Estimating the Lowest Latitude of Baroclinic Growth

Abstract

Abstract Midlatitude storm tracks are the most prominent feature of the midlatitude climate. The equatorward boundary of the storm tracks marks the transition from the dry subtropics to the temperate midlatitudes. This boundary can be estimated as the lowest latitude of efficient baroclinic growth. Scaling theories for the lowest latitude of baroclinic growth were previously suggested based on the domain-averaged parameters of the Eady growth rate and supercriticality. In this study, a new estimate for the lowest latitude of baroclinic growth is proposed, based on the assumption that baroclinic growth is limited by the vertical scale of eddy fluxes. An equation for the eddy displacement flux is obtained from which the vertical scale is calculated, given the zonal-mean zonal wind and temperature profiles. It is found that the vertical scale of the eddy displacement flux and the observed baroclinic conversion rate decrease rapidly toward the equator around the same latitude. The seasonal cycle of the lowest latitude of baroclinic growth, calculated from the observed baroclinic conversion rate, is compared with the theoretical estimates. The estimates based on the vertical scale of the eddy displacement flux and supercriticality agree well with the observed lowest latitude of baroclinic growth. In contrast, the estimate based on the Eady growth rate is located around 10°–15° equatorward. The estimate of the lowest latitude of baroclinic growth may be used in future studies for explaining variations in the properties of the storm track, the Hadley cell edge, and the subtropical jet. Significance Statement The lowest latitude of baroclinic growth marks the transition from the dry and stable subtropics to the moist and variable midlatitudes. Estimating this latitude based on mean-flow variables can potentially advance the theoretical understanding of the latitudinal structure of the atmospheric circulation around the subtropics and midlatitudes. This study suggests a new method for estimating the lowest latitude of baroclinic growth, which is found to predict the observed lowest latitude of baroclinic energy conversion relatively well, compared with the traditional prediction based on the Eady growth rate.