Baroclinic stability at low latitudes: Explaining subtropical jet variability in observations

The subtropical jet is characterized by a strong vertical shear and is usually located far equatorward of the maximum surface westerlies. In some cases, strong westerlies develop below the subtropical jet, indicating that momentum flux convergence by baroclinic eddies contributes to the jet driving. In this work we examine this variability of the subtropical jet in light of baroclinic instability theory. According to linear baroclinic instability theory, the vertical scale of eddy fluxes decreases toward the equator, making the eddies effectively stable at low latitudes. This stabilizing effect enables the subtropical jet to be maintained by angular momentum advection from the tropics, without the development of baroclinic eddies and surface westerlies below the jet. The dimensionless Charney number is used as an indication for the degree of baroclinic stability at low latitudes. This number incorporates the stabilizing effect at low latitudes, in contrast with the commonly used measure for baroclinicity – the Eady growth rate. It is found that the Charney number performs better than the Eady growth rate in estimating the lowest latitude of baroclinic growth and explaining subtropical jet variability.