JP3.7
Influence of the Amazon/Orinoco plume on Atlantic hurricanes
Kerry H. Cook, Cornell University, Ithaca, NY; and E. K. Vizy
The Amazon/Orinoco plume is the freshwater discharge of the Amazon and Orinoco Rivers into the western tropical Atlantic Ocean, which often generates warm SSTAs over 1 K. Regional model simulations are used to understand the influence of the plume SSTAs on climate and weather. Two pairs of regional model simulations with and without plume SSTs are discussed - the 1999 summer season and a case study of Hurricane Dennis in 2005. For the 1999 summer season, during which the plume was strong and well-defined, the presence of the plume is associated with a strengthening of the western Atlantic ITCZ and increased precipitation in the eastern Caribbean in the model simulations. In addition, the plume SSTAs cause a redirection of hurricane tracks, shifting storm tracks in the western Atlantic to the east and directing more storms into the Gulf of Mexico. In the simulation with the plume SSTAs, the number of hurricane-strength storms doubles. This increase in storm intensity does not occur directly in association with warm plume SSTAs. Instead it is associated with a modulation of the large-scale background steering flow, which is generally weaker when the plume SSTAs are present, slowing the propagation speed of the storms and allowing for the potential of further storm enhancement over the relatively warm waters of the Caribbean Sea and Gulf of Mexico. A slight increase in the numbers of storms in the simulation with the plume is not significant. Hurricanes Floyd and Gert are captured in the model simulation, identifiable from the waves from which they develop, and their simulated paths are improved when plume SSTAs are included. The same is true in the case study of Hurricane Dennis, in which the presence of plume SSTAs produces a storm track and intensity history much closer to the observations. The changes simulated in response to the plume SSTAs are related to the classic Gill/Matsuno response to off-equator tropical SSTAs, with important secondary responses to changes in diabatic heating distributions. The strengthening of the western Atlantic ITCZ and increased precipitation in the eastern Caribbean can be attributed to the Kelvin wave part of the response, which causes increased convergence in the vicinity of the SSTAs. Changes in the hurricane tracks are related to the Rossby wave part of the response, which places anomalous cyclonic flow to the northwest of the SSTAs, i.e., over the western Caribbean and the Gulf of Mexico, weakening the North Atlantic subtropical high in this region.
Joint Poster Session 3, Tropical Cyclones and Climate Change Poster Session
Monday, 21 January 2008, 2:30 PM-4:00 PM, Exhibit Hall B
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