Inertial Available Kinetic Energy and the Dynamics of Tropical Plume Formation

Abstract

Tropical plumes are identified in satellite data as elongated cloud bands originating from convective activity along the intertropical convergence zone (ITCZ), often extending far into the subtropics and middle latitudes. Many previous studies consider tropical plumes as a product of quasigeostrophic or convergent forcing. Here the authors consider the view that a tropical plume is the upper branch of an enhanced thermally direct circulation driven by latent heat released along the ITCZ. In this way, tropical plume formation is strongly tied to deep cumulus convection and inertial processes. Observations of plume development show that as a midlatitude wave nears a subsequent plume genesis region, a northward advection of upper-tropospheric, low potential vorticity (potential vorticity unit ? 1) occurs as anticyclonic flow intensifies southeast of the midlatitude wave. As this low potential vorticity (PV) ridges over and straddles the ITCZ, plume genesis occurs. Plume development occurs about 1?2 days prior to the midlatitude wave?s more direct impact on the ITCZ environment as it moves to within 5°?10° of the ITCZ. However, as the midlatitude wave nears the ITCZ, an equatorward advection of high PV occurs to end plume development. Thus, a midlatitude wave both indirectly causes tropical plume formation and appears directly responsible for plume demise. As the low PV advects across the ITCZ, the meridional inertial stability gradient equilibrates. Under these conditions, it is hypothesized that the work requirements of deep ITCZ convection to spread its outflow and force compensating subsidence ease as inertial stability lowers. In the event that convection transports easterly boundary layer momentum to a level of strong convective outflow, it is found that regions poleward of the ITCZ become dynamically preferred for outflow as convectively generated (negative) PV lowers inertial stability there more than equatorward. Thus, convective-scale processes are suggested to be critical to plume formation. The diagnostic parameter ?inertial available kinetic energy? (IAKE), computed on the 340-K isentrope surface, reveals much reduced inertial stability as PV lowers across the ITCZ in conjunction with tropical plume formation. With an easterly (downgradient for the ITCZ environment) convective momentum transport, IAKE becomes positive in the poleward direction in the plume genesis region, suggesting an inertial instability relative to convective updrafts. Theoretically, ITCZ convection in these instances may use convective available potential energy in the presence of IAKE to explosively develop, forming a tropical plume.