Chapter 1 | | ORIGIN AND RATIONALE |
The tropical Pacific Ocean exhibits several intriguing features. For nearly 50% of the tropical
Pacific (30ºN to 30ºS) SSTs are within a narrow range between 300 K and 303 K, and less than
0.01% shows SSTs in excess of 304 K (see Figure 4). Furthermore, deep intensive convection
reaching altitudes of 18 to 20 km is triggered only when SSTs exceed 300 K. Extended anvil
cirrus clouds, associated with deep convection, form primarily over such warm oceans and can
exceed 105 km2 in area. In these warm ocean regions, monthly and annual mean precipitation
exceeds local evaporation by a factor of nearly two. The atmosphere above the warmest waters of
the Pacific is also very humid, with maximum atmospheric trapping of long-wave (or infrared, IR)
radiation. Given these observations,
- Why do maximum SSTs in the tropical oceans remain within a few degrees of the 300 K
threshold SST for deep convection?
In regions of deep convection and warm SST, the reduction of outgoing long-wave energy by the
atmospheric water vapor increases so rapidly with SST that it exceeds the rate of increase of
blackbody surface emission, thereby producing a super-greenhouse effect. In other words, the
surface-atmosphere system in the warm pool loses its ability to radiate the excess energy to space.
Without some negative feedback process, this super-greenhouse effect would produce a runaway
warming. This raises the following question:
- What are the restoring forces that limit SST and deep convection to observed tropical Pacific values?
Ramanathan and Collins (1991) used the Earth Radiation Budget Experiment (ERBE) data to
examine this question. They found that the warm ocean was also covered by thick anvil clouds,
which reflect a significant amount of solar energy back to space. They also concluded that the
anvils produced by deep convection act like a thermostat to regulate the flow of solar energy to the
ocean and limit SSTs to values between 303 K and 305 K. Furthermore, the hypothetical
maximum equilibrium SSTs on the planet, which would be obtained in the absence of ocean heat
transport away from the warm ocean, should be less than 305 K for present-day boundary
conditions. The highest SSTs observed are, in fact, less than 305 K (see Figure 4).
The role of evaporative cooling in regulating SSTs is a matter of considerable debate. Some have
that an increase in evaporative cooling in the warmer oceans is the dominant mechanism
limiting SSTs. Others have concluded that evaporation, by itself, is not sufficient to explain the
observed SSTs. It is clear that detailed in-situ and surface observations are required to settle this
issue. The CEPEX objective is to obtain those measurements necessary to establish the respective
roles of cirrus radiative effects and surface evaporation in limiting maximum surface temperature in
the equatorial Pacific.
Table 1 ,
Table 2
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