Localized Driving of the Subtropical Jet by Tropical Convection: An Idealized Modelling Study.

An idealized model is used to examine the driving influence of localized tropical convection on the wintertime subtropical jet. To avoid preferred convective regions, and instead focus on the response to spontaneously occurring convection, the model is run with fixed, zonally symmetric sea surface temperatures under perpetual solstice conditions. A combination of three complementary analyses is used here: 1) a zonal recentering of the daily data around the longitude of maximum tropical diabatic heating to allow a focus on the strongest convective events, 2) a lag regression onto tropical diabatic heating to examine the time evolution of the circulation response to convection, and 3) an idealized experiment with a switch-on tropical diabatic heating perturbation designed to mimic deep convection. The qualitative picture that emerges suggests that deep convection in the summer hemisphere drives an anomalous localized Hadley cell that crosses into the winter hemisphere and drives a locally strengthened subtropical jet downstream via advection of angular momentum. A key feature associated with this picture is a "pocket" of reduced and homogenized angular momentum through which the local cross-equatorial Hadley cell flows. Momentum fluxes associated with both the divergent overturning circulation and rotational eddies drive this pocket, thus highlighting the complexity in interpreting the angular momentum budget due to the inherent zonally asymmetric and temporally varying nature of tropical convection and associated Hadley cell. Overall, the zonal-mean tropical circulation can be considered a superposition of times and regions with strong convective activity and thus a locally strengthened Hadley cell and subtropical jet, as well as times and regions with weak convective activity. Significance Statement: This study examines how localized and temporally varying tropical deep convection in the summer hemisphere leads to the formation of a winter hemisphere subtropical jet, given that traditional theories cannot take such space–time inhomogeneities into account. Using targeted idealized experiments and methodologies, we examine the mechanisms at work and find a robust qualitative picture: The longitudinally averaged tropical–subtropical circulation can be considered as an accumulation of times and regions with strong convection and associated circulation, as well as periods with weak convection.