In meteorology, air flowing longitudinally, across the parallels of latitude. Meridional flow occurs in atmospheric cells, resulting in part from longitudinal temperature variations. See Song et al. (2008) J. Climate 21, 19. The meridional temperature gradient is the change in temperature along a meridian. Von Storch (2008) J. Climate 21, 18 finds that an increase in the meridional gradient of sea surface temperatures strengthens the atmospheric circulation.