Scientific basis for the emergence of deep convection in the tropics at or above 28 ¿C sea-surface temperature (SST), and its proximity to the highest observed SST of about 30 ¿C, is explained from first principles of moist convection and TOGA-COARE data. Our calculations show that SST of 28--29 ¿C is needed for charging the cloud-base airmass with the required moist static energy for clouds to reach the upper troposphere (i.e., 200 hPa). Besides reducing solar irradiation by cloud-cover, moist convection also produces cool and dry downdrafts, which promote oceanic cooling by increased sensible and latent heat fluxes at the surface. Consequently, the tropical ocean seesaws between the states of net energy absorber before, and net energy supplier after, the deep moist convection, which causes the SST to vacillate between 28¿ and 30 ¿C. While dynamics of the large-scale circulation embodying the easterly waves and Madden-Julian Oscillations (MJOs) modulate moist convection, we show that the quasi-stationary vertical profile of moist static energy of the tropics is the ultimate cause of the upper limit on tropical SSTs. ¿ 1999 American Geophysical Union