@Scherrer2012; @Schou2012] onboard the Solar Dynamics Observatory [SDO, @Pesnell2012].

Divergence maps of supergranular flows {#secDataIntro}
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We use maps of the horizontal near-surface divergence that were obtained by [@Langfellner2018] using Dopplergrams and intensity images from HMI/SDO. The data cover 365 days from May 2010 through April 2011.

Regions of size $\sim 180 \times 180 \,\rm{Mm}^2$, which are centered at the equator, were tracked for eleven days from about $70\degr$ east to $70\degr$ west at the surface rotation rate [@Snodgrass1984]. Dopplergrams and intensity maps were remapped and each eleven-day time-series was subdivided into 33 eight-hour segments. The seventeenth segment crosses the central meridian.

Using the intensity images and LCT of granules [@November1988], [@Langfellner2018] obtained estimates of the two horizontal flow components for the same spatial and temporal coverage. This results in maps with $97\times97$ pixels, each of size $(5\times 0.348\,\rm{Mm})^2$. These maps span a slightly smaller region of size $\sim 170 \times 170 \,\rm{Mm}^2$. [[From these maps, w]{}]{}e computed the horizontal divergence by taking derivatives using Savitzky-Golay filters [@Savitzky1964]. In the Savitzky-Golay filters, we used a different filter width (3 instead of 15 pixels) and polynomial order (2 instead of 3) compared to [@Langfellner2018] to keep the smoothing as small as 