High-energy atmospheric muon neutrinos are detected by the IceCube Neutrino Observatory with
a high rate of almost a hundred thousand events per year. Being mainly produced in meson
decays in cosmic-ray-induced air showers in the upper atmosphere, the flux of these neutrinos
is expected to depend on atmospheric conditions and thus features a seasonal variation. The
correlation between temperature fluctuations and variations of the neutrino rates can be described
with a slope parameter 𝛼, whose previous measurement with 6 years of IceCube data indicated
a discrepancy to theoretical expectations. In this work, we present an update of the previous
analysis, extending the statistics to 12-years of IceCube data, as well as adding a region in the
Northern Hemisphere to the analysis. We estimate the slope parameter in the Southern Hemisphere
to be 𝛼 = 0.325 ± 0.022, which confirms the previous observation of the tension between the
theoretical predictions and experimental measurements with significance > 3𝜎. Furthermore,
the seasonal variation in the Northern Hemisphere has also been observed for the first time, with
𝛼 = 0.731 ± 0.222. Investigations into systematic effects reveal that the observations not only
show a weaker correlation compared to the predictions, but also deviate from the expected linear
relation between the atmospheric neutrino flux and the atmospheric temperature.