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Saturday, December 19, 2009

Blame El Nino for 2009 tropical season (Part 3)

In this essay, we have attempted to explore some of the effects of El Nino on the formation and intensity of tropical storms and hurricanes. Part 1 and Part 2 both focused on the mild Atlantic hurricane season we experienced in 2009. The effect of El Nino on the Atlantic season was a marked increase in vertical wind shear which generally tended to prevent storm formation or break up a storm before it could it could get too strong or last too long. Of course there were a few exceptions but this was generally true.

The story in the Pacific basin was quite different, of course. As we documented in our 2009 Pacific Tropical Wrap-up, activity in both the Eastern Pacific Ocean and the Western North Pacific Ocean were higher than typical.

The Eastern Pacific saw 18 names storms with 7 hurricanes including 4 major hurricanes. During the previous 10 years, the average number of named storms per year was just under 15 with slightly less than 7 hurricanes. Interestingly enough, the years preceeding 1997 had more named storms than 1997 - 2008 but less than the 18 storms of this past year AND more of those storms became hurricanes (9.2 on average compared to 7 this year).

This increase in storm activity can easily be linked to El Nino. The most well known affect of El Nino is the movement of warm water from the western Pacific to the eastern Pacific along the equator. This pool of warmer than normal water extends fairly far north. Its volume is larger than all of the water in the great lakes and it is larger in area than that of the continental US.

Seven out of nineteen named storms formed in the eastern Pacific off the coast of Mexico (this includes Tropical Storm Lana which formed as a depression in the eastern Pacific and travelled to the Central Pacific towards Hawaii). Additionally 11 eastern Pacific storms actually formed as tropical depressions or tropical lows in the Atlantic basin and maintained enough tropical characteristic that once they reached the Pacific Ocean, they could develop into full fledged storms.

In addition to the warm water pool shifting from west to east during an El Nino event, the atmospheric pressure in the region also shifts in the opposite direction. This effect is known scientifically as the El Nino/Southern Oscillation (ENSO). The pressure is typically higher in the eastern Pacific than in the west. This pressure gradient generates the tradewinds across the ocean.

During an El Nino event, the pressure in the east decreases and the pressure in the west increases. This reduction in the tradewinds may be the driving force that pulls the warmer water eastward. (Or rather the tradewinds drive the water westward where it is heated and the lack of tradewinds allows this warm water to slosh back to the east). Researchers do not know if the pressure changes causes the water temepratures to shift or if the temperature shifts cause the reduction in the trade winds.

El Nino / Southern Oscillation (ENSO) Phenomenon (Max Planc Intitut fur Meteorologie)
The 'Southern Oscillation' is a see-saw in air pressure between the low pressure over south-east Asia and the higher pressure over the south-east Pacific. This pressure difference determines the strength of the trade winds along the equator in the Pacific. We know, however, that the surface temperature of the equatorial Pacific is affected by the strength of the trade winds. The trade winds force cold water to upwell to the surface along the coast of South America and the equator in the eastern Pacific - this is the cause of the relatively low sea surface temperatures in these regions.

The warming of the east Pacific that comes at the start of an El Niño event reduces the east-west temperature difference along the Equator, which in turn reduces the pressure difference between these areas – the pressure in the west increases from its low state and the high pressure in the east sinks. This weakens the trade winds, resulting in less cold water being upwelled in the eastern Pacific. As less cold water is being upwelled, sea surface temperatures in this region warm more, further reducing the east-west pressure gradient and the trade winds. This cycle of interactions between the ocean and atmosphere eventually results in a full El Niño event, with unusually high temperatures in the east Pacific and the stilling of the trade winds. A La Niña event develops in an analogous fashion, but with the processes happening in the opposite way (cooling water in the east strengthens the pressure difference and the trade winds and cools further).

Regardless of whether the wind or the temperature shift is the cause and which is the effect, one thing is for certain. As the trade winds die down, the amount of vertical shear decreases significantly. So in the eastern Pacific you have warmer sea surface temperatures with lower wind shear. Perfect conditions for tropical storm formation and development.

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