Observational Evidence for the Mutual Regulation of the Tropical Hydrological Cycle and Tropical Sea Surface Temperatures

From the analysis of surface, upper-air, and satellite observations it is suggested that the hydrological cycle associated with the Madden–Julian oscillation acts in the mode of a self-regulating oscillator. The regulation occurs as a feedback between hydrological processes in the atmosphere; radiation processes; and the dynamical movement of air over the tropical oceans controling variations of rainfall, cloudiness, and sea surface temperature (SST) on time scales varying between 30 and 60 days. The conjectured feedback occurs in three main phases: (i) the destablization phase: the atmosphere becomes increasingly unstable by the combination of radiative cooling of the upper troposphere, the gradual build up of shallow convection, and the warming of the SSTs under near-clear-sky and calm conditions; (ii) the convective stage: large-scale convection develops over the region resulting in widespread heavy precipitation, deepening of the oceanic mixed layer, cooling of the SST, and moistening of the upper troposphere; and (iii) the restoring phase: the combination of continued cooling of the SSTs maintained by the strong low-level winds and reduced solar heating, with the radiative heating of the upper atmosphere by high clouds sustained by high humidity, are major factors in stabilizing the atmosphere, suppressing convection, bringing an end to the cooling of the SSTs, and eventually leading to a calming of the winds, dissipation of the thick upper-level clouds, and a restoration of the cycle to its warming phase.