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Subject: G2) How does El Niño-Southern Oscillation
affect tropical cyclone activity around the globe?
Contributed by Chris Landsea
El Niño/Southern Oscillation (ENSO) - During El Niño
events (ENSO warm phase), tropospheric vertical shear
is increased inhibiting tropical cyclone genesis and
intensification, primarily by causing the 200 mb (12 km or
8 mi) westerly winds to be stronger (Gray 1984). La Nina
events (ENSO cold phase) enhances activity. Recently, Tang
and Neelin (2004) also identified that changes to the moist
static stability can also contribute toward hurricane changes
due to ENSO, with a drier, more stable environment present
during El Nino events.
Reference:
Tang, B. H., and J. D. Neelin, 2004: ENSO Influence on
Atlantic hurricanes via tropospheric warming. Geophys. Res. Lett.:
Vol 31, L24204. "
The Australian/Southwest Pacific shows a pronounced shift
back and forth of tropical cyclone activity with fewer tropical
cyclones between 145° and 165°E and more from 165°E
eastward across the South Pacific during El Niño (warm
ENSO) events. There is also a smaller tendency to have the
tropical cyclones originate a bit closer to the equator. The opposite
would be true in La Niña (cold ENSO) events. See papers by
Nicholls (1979),
Revell and Goulter (1986),
Dong (1988), and
Nicholls (1992).
The western portion of the Northeast Pacific basin (140°W to
the dateline) has been suggested to experience more tropical
cyclone genesis during the El Niño year and more tropical
cyclones tracking into the sub-region in the year following an
El Niño (Schroeder and Yu 1995)
, but this has not been completely documented yet.
The Northwest Pacific basin, similar to the Australian/Southwest
Pacific basin, experiences a change in location of tropical cyclones
without a total change in frequency. Pan
(1981), Chan (1985),
and Lander (1994) detailed
that west of 160°E there were reduced numbers of tropical
cyclone genesis with increased formations from 160E to the
dateline during El Niño events. The opposite occurred
during La Niña events. Again there is also the tendency
for the tropical cyclones to also form closer to the equator
during El Niño events than average.
The eastern portion of the Northeast Pacific, the Southwest Indian,
the Southeast Indian/Australian, and the North Indian basins
have either shown little or a conflicting ENSO relationship
and/or have not been looked at yet in sufficient detail.
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