Mean mode frequency variation as a function of time. (Top, left) for the l= 0 modes for the global fit (continuous line) and the local fit (dashed line). (Top, right) for the l= 1 modes for the global fit (continuous line), the local phase fit (dashed line), the global fit of north-south difference (green line) and the global fit of east-west difference (green dashed line). (Bottom, left) for the l= 2 modes for the global fit (continuous line), the local phase fit (dashed line), the global fit of north-south difference (orange line) and the global fit of east-west difference (orange dashed line). (Bottom, right) for the l= 3 modes for the local phase fit (dashed line), the global fit of north-south difference (red line) and the global fit of east-west difference (red dashed line). The l = 3 modes are not shown because they are too faint to give a meaningful full-disk signals.