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

Extratropical weather perturbations have been linked to Atlantic tropical cyclones (TCs) activity in observations. However, modeling studies of the extratropical impact are scarce and disagree about its importance and climate implications. Using a nonhydrostatic regional atmospheric model, we explore the extratropical impact by artificially suppressing extratropical weather perturbations at the tropical-extratropical interface. Our 22-year simulations of August-October suggest that the extratropical suppression adds 3.7 Atlantic TCs per season on average, although the response varies among individual years. The TC response mainly appears within 30-40 degrees N, where tropical cyclogenesis frequency quadruples compared to control simulations. This increased cyclogenesis, accompanied by a strong increase of midtropospheric 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. Plain Language Summary Recent observational studies suggest that Atlantic hurricane activity is strongly affected by extratropical weather processes, but modeling studies disagree about the importance of such an impact. Using a regional atmospheric model, we conduct idealized experiments to explore the extratropical impact on Atlantic hurricane activity. Our simulations of 22 hurricane seasons show that artificially suppressing extratropical weather perturbations at the northern boundary in the model can increase hurricane activity significantly. The response of the model's hurricane activity varies from season to season, which may contribute to the different responses found among earlier modeling studies. Notably, the strongest response of hurricane activity appears within 30-40 degrees N, where the number of hurricane formations quadruples compared to reference simulations. The perturbation reduction also helps hurricanes to move more slowly and smoothly with fewer disruptions from the high latitudes. These experiments, though idealized, suggest that the extratropical atmospheric circulation plays an important moderating role in limiting Atlantic hurricane formation. These experiments also could have implications for hurricane activity in the distant past or in coming decades, for example, if climate warming were accompanied by a weakening or poleward shift of extratropical weather systems.