The role of wave energy accumulation in tropical cyclogenesis over the tropical North Atlantic

Abstract

A hierarchical modeling approach is used to study the process by which interactions of easterly waves with the background flow can result in a reduction in the longitudinal and vertical scale of the waves. Theory suggests that in flows that possess a negative longitudinal gradient (U x \ 0) there is a reduction of longitudinal and vertical group speeds and an increase in regional wave action density (or ''wave energy''). Relative vorticity increases locally leading to an increase in the likelihood of tropical cyclogenesis near the wave axis. Opposite impacts on the structure of the waves is expected in a U x [ 0 domain. In the simplified framework of a free-surface and divergent shallow water model, Rossby wave properties are tracked through a range of background flow scenarios to determine the important scales of interaction. The importance of wave energy accumulation for tropical cyclogenesis is then studied in a full physics and dynamics model using a nested regional climate model simulation, at 12 km horizontal grid spacing, over the tropical North Atlantic region for the entire 2005 hurricane season. The dynamical environment within which 70% of easterly waves formed tropical cyclones exhibits coherent regions in which easterly winds increase towards the east, consistent with the occurrence of wave energy accumulation.

Convectively coupled Kelvin and easterly waves in a regional climate simulation of the tropics

“…These results are consistent with Holland (1995)'s idea that TC genesis from easterly waves is ultimately caused by the disturbance entering a region of stretching deformation, where dU/dx ( 0, such that wave energy is enhanced on small scales via ''wave accumulation'' (Webster and Chang 1988). Similar evidence for wave accumulation playing an important role in TC genesis can be found in the recent WRF study by Done et al (2009).…”

Interannual Variability of the Atlantic Hadley Circulation in Boreal Summer and Its Impacts on Tropical Cyclone Activity

“…The extended area of absolute vorticity gradient reversal implies a stronger instability and thus a better chance for wave growth. Figure 11 also shows that the easterly jet weakens more sharply over the Atlantic as extending westward from the West African coast in the positive phase of M1, which implies stronger wave accumulation at the jet exit region (Webster and Chang 1988;Done et al 2011). The vertical cross sections of the zonal wind show that the low-level westerly flow is enhanced in the positive phase of M1 (Figs.…”

Evaluation of a unique approach to high-resolution climate modeling using the Model for Prediction Across Scales – Atmosphere (MPAS-A) version 5.1

“…The SSTs in ERA-I have, however, been derived from several different datasets over the years (Dee et al, 2011). For reanalysis times after February 2009, ERA-I surface fields originate from the Operational Sea Surface Temperature and Sea-Ice Analysis (OSTIA; Donlon et al, 2012). To maintain consistency between all simulations, the OSTIA SST, interpolated from its native 0.05 • horizontal grid spacing to the ERA-I grid, is used for simulation years prior to 2009.…”