Tropospheric temperature control and its effect on precipitation on the northern coast of Peru

The sea surface temperature threshold (Tc) is a necessary but not sufficient condition to activate deep convection in tropical oceans. Through the use of hierarchical clustering analysis, as a tool for seasonal forecasting, it was determined that the most important variables in the control of deep convection in summer on the northern coast of Peru are sea surface temperature (SST) and tropospheric temperature (TT). A sea surface temperature threshold that varies interannually (Tcd) is proposed, Tcd = Tc + βṡTT', which is a function of tropospheric temperature anomalies and β is calculated from the relationship between Tc and TT in a simulation with initially-perturbed atmosphere using SST prescribed to increase the number of tropospheric warming and cooling interannual events over the tropics (20°N - 20°S); and the difference SST - Tcd was evaluated as a measure of stability. It was proved that precipitation on the northern coast of Peru is controlled by this measure of stability over the Niño 1+2 region. Thus, the probability of precipitation being above normal increases (~ 100%) when SST > Tcd, while decreases (~ 30%) when SST < Tcd. Finally, it is concluded that the threshold for activation of deep convection (measured through Tcd) is controlled by tropospheric temperature disturbances. This atmospheric mechanism, according to simulations and observations, is more sensitive to tropospheric cooling, leading to instability and stimulating deep convection, than to tropospheric warming which inhibits deep convection.