| |
ENSO and North Atlantic HurricanesAtlantic WindshearENSO events alter the global circulation and change the upper level winds over the Atlantic (Arkin, 1982). During a El Niño year , the difference between upper and lower level winds, called wind shear, is larger in most of the tropical Atlantic, which inhibits the formation of hurricanes, while the opposite happens in a La Niña year. In Figure 8 the wind shear magnitude of composites for El Niño and La Niña years is shown. The main development region (MDR) (Goldenberg and Shapiro, 1996), where most hurricanes form is also shown in red. Wind shear magnitudes larger than 7.5-10m/s in the MDR region are unfavorable for hurricane development (Zehr 1992; DeMaria et al., 1993; Landsea et al., 1998). Higher values of wind shear are not shaded in Figure 8, the El Niño composite shows a larger white area in the MDR than the La Niña composite, indicating higher values of wind shear. In El Niño years there is a region of low shear on a latitude belt more north of the MDR region, this pattern leads to formation of less hurricanes in general and fewer storms that have African blasterly waves as their primary source, as was the case of 1991 (Pasch and Avila, 1992). In 1995, for instance, the Atlantic hurricane season was very active
(La Niña year) (Lawrence et al., 1998) with hurricanes
forming in latitudes equatorward of 25oN (Landsea et al., 1998) and a very
large number of them originated from African easterly waves (Pasch et al.,
1998). In contrast in 1997 (an El Niño year), the Atlantic tropical
cyclones were relatively few in number, short lived, and weak compared
to the long-term average, besides all but one tropical cyclone formed
at relatively high latitudes (between 22o-32oN) (Rappaport, 1999).
The composite of the average first positions in El Niño, neutral
and La Niña years also shows this latitudinal shift clearly.
|
