At the end of the 20th century and beginning of the 21st century empirical climate researchers made a startling discover concerning the natural regulation of sea surface temperature. The research began with group at NASA under the leadership of Y.C. Sud investigating why the surface temperature of the mid-Pacific region stays within a relatively narrow range of 28°C to 30°C. This region of the Pacific is probably the warmest spot in the world's oceans but the temperature appears to be regulated by some mechanism. This was a significant topic for research because the fluctuations in the tropical Pacific are responsible for the El Niño Southern Oscillation (ENSO). What Sud et al. found was that higher temperatures stimulate increased evaporation leading to rising moist air. The rising warm air is compensated for by downdrafts of cool dry air from the upper tropospher which cools the sea surface.

The Sud study which was published in 1999 was followed by another NASA study of the mid-Pacific under the leadership of Richard Lindzen of M.I.T. The usual process above oceans is that the rising air cools and water droplets condense out as cumulus clouds. Some rain falls but much of the condensed moisture is carried to higher altitudes where the cloud droplet freeze into ice crystals. This forms cirrus clouds. These cirrus clouds are effective in capturing the thermal radiation from the sea surface and feeding it to the lower atmosphere and the sea surface. This would be a positive feedback. What the Lindzen group found is that as the sea surface temperature rises above 28°C the positive feedback turns into a negative feedback. The higher temperature results in heavier loading of the rising air with water vapor. This leads to denser formations of cloud droplet and hence more rainfall. This heavier rainfall results in fewer droplets continuing their rise to the upper atmosphere and less cirrus cloud cover. Fewer cirrus clouds mean more of the thermal radiation from the sea surface escapes into space. This is a negative feedback effect. Its existence was established by putting together the information on cloud cover from one satellite with the data on sea surface temperature from another satellite.

The Lindzen group study was published in 2001. In 2002 two more studies were published. One was by another NASA research group, under the leadership of Bruce Weilicki. The other study was conducted under the leadership of Jun Chen of Columbia University. These two studies estimated the magnitude of the heat energy loss due to the hole opened in the cloud cover by the effect. The area of the Pacific is so vast that the amount of heat energy escaping due to this opening up of a hole in the cloud cover is on the order of magnitude of the heat energy that would be captured by a doubling of the CO2 level for the Earth.

(To be continued.)

Sources:

• Y.C. Sud, G.K. Walker and K.-M. Lau, "Mechanism Regulating Sea-Surface Temperature and Deep Convection in the Tropics," Geophysical Research Letters vol.26, no. 8, (April 15, 1999), pp. 1019-1022.
• Richard S. Lindzen, Ming-Dah Chou and Arthur Y. Huo, "Does the Earth Have an Adaptive Infraed Iris?" Bulletin of the American Meteorological Society, Vol. 82, no. 3 (March 2001), pp.417-432.
• Jun Chen, Barbara E. Carlson and Anthony D. Del Genio, "Evidence for the Strengthening of the Tropical General Circulation in the 1990's," Science, vol. 295 (February 1, 2002), pp. 838-841.
• Bruce A. Weilicki et al., "Evidence for Large Decadal Variability in the Tropical Mean Radiative Energy Budget," vol. 295 (February 1, 2002), pp. 841-844.