Orographic Vorticity Streamers and Caribbean Tropical Cyclogenesis: Hurricane Delta (2020)

Interaction of the base-state flow with the elevated topography of northwestern South America has been observed to form elongated, low to middle tropospheric vorticity structures (streamers) emanating in the mountain wake, a phenomenon that has been noted by studies in other geographic contexts. African easterly waves (AEWs) and other transient perturbations can favor such interactions in Venezuela and Colombia. Anomalous southerly flow associated with AEWs is hypothesized to be favorable for producing vorticity streamers which are advected northward to the Caribbean Sea. The low-level vorticity is entrained into the AEW, leading to axisymmetrization and increasing the likelihood of tropical cyclogenesis. Using gridded reanalysis data and sensitivity experiments with a convection-permitting numerical model, we found that orographic vorticity ingestion was integral in the formation of hurricane Delta (2020). Vorticity was sourced from frictional processes along the slope, boundary layer separation at the mountain range terminus, and hydraulic effects associated with horizontal flow splitting around the orography. Moist diabatic production of potential vorticity was evident downstream, further supporting the horizontal expansion and elongation of filaments. Large-scale deformation (strain) was found to be supportive for maintaining filament structure and preventing destabilization until reaching the central Caribbean, consistent with evolution in reanalysis.