Based upon such considerations, Ramanathan and Collins arrive at an equation for the surface-atmosphere column energy balance, from which they conclude that the cirrus anvils limit the warm pool SSTs to about 303 to 305 K.
The essential point of the hypothesis is that a thermostat mechanism operates not only over the "normal" warm pool location but wherever the ocean waters are warmest and are accompanied by deep convection in the troposphere, as in the central Pacific Ocean during El Niño events.
Cloud radiative forcing is the net of the long-wave and short-wave effects.
Let us now consider the five factors involved in the thermostat hypothesis.
The thermostat mechanism operates in regions of deep convective clouds, with bases at between the surface and 3 km and tops at between 12 and 18 km.
This dual tropospheric, long-wave radiative heating and surface, short-wave radiative cooling role of cirrus is the central premise of the thermostat hypothesis proposed by Ramanathan and Collins (1991).
The thermostat mechanism relies upon five factors:
The following convention is adopted to represent radiation fluxes: Long-wave flux (also referred to as irradiance) is represented by the letter F.
Ramanathan and Collins used the 1987 event (also referred to as the 19861987 event) as a natural experiment to develop their thermostat hypothesis, because simultaneous observations of SST and cloud radiative forcing were available only for the period from 1985 to 1989.
In addition, the existence of the thermostat hypothesis depends upon (and came about because of studies of) cloud radiative forcing behavior during El Niño events, and this is discussed in the last section.
The symbols and the various terms used here and elsewhere in the text are defined in Table 1.
Because the nature of radiative processes in tropical cirrus clouds is central to a thermostat mechanism, the recent research results on cirrus are reviewed briefly to put the hypothesis into context.