Tropical Tropospheric Temperature and Precipitation Response to Sea Surface Temperature Forcing

Abstract

During an El Nino event, there are substantial tropospheric temperature anomalies across the tropics associated with sea surface temperature (SST) warming in the central and eastern Pacific. The typical spatial scale for teleconnection response of tropospheric temperature tends to be large. On the other hand, the precipitation response exhibits strong compensation between positive response over warm SST anomalies and a complex negative response remotely. The tropical spatial averages of tropospheric temperature and precipitation thus yield an interesting contrast in behavior. Anomalies of tropical averaged precipitation for 3-month averages appear quite scattered in relation to tropical SST anomalies, while the tropical mean tropospheric temperature obeys an approximately linear relationship to SST. This different behavior of tropical mean precipitation and tropospheric temperature in relation to SST is examined in detail using observational data, GCM simulations and idealized experiments with the quasi-equilibrium tropical circulation model (QTCM). Theoretical understanding is provided through a simple analytical model, which suggests that the integral constraint on tropical average precipitation is dominated by dry static energy transport into or out of the tropics. Convection acts to keep tropospheric temperature in quasi-equilibrium (QE) with boundary layer moist static energy, which is in turn held toward SST by surface fluxes. To maintain QE, the tropical average convective heating (i.e., precipitation) anomalies react to oppose any processes that tend to cool the tropical troposphere. Thus, while tropical average tropospheric temperature is closely related to SST, unrelated heating or cooling anomalies such as those due to the tropical-midlatitude transports can create large scatter in tropical average precipitation anomalies.