21.  As surface water cools, it becomes more dense, sinks and is replaced by warmer water from below.

22.  As they falter warm Pacific surface waters that usually build up near Asia begin flowing eastward toward South America, bringing usually warm currents to the region.

23.  As surface water cools in winter, it becomes dense and sinks, with slightly warmer water from below rising to the surface to replace it.

24.  As the surface water warms, it becomes less dense and its volume increases, adding moisture to the atmosphere.

25.  As warm surface water sank in response to the stirring, cooler water upwelling from the depths replaced it.

26.  As surface water cools, it becomes denser, sinks and is replaced by warmer water from below.

27.  Atmospheric circulations direct water near the Equator west and then north, allowing the warmest surface waters to accumulate on the western edge of these oceans.

28.  Back then, Mars may have had a warmer, denser atmosphere that could sustain surface water.

29.  Besides, he said, surface water is not readily available in some areas.

30.  Because the outflow of cold deep water has diminished, the influx of warm surface water that usually replaces it must have also declined.