Storms And Hurricanes
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|El Niño and Tropical Cyclones|
Item 2 24-MAR-1998 17:20 Ann Close (aclose)
The following is modified from a publication of the Risk Prediction Initiative entitled, "Tropical Cyclones and Climate Variability: A Research Agenda for the Next Century". It describes the effects of El Nino on hurricane activity. If you're interested in receiving a copy of this publication, please email me at email@example.com.
El Niño greatly alters global atmospheric circulation patterns, affecting tropical cyclone activity around the globe. It affects tropical cyclone frequencies mainly by altering relative vorticity patterns in the lower atmosphere and by changing the vertical shear profile. High levels of relative vorticity impart spin to a tropical cyclone’s inflowing winds, allowing a storm to develop its characteristic pinwheel shape. Atmospheric pressure changes associated with ENSO also affect tropical cyclone frequencies. Over the western Pacific, high air pressure during an El Niño increases atmospheric stability and decreases convection, thereby reducing the frequency of tropical cyclones.
During an El Niño, tropical cyclones decrease in number near Australia (90°E to 165°E), but increase in the South Pacific east of 165°E. Tropical cyclones also tend to originate closer to the equator. The opposite is true during La Niña. Changes in tropical cyclone frequency during El Niño occur because a persistent low-pressure area known as the monsoon trough weakens near Australia but strengthens farther east. Thus widespread rotation and moisture convergence decrease near Australia, but increase in the South Pacific. The Northern Territory of Australia experiences a local change that opposes the regional tendency: fewer tropical cyclones and landfalls occur during La Niña than El Niño years. This occurs because the monsoon trough, albeit stronger, moves over land during La Niña. This steers incipient tropical depressions away from the oceanic moisture and heat source they need in order to develop.
The Northwest Pacific basin experiences a similar change in the location of tropical cyclone formation without a change in total frequency. During El Niño, fewer tropical cyclones form west of 160°E, but more develop from 160°E to just east of the dateline The opposite occurs during La Niña. Also during La Niña, more tropical cyclones form during mid-season in subtropical latitudes (20 to 30°N). Changes in the location and strength of the monsoon trough again appear to dictate the tropical cyclone variations, though this possible effect is undocumented.
In the western portion of the Northeast Pacific basin near Hawaii (140°W to the dateline) more tropical cyclones form during El Niño. In the year following an El Niño, more tropical cyclones track into the sub-region. Researchers have yet to analyze the opposite effects of La Niña, and the mechanism for such changes is unclear at this time.
While the just cited studies show that ENSO alters the frequency of tropical cyclone formation via changes in local conditions, the Atlantic basin feels the effects of ENSO remotely through changes in vertical shear. Vertical shear increases during El Niño events, primarily due to increased westerly winds in the upper troposphere (Figure 9). During La Niña events, westerly winds at this level decrease, thus reducing vertical shear. Due to these effects, it is two times more likely for two or more landfalling hurricanes to strike the US during a neutral year than during an El Niño year.
In the remaining basins—the eastern portion of the Northeast Pacific (the North Pacific Ocean from 140 °W to North America), the Southwest Indian, and the North Indian—no apparent correlation exists between ENSO and variations in tropical cyclone activity. This may be a statistical artifact of the small number of tropical cyclones in these regions. Because the number of storms is small, variations in storm frequency due to ENSO may have the same order of magnitude as variations due to other natural causes and thus may not be statistically significant.
Is that Risk Prediction Initiative the effort that BBSR is involved in?
Appreciate this additional information about El Nino and La Nina's effects on hurricanes/cyclones. Thanks.