Abstract
Although Earth’s troposphere does not superrotate in the annual-mean,
for most of the year – from October to May – the winds of the tropical
upper troposphere are westerly. We investigate this seasonal
superrotation using reanalysis data and a single-layer model for the
winds of the tropical upper troposphere. The temporal and spatial
structures of the tropospheric superrotation are characterized, and the
relationships between the superrotation and the leading modes of
tropical interannual variability are quantified. It is also shown that
the strength of the superrotation has remained roughly constant over the
past few decades, despite the winds of the tropical upper troposphere
decelerating (becoming more easterly) in other months. The underlying
dynamics of the seasonal superrotation are studied using a combination
of momentum budget analysis and numerical simulations with an
axisymmetric, single-layer model of the tropical upper troposphere.
Momentum flux convergence by stationary eddies accelerates the
superrotation, while cross-equatorial easterly momentum transport
associated with the Hadley circulation decelerates the superrotation.
The seasonal modulations of these two competing factors shape the
superrotation. The single-layer model is able to qualitatively reproduce
the seasonal progression of the winds in the tropical upper troposphere,
and highlights the northward displacement of the Intertropical
Convergence Zone in the annual-mean as a key factor responsible for the
annual cycle of the tropical winds.