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

TL;DR

This study examines the dynamics of East Atlantic seed vortices using high-resolution models, revealing how differences in convective organization affect vortex development and hurricane formation.

Contribution

It demonstrates that explicit convection models may underestimate hurricane activity due to deficiencies in simulating mesoscale convective systems and vortex mass flux profiles.

Findings

Higher-resolution explicit convection simulation produces fewer, weaker hurricanes.

Differences in convective organization explain variations in vortex evolution.

Underestimation of MCS stratiform components affects vortex development.

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-only 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-derived climatologies. Despite the simulations displaying comparable continental vortex populations, we show that the highest-resolution simulation with explicit convection produces fewer, weaker hurricanes than coarser, parameterised counterparts due to a failure to amplify vortices crossing the West African coastline. We identify a failure to maintain strong top-heavy mass flux profiles experienced by seeds as the primary cause, demonstrating profiles' roles in low-level circulation development through vortex stretching. 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 from underestimation of MCS stratiform components, a bias shared with other explicit convection models; and a latitudinal offset between offshore seed vortex and MCS trains.