphere
of the summer hemisphere. For SH, a secondary maximum of 275 m2/s2 at 65°S is
observed in the 0.5-0.05 hPa layer. Foci of high values of the semidiurnal tide are
located in the upper stratosphere-lower mesosphere symmetrically relative to the
equator by 45-55° in the winter hemisphere and reach 220 m2/s2. In the structure of
the third-day tide, two latitudinal maxima are distinguished for both five-month
periods at the upper boundary of the considered layer of 0.01 hPa. In the November-
March period, one focus (70 m2/s2) is located in the moderate latitudes of the NH
with a center of about 50°N, and the other, more intense (90 m2/s2) about 15°S. For
the period May-September, a certain mirror symmetry of the latitudinal position of
these foci is observed: 60 m2/s2 about 55-60°S and more intense (70 m2/s2) about
20°N Among the tides, the largest (negative) trend (up to -25 m2/s2/10 years at 45-
55°N) has a semidiurnal tide. Planetary waves (7-38 days) for wave number k = 1

have two latitudinal maxima in stratosphere and lower mesosphere of moderate and
polar latitudes of winter hemisphere. Standing waves predominate in the NH, while
eastward-propagating waves predominate in the SH. The highest spectral values are
observed for standing waves S. At the same time, the mid-latitude maximum of
waves S (50 m2/s2) is weaker than the polar (70 m2/s2). The same ratio is noted for
E waves (35 and 50 m2/s2 for moderate and polar latitudes, respectively) and
similarly for W waves (30 and 45 m2/s2). For a wavenumber k = 2, the standing
wave intensity in the NH is about 2 times higher than in the SH. Moreover, the
configuration of the high-latitude distribution is significantly different from waves
with k = 1, especially for SH. For k = 2, there is practically no source of high values
for all types of waves near the pole. In the upper troposphere of both hemispheres,
for wavenumbers greater than 3, high values of the E wave spectra (up to 45 m2/s2)
are observed in subtropical and temperate latitudes.