  Intuitive entropic interpretations of the thermoelectric effect in metals
predict an isotropic Seebeck coefficient at high temperatures in the incoherent
regime even in anisotropic metals since entropy is not directional.
$\mathrm{Sr}_2\mathrm{Ru}\mathrm{O}_4$ is an enigmatic material known for a
well characterised anisotropic normal state and unconventional
superconductivity. Recent ab-initio transport calculations of
$\mathrm{Sr}_2\mathrm{Ru}\mathrm{O}_4$ that include the effect of strong
electronic correlations predicted an enhanced high-temperature anisotropy of
the Seebeck coefficient at temperatures above 300 K, but experimental evidence
is missing. From measurements on clean $\mathrm{Sr}_2\mathrm{Ru}\mathrm{O}_4$
single crystals along both crystallographic directions, we find that the
Seebeck coefficient becomes increasingly isotropic upon heating towards room
temperature as generally expected. Above 300 K, however, $S$ acquires a new
anisotropy which rises up to the highest temperatures measured (750 K), in
qualitative agreement with calculations. This is a challenge to entropic
interpretations and highlights the lack of an intuitive framework to understand
the anisotropy of thermopower at high temperatures.
