Oceanic thermostat

E. Linacre and B. Geerts

7/'98

The sea surface never exceeds 30° C on Earth, except in some small, shallow seas surrounded by land. This observation suggests that there is some regulatory negative feedback on sea surface temperature (SST)(1). At least two processes offer a negative feedback. Firstly, a 1°C increment in SST, when the SST is near 30°C, enhances the evaporation rate 67% more than the same SST increment over an ocean that is 20°C and three times more from an ocean at 10°C. This is because the saturation vapour pressure increases exponentially with temperature (Note 4.C). Evaporation implies latent heat loss, and this implies a control on the SST. Secondly, as a consequence of the evaporation, a high SST leads to increased convection of moist, warm boundary layer air, and hence more cloud, which automatically cuts off further solar heating of the sea surface.

Recent measurements over the west Pacific warm pool indicate that both processes contribute to ocean thermoregulation, and that their relative importance depends on the SST; the cloud effect is dominant at low temperatures and the evaporation effect at higher temperatures (2). So the evaporation effect is the important one near 30° C. However, the matter is complicated by the influence of global winds on cloud formation.

Incidentally, the evaporation hypothesis is like that advanced by Priestley and Taylor (3), arising from papers by Linacre (4,5) on the tendency of well-watered leaves in full sunlight to differ from the air temperature in the direction of 30° C.

 

References

  1. Waliser, D.E. 1996. Some considerations on the thermostat hypothesis. Bull. Amer. Meteor. Soc., 77, 357-60.
  2. Zhang, G.J. and R.L. Grossman 1996. Surface evaporation during the Central Equatorial Pacific Experiment. J. Climate, 9, 2522-37.

  3. Priestley, C.H.B. and R.J.Taylor 1972. On the assessment of surface heat flux and evaporation using large-scale parameters. Mon. Weather Rev., 100, 81-92.

  4. Linacre, E.T. 1964. A note on a feature of leaf and air temperatures. Agric. Meteor., 1, 66-72.

  5. Linacre, E.T. 1967. Further notes on a feature of leaf and air temperatures. Archiv. Meteor. Geophys.s Biollim., B15, 422-36.