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physical perspective. We now need to undertake detailed in situ measurements to address issues that are relevant to the climate problem in general and to GCMs in particular.  In situ observations with high precision and high absolute accuracy of the microphysical, thermodynamic, and spectral radiative fluxes are needed.

But, where do we start?  Because of the immense complexity and the global nature of the problem, we should avoid the temptation to attack all of the issues but focus instead on a key region, which will provide unique insights into the global warming issue.  In particular, we need a region subject to a natural climate change experiment that exhibits large oscillations in surface temperature, cloudiness, water vapor, and radiative fluxes. 

The Tropical Western Pacific: An Ideal Greenhouse Laboratory

Nearly all of the radiative and cloud feedback processes involving the ocean/atmosphere system are manifested in the western tropical Pacific ocean shown schematically in figure 1.  This region has the largest pool of warm water found anywhere on the planet, accompanied by unstable thermodynamic and radiation regions.  For example, maximum water vapor concentrations and greenhouse effect are found over the western Pacific.  Furthermore, the climate of this coupled ocean/atmosphere system is regulated in part by the feedback between sea surface temperature (SSTs), cloud-radiative interactions, the water vapor greenhouse effect, and convection.  Since SSTs in the western Pacific exceed 30 C, it is an excellent region of focus for understanding the feedbacks and the evolution of energy fluxes in a warmer planet with enhanced greenhouse gases.  In this regard, it is important to note that the tropical Pacific has witnessed statistically significant increases in annually averaged SSTs during the last decade.

Lastly, during El Nino events, which occur once every two to six years, the equatorial Pacific warms abruptly by 2-5 C, accompanied by significant increases in the water vapor, atmospheric and cloud greenhouse effect, and reflection of solar radiation by clouds.  Hence, the tropical Pacific is an ideal laboratory for studying and unravelling the problem of cloud feedback and climate change.

The Coupled Ocean/Atmosphere System

The warmest waters on the planet (SST > 30°C) are found in the tropical western Pacific ocean.  Maximum trapping of IR radiation by the atmosphere and clouds occur in this region, giving rise to a so-called super greenhouse effect.  The IR and solar radiation heat the ocean.  As the ocean warms, the atmosphere above it becomes thermodynamically unstable.  When SSTs exceed 27°C, this instability becomes so large that deep convection sets in to release the energy.  This cumulus-nimbus convection occurs throughout the year, with the cloud tops reaching 16-19 km—maximum altitudes and depths for clouds anywhere in the planet.  The deep clouds vertically transport enormous amounts of vapor and latent energy from the oceanic boundary layer to the upper troposphere.

The heat supplied to the atmosphere in the western Pacific by deep convection gives rise to upward motions (on a spectrum of spatial scales) that constitute the rising branch of the Hadley cell.  The air moves eastward and poleward aloft.  The eastward-moving parcel along the equator sinks over the eastern Pacific and moves back towards the western Pacific, completing the so-called Walker cell.  The poleward-moving parcel sinks around the subtropical Pacific and returns equatorward (the trades), competing the so-called Hadley cell.  The Hadley and the Walker cells are the conveyor belts that cycle water vapor in the tropics.

Cirrus clouds exceeding 10[7] km[2] in areal extent are formed by ice detrained by convection.  The ice content of the cirrus is so large that they are opaque in the IR and have such large albedos that they reduce the solar energy reaching the ocean surface by nearly a factor of two.  Thus, these clouds, which owe their existence to warm SSTs and convection, can play a significant role in regulating sea-surface temperatures.