عنوان مقاله [English]
Major hurricanes occur in the Atlantic Ocean every year over a seasonal period known as the Atlantic hurricane season. There is evidence to suggest that the hurricanes can be affected by the North Atlantic Oscillation (NAO), a low-frequency phenomenon occurring in a large region over the North Atlantic. In turn, hurricanes can affect the North Atlantic storm track by transition to extratropical cyclones in the midlatitude regions. The objective here is to investigate the relationship between hurricanes and the North Atlantic storm track through NAO index. For this, the correlation coefficient between daily NAO index and 6-hourly “accumulated cyclone energy” (ACE) index related to the hurricanes are computed and analyzed for July to October 2017–2019. Then, in the dynamical study using JRA-55 data and from the energy point of view, the vertically-averaged “eddy kinetic energy” (EKE) and the main terms involved in its dynamical evolution are computed for the hurricane season. Also, by selecting one of the major hurricanes in September, which has different conditions in terms of being affected by the North Atlantic Storm track and entering the midlatitudes, the relationships between hurricanes and the North Atlantic storm track are further investigated.
Results show that when hurricanes are active for only about a week, they are limited to the subtropical region and have a higher correlation coefficient (about 95%) with NAO. But when hurricanes are active for more than a week and involve an extratropical transition phase, they have a relatively lower correlation coefficient with NAO. Also, the long-term statistical study (1995–2019) shows that although the number of hurricanes in the positive phase of NAO is about 7% more than that in the negative phase, but the relative prevalence of the negative phase of NAO at the time of hurricane activity in the main development region is slightly higher than that of the positive phase. In addition, hurricanes in which all activity is in the positive phase of NAO stretch to the east coast of the United States and are reinforced there, while hurricanes that all of their activity coincide with the negative phase of the NAO, occur in the Sargasso Sea and the CapeVerde regions. Therefore, NAO phases affect hurricane track during extratropical transition.
The monthly mean values of the vertically-averaged EKE and the main dynamical terms of its time evolution equation in September show that eddy activity is weak during summer in the hurricanes activity zone; however, in the east coast of the United States and Canada, there are significant changes in the dynamical terms. Also, in the extratropical transition, the dynamical terms determining EKE evolution at the entrance of the Atlantic storm track have large amounts in the Labrador Sea due to deep convection, which suggests a significant energy exchange with hurricanes in this area. Another result is that baroclinic conversion and divergence of ageostrophic geopotential flux are the most important terms determining EKE evolution. Also, geographical location of the hurricanes during transition has a significant effect on the changes of EKE. If hurricanes are intensified in the east coasts of North America and Canada and occur at the same time in the positive phase of NAO, they could play a very important role in strengthening the North Atlantic storm track.