We examined the impact of ENSO on the activity of tropical cyclones (TCs) in the North Atlantic using 71-yr outputs simulated in a high-resolution (T213) coupled ocean-atmosphere general circulation model (CGCM). The high-resolution model simulated TCs with a maximum surface wind speed at 10 m more than 17 m/s, although there are no TCs with a maximum surface wind more than 43 m/s belonging to the category 2 on the Saffir-Simpson scale. The model also simulated quasi-periodic (about 5 years) ENSO-like events with amplitudes of Nino34 SST anomaly twice as large as observed. The spatial structure in the model anomalies shows good agreement with the observed one.
As in the observations, the annual frequency of model TCs as well as hurricanes over the North Atlantic increases (decreases) in model La Nina (El Nino) years. The increase (decrease) in model TCs is related to the reduction (enhancement) of vertical wind shear over the North Atlantic induced by a cold (warm) SST anomaly in the eastern equatorial Pacific. The upward trend found in model TC frequency, on the other hand, seems to be related to SST warming in the North Atlantic.
Recent papers have been focusing on the association between TC activities and global warming. Emanuel (2005) and Webster et al. (2005) suggested that the recent upswing in Atlantic TC activity since 1995 is related to SST warming in the North Atlantic, resulting from anthropogenic climate change, while Goldenberg et al. (2001) and Landsea et al. (2006) attributed the change to natural climate variability termed the AMO. On the other hand, Knight et al. (2005) suggested that changes in Atlantic thermohaline circulation could cause the AMO but the associated SST changes may be not enough to explain the recent increase in SST over the North Atlantic. It is certainly necessary to enhance our understanding of how global warming affects TC activities from both observational and modeling perspectives. Although the relationship between ENSO and TC frequency in the North Atlantic is simulated reasonably well in this high-resolution CGCM, there is still the deficiency in simulated TC intensity, therefore suggesting that further finer-resolution model would be required to simulate realistic TC intensity. Here we have demonstrated that the use of a finer-resolution GCM could be a powerful tool for understanding the future TC activities.
Acknowledgments The authors would like to thank Dr. M. J. McPhaden for helpful suggestions. This work is supported by the Research Project for Study on extreme weather events and water-related disasters due to Climate Change and Study on long term prediction of typhoon disasters under NIED, and Suitable Coexistence of Human, Nature, and the Earth under the Japanese Ministry of Education, Sports, Culture, Science, and Technology. The work is partially supported by Global Environment Research Fund of the Ministry of the Environment, Japan.
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