Halliday & Matthiessen (1999: 117):
enhancement is a circumstantial or qualifying relation between figures: it is, in a sense, extension plus a circumstantial feature — ‘and’ + time (‘and then’, ‘and at the same time’, etc.), ‘and’ + manner (‘and in the same way’, ‘and likewise’), ‘and’ + cause (‘and therefore’, etc.), etc.: it is autumn, so the leaves are turning brown.