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Impacts of Extratropical Weather Perturbations on Tropical Cyclone Activity: Idealized Sensitivity Experiments with a Regional Atmospheric Model

January 7th, 2020

Key Findings

  • This study simulated the influence of extratropical weather variability on tropical cyclone formation using an idealized experimental design, perturbing the northern boundary conditions of a regional TC-permitting atmospheric model.
  • Artificial suppression of extratropical weather perturbations along 45°N elevates hurricane activity at 30°N–40°N in idealized experiments
  • The response is related to reduced equatorward mixing of extratropical dry air, which could occur to some degree in altered climate states
  • The experimental results suggest mechanisms through which extratropical atmospheric variability could affect tropical cyclogenesis

Gan Zhang, Thomas R. Knutson, and Stephen Garner. Geophysical Research Letters. DOI: 10.1029/2019GL085398

Recent observational studies suggested that Atlantic hurricane activity is strongly affected by weather processes outside of the tropics, but modeling studies reported divergent findings regarding the importance of such an impact. Using a regional atmospheric model with imposed boundary conditions, the authors conducted idealized experiments to explore whether and how extratropical weather perturbations affect Atlantic hurricane activity.

In this study, 22-year simulations of August–October suggest that extratropical suppression of weather perturbations adds ~3.7 Atlantic hurricanes, or tropical cyclones (TC) per season on average, although the response varies among individual years. The TC response mainly appears within 30°N–40°N, where tropical cyclogenesis frequency quadruples compared to control simulations. This increased cyclogenesis, accompanied by a strong increase of mid‐tropospheric relative humidity, arises as the perturbation suppression reduces the extratropical interference of TC development. The suppression of extratropical perturbations is highly idealized but may suggest mechanisms by which extratropical atmospheric variability potentially influences TC activity in past or future altered climate states.

This study helps to clarify the sensitivity of Atlantic hurricane activity to extratropical weather perturbations. The extratropical impact is an important contributor to uncertainties in hurricane predictions but had not been well understood. These findings suggest that the extratropical impact on the variability and future changes of Atlantic hurricane activity can be profound. Together with our other ongoing studies, this study may lead to more reliable predictions of Atlantic hurricane activity on seasonal to centennial time scales.

Tropical cyclone track density (left), genesis locations (middle), and lysis locations (right) during 1995–2016. From the top to the bottom, the subplots show (a,d,g) the observation, (b,e,h) the RAS‐Default experiment, and (c,f,i) the RAS‐Ncalm experiment. The track density panels (unit: 6‐hourly counts per season) show the climatologies (shading) and the differences from observation (black contour in (b) and (c)). In (d)–(f), genesis locations are denoted with red dots, with the number of cyclogenesis occurrences within each 10° latitudinal band (e.g., 10–20°N) indicated on the right. In (g)–(i), lysis locations are denoted with purple dots, with the number of lysis at marked for 10° latitudinal bands (except for 40–45°N). For the simulation results, the genesis and the lysis locations correspond to the first and the last locations where a vortex meets the TC criteria described by Knuton et al. (2007), respectively. The blue shading between 40°N and 45°N in the RAS‐Default and RAS‐Ncalm experiments indicates the nudging region at the northern lateral boundary, which rapidly dampens small‐scale perturbations (e.g., TCs) that enter the region.