Cause of the intense tropics-wide tropospheric warming in response to El Niño
During El Niño events, a strong tropics-wide warming of the free troposphere is observed (order 1K at 300hPa). This warming plays an important role for the teleconnection processes associated with El Niño but it remains unclear what initiates this warming. Since convective quasi-equilibrium only holds in regions of deep convection, the strong free tropospheric warming implies that the warmest surface waters (where atmospheric deep convection occurs) must warm during El Niño. We analyze the evolution of the oceanic mixed layer heat budget over El Niño events as function of sea surface temperature (SST). Data from the ECMWF Reanalysis 5 (ERA-5) and an unforced simulation of a coupled climate model both confirm that SSTs during an El Niño event increase at the high end of the SST distribution. The data shows that this is due to an anomalous heat flux from the atmosphere into the ocean caused by a decrease in evaporation due anomalously weak low-level winds, i.e. relative to the wind speed observed in the domain of deep convection in the climatological base state. It is hypothesized that the more zonally symmetric circulation during El Niño is responsible for the weakening of low-level winds. The result of a substantial heat flux into the ocean in the domain of atmospheric deep convection (the opposite of the canonical heat flux out of the ocean into the atmosphere observed in the cold Eastern Pacific) caused by a decrease in low-level wind speed implies that the prominent tropospheric warming results from mechanical forcing.