Dynamics of East Atlantic seed vortex populations in global km-scale models

Abstract

Africa is the primary source of cyclonic vortices over the tropical Atlantic. Over both land and sea, these vortices are entwined with deep convective activity, with the majority being African Easterly Wave troughs. Their convective interactions have downstream impacts, since the same vortices provide the seed population for Atlantic basin tropical cyclone (TC) genesis. Understanding the dynamics of East Atlantic seed populations, particularly the processes that distinguish vortices which undergo cyclogenesis, is crucial for understanding the formation of Atlantic hurricanes and model representations of their populations. Here we investigate these questions in three one-year, atmosphere-land global km-scale Met Office Unified Model simulations. We use objective tracking algorithms to independently identify seed vortices, easterly waves, TCs, and Mesoscale Convective Systems (MCSs), benchmarking against reanalysis and satellite observations. The simulations display equivalent continental and offshore seed vortex frequencies, however the high-resolution, explicit convection simulation produces fewer, weaker hurricanes. Parameterised convection counterparts show stronger vortex amplification of seeds crossing the West African coastline, and more hurricanes form offshore. In the explicit simulation, we identify a failure to maintain strong vertical mass flux profiles experienced by seeds as the primary cause of weak vortex development, finding no low-level circulation development offshore about seed vortices. Using MCS tracks, we show that systematic differences in convective organisation between the simulations can explain the differences in mass flux profiles, and thus vortex evolution. Deficiencies in the explicit simulation stem primarily from underestimation of MCS anvils and stratiform rainfall, a long-standing systematic bias common amongst explicit convection models.