Downwelling means a tendency to converge, and upwelling means divergence. The small scale convection rolls intervene with the main trend following the large convection cells of lower mantle. That main line lies through the middle of the country from SW to NE. The two southern volcanic zones, called WVZ and EVZ, are found within the same settings, namely with upwelling to the west and downwelling at the eastern side. The reason is conflict between the tectonic drift to the NW and the opposite trend resulting from the upwelling and downwelling lines.
The MIVZ in central Iceland actually also has this setup, but to the NE and SW. The shift of tightness of the polygons at 65°N results in almost continuous volcanic areas from Snæfellsjökull to Snæfell. Then there is the NS-axis from Öræfajökull to Axarfjörður, besides the less known NS axis from Eyjafjallajökull through Hekla to Drangey.