Extratropical Impacts on Atlantic Tropical Cyclone Activity

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Numerical experiments are carried out to explore the impacts of local and remote forcing on the interannual variability of tropical cyclone (TC) frequency. The first two groups of experiments focus on the regional simulations of Atlantic TCs, and the lateral boundary conditions and sea surface temperature (SST) are specified to investigate the relative importance of remote and local forcing. The results suggest that remote processes outside the North Atlantic, particularly extratropical processes, play an important role in modulating Atlantic TC frequency and that the remote impacts may exceed the impacts of local SST in some years. The total TC frequency in the northern tropics is explored in the third group of experiments. In contrast to the North Atlantic, tropical SST plays a dominant role in modulating the total TC frequency in the northern tropics. The difference may help to explain the uncertainties in the projections of future Atlantic TC frequency.

Section: Introductionmentioning

confidence: 99%

Relative Impacts of Local and Remote Forcing on Tropical Cyclone Frequency in Numerical Model Simulations

Chang

Wang

2018

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“…The predictability, at least for short‐range predictions, appears largely limited by the atmospheric environment variables rather than the SST. Consistent with this finding, earlier studies suggest that the variability of extratropical atmosphere is not strongly constrained by the oceanic forcing (Kushnir et al, ; Lee et al, ; Sutton & Hodson, ) and is associated with lower prediction skills of TC activity (Bell et al, ; Fitzpatrick et al, ; Li et al, ; Wang et al, ; G. Zhang, et al, ). Overall, the skill of the FLOR‐FA prediction appears higher in regions where the potential predictability is higher.…”

Section: Summary and Discussionmentioning

confidence: 52%

Dynamical Seasonal Prediction of Tropical Cyclone Activity: Robust Assessment of Prediction Skill and Predictability

Zhang

Murakami

Gudgel

et al. 2019

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Improving the seasonal prediction of tropical cyclone (TC) activity demands a robust analysis of the prediction skill and the inherent predictability of TC activity. Using the resampling technique, this study analyzes a state‐of‐the‐art prediction system and offers a robust assessment of when and where the seasonal prediction of TC activity is skillful. We found that uncertainties of initial conditions affect the predictions and the skill evaluation significantly. The sensitivity of predictions to initial conditions also suggests that landfall and high‐latitude activity are inherently harder to predict. The lower predictability is consistent with the relatively low prediction skill in these regions. Additionally, the lower predictability is largely related to the atmospheric environment rather than the sea surface temperature, at least for the predictions initialized shortly before the hurricane season. These findings suggest the potential for improving the seasonal TC prediction and will help the development of the next‐generation prediction systems.

“…In some cases, the lack of the extratropical influence allows the Fujiwhara effect to dictate storm motion. The difference might arise because they emphasized lateral boundary forcing from the 2013 season, which featured extratropical anomalies that were exceptionally strong and extended into the deep tropics (Bell et al, 2014;Zhang et al, 2016;Zhang et al, 2017). Finally, storm lysis in the Ncalm simulations occurs farther east and less frequently along the northern boundary than in the Default simulations.…”

Section: Responses To Modified Lateral Boundary Forcing: Climatologymentioning

confidence: 94%

“…Since the Default and the Ncalm experiments share the same SST forcing, the environmental changes can be causally attributed to the suppression of the weather perturbations at the northern boundary. The moistening tendency may occur because the suppression of the tropical-extratropical interaction reduces the mixing of the high-latitude dry air into the subtropics (e.g., Dunion, 2011;Galewsky et al, 2005;Zhang et al, 2016). The moistening tendency may occur because the suppression of the tropical-extratropical interaction reduces the mixing of the high-latitude dry air into the subtropics (e.g., Dunion, 2011;Galewsky et al, 2005;Zhang et al, 2016).…”

Section: Responses To Modified Lateral Boundary Forcing: Climatologymentioning

confidence: 99%

“…Based on idealized sensitivity experiments with the 1982 and 1995 hurricane seasons, they suggested that the high-frequency "weather" variations, defined as anomalies relative to seasonal mean fields in the domain interior and at the lateral boundaries, are only of secondary importance for the observed interannual variability in TC statistics. However, there is substantial overall evidence from previous studies suggesting that TCs are sensitive to the weather perturbations contained in the lateral boundary forcing (Hanley et al, 2001;Leroux et al, 2016;Peirano et al, 2016;Zhang et al, 2016Zhang et al, , 2017. They found that the skill of the two simulations did not differ much and suggested that the interannual variability of TC activity was generally insensitive to the lateral boundary forcing.…”

Section: Introductionmentioning

confidence: 97%

See 1 more Smart Citation

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

Zhang

Knutson

Garner

2019

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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°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 SummaryRecent 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°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.