THE ROLE OF TROPICAL-EXTRATROPICAL INTERACTION IN THE MAINTENANCE OF TROPICAL SST

There is considerable evidence indicating that the tropical sea surface temperature (SST) varies relatively little even in the face of large fluctuations of the overall climate. This is remarkable considering that interaction between lower tropospheric humidity and infrared radiation is known to produce a positive and hence destabilizing feedback on SST in the Tropics, and suggests that the compensating negative feedbacks must be very strong. Tropical SST is essentially determined by the balance of net downwelling radiation and evaporation. The latter is proportional to U(q₀-q_s), where U is surface windspeed and q₀-q_s is the difference in specific humidity accross the surface layer. Assuming constant surface relative humidity, the Clausius-Clapeyron relation indicates that q₀-q_s increases exponentially with SST, giving a negative feedback on SST. However, there is evidence (from observations and GCM modeling) that U is also positively correlated with SST. In principle this could give an additional negative fedback. Here, the magnitude of this feedback is assessed using a simple but relatively realistic radiative-convective model of the Tropical atmosphere. Results indicate strong dependence on the strength of the coupling between SST and U. When coupling is weak, the wind feedback is small relative to the humidity feedback. As coupling strength increases, the wind feedback grows while the humidity feedback decreases and actually becomes destabilising. A simplified version of the model is formulated and used to account for this changeover. Overall, the results suggest that tropical-extratropical coupling could be important in stabilising the Tropics and should be studied in more detail.