El Nino and La Nina events and North Atlantic tropical cyclones
Hildebrand, Paula E.
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We have examined the impacts of El Nino (EN) and La Nina (LN) events on North Atlantic tropical cyclones (TCs), and the physical mechanisms that produce these impacts. We constructed composites of best- track data from the National Hurricane Center and reanalysis data from the National Centers for Environmental Prediction based on ten EN and ten LN events that occurred during 1970-1999. We analyzed the differences in the composite TC numbers, formation sites, and tracks during EN and LN events for several periods within the North Atlantic TC season (June-November). The largest differences occurred in the middle (July-September) and late (September-November) portions of the TC season. Throughout almost all of the season, there were more TC formations during LN events than during EN events, especially in the tropical North Atlantic (about 1O-20 deg N). However, in the late season, there were more formations during EN events in the subtropical North Atlantic (about 20-30 deg N). The formation site differences appear to have been mainly the result of lower vertical shear in the tropics during LN events, and lower vertical shear in the subtropics during EN events. The vertical shear differences in the tropical North Atlantic were mainly the result of anomalies in upper tropospheric heights and the tropical easterly jet associated with variations of the Asian summer monsoon. The vertical shear differences over the subtropical North Atlantic were mainly the result of an extratropical anomalous wave train extending from the western North Pacific to the North Atlantic. The differences in formation sites appear to have led to TCs with longer tracks, longer residence times within a tropical environment, and greater intensities during LN events. There were a larger (smaller) number of TCs making landfall in the Gulf of Mexico and eastern U.S. during LN (EN) events during the late season.
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