The Response Time of the Temperature of the Equatorial Troposphere to ENSO Heating

Abstract

Air temperature anomalies, averaged over the troposphere to 200 mb and around the earth from 10°S to 10°N, lag the similarly averaged El Niño?Southern Oscillation (ENSO) atmospheric latent heating anomalies by about one month. Most of the latent heating is balanced by vertical adiabatic cooling although the zonally averaged imbalance is larger than is typical locally in the Tropics. The excess latent heating heats the atmosphere and generates a temperature anomaly. As the temperature anomaly rises, the atmosphere loses heat until the residual heating is balanced by anomalous cooling. By then the temperature anomaly is typically about 0.4°C. Analysis of the thermodynamic energy equation shows that the ENSO heat loss is highly linearly correlated with the air temperature anomaly averaged over the equatorial troposphere; that is, the adjustment to the residual anomalous heating (or cooling) is Newtonian. Consistent with the observed one-month lag, the Newtonian e-folding time is about 35 days. Similar results apply for latitude bands 5°S?5°N and 15°S?15°N (Newtonian cooling times of 29 and 46 days, respectively). The heat loss is mainly through meridional sensible heat flux rather than radiation. Much of the anomalous cooling is due to the mean meridional flow that diverges more temperature anomaly aloft than it converges near the surface.