Monday, March 26, 2007
A DEVELOPING LA NINA
Over the winter there was an anomalous warming of the waters off the equatorial Pacific Ocean off the South American coast. This is referred to as El Nino conditions. NOAA's Climate Prediction Center, which is part of the National Weather Service, declares the onset of an El Nino episode when the 3-month average sea-surface temperature departure exceeds 0.5oC in the east-central equatorial Pacific [between 5oN-5oS and 170oW-120oW].
As we head into spring the opposite conditions look to be developing; there is now anomalous cooling of the waters over the same region and this is referred to as La Nina conditions. One of the best correlations between weather events and an El Nino or La Nina pattern exists with Hurricanes.
La Nina produces easterly wind departures at upper levels of the atmosphere and westerly wind departures at lower levels, across the eastern tropical Pacific Ocean and tropical Atlantic. Over the eastern Pacific these wind patterns are in phase with those normally seen in the region, resulting in higher vertical wind shear. The eastern Pacific hurricane season is typically less active during La Niña because of the expanded area of high vertical wind shear.Across the tropical Atlantic these same wind patterns are opposite to those normally observed, and result in lower vertical wind shear. There tend to be more Atlantic hurricanes during La Niña because of this expanded area of low vertical wind shear.El Niño and La Niña also influence where the Atlantic hurricanes form. During El Niño fewer hurricanes and major hurricanes develop in the deep Tropics from African easterly waves. During La Niña more hurricanes form in the deep Tropics from African easterly waves. These systems have a much greater likelihood of becoming major hurricanes, and of eventually threatening the U.S. and Caribbean Islands.
Last year early forecasts were for a severe hurricane season, this was made before El Nino conditions developed. This is now the most attributable factor, as to why last year was so non-eventful. Hurricane season starts in June.
As we head into spring the opposite conditions look to be developing; there is now anomalous cooling of the waters over the same region and this is referred to as La Nina conditions. One of the best correlations between weather events and an El Nino or La Nina pattern exists with Hurricanes.
La Nina produces easterly wind departures at upper levels of the atmosphere and westerly wind departures at lower levels, across the eastern tropical Pacific Ocean and tropical Atlantic. Over the eastern Pacific these wind patterns are in phase with those normally seen in the region, resulting in higher vertical wind shear. The eastern Pacific hurricane season is typically less active during La Niña because of the expanded area of high vertical wind shear.Across the tropical Atlantic these same wind patterns are opposite to those normally observed, and result in lower vertical wind shear. There tend to be more Atlantic hurricanes during La Niña because of this expanded area of low vertical wind shear.El Niño and La Niña also influence where the Atlantic hurricanes form. During El Niño fewer hurricanes and major hurricanes develop in the deep Tropics from African easterly waves. During La Niña more hurricanes form in the deep Tropics from African easterly waves. These systems have a much greater likelihood of becoming major hurricanes, and of eventually threatening the U.S. and Caribbean Islands.
Last year early forecasts were for a severe hurricane season, this was made before El Nino conditions developed. This is now the most attributable factor, as to why last year was so non-eventful. Hurricane season starts in June.
Posted by Ted Zarras at 6:45 PM
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