NASA - National Aeronautics and Space Administration

NASA oceanographers agree that the recent La Nina in the eastern Pacific Ocean is not expected to have an effect on the Atlantic hurricane season this year. That is good news, because normally a La Niña tends to increase Atlantic hurricane activity and decrease Pacific Ocean hurricanes.

Image right: This is an image of sea surface height measurements from NASA’s Topex/Poseidon and Jason satellites, during Hurricane Rita's trek in the Gulf of Mexico in Sept. 2005. This image shows ocean circulation patterns. Florida (right) and Texas and Mexico (left) are colored in gray. Red indicates strong circulation of warm waters. Sea surface height is a useful measure of potential hurricane activity because storm-fueling warm water is higher than surrounding cooler water. The area shown in red is approximately 35 to 60 centimeters (roughly 13 to 23 inches) higher than the surrounding Gulf. Credit: NASA/JPL/University of Colorado CCAR

Although La Nina occurs in the Pacific, it affects weather in the Atlantic Ocean as well, through changes in the winds. La Niña changes the wind patterns in the upper and lower levels of the atmosphere, which make it easier for hurricanes to form in the Atlantic and harder in the eastern Pacific. In the Atlantic, the winds that would normally tear a hurricane's circular motion apart are lessened but they increase in the eastern Pacific.

The National Oceanic and Atmospheric Administration's (NOAA) Climate Prediction Center is the federal agency that monitors La Nina conditions such as cooler than normal sea surface temperatures, precipitation and winds. According to their latest report on April 6, 2006, sea surface temperatures were warming back to normal. That latest report stated that during the month of April, sea surface temperatures were slightly cooler than normal in the extreme eastern equatorial Pacific, and conditions returned to near average in that region.

Image/animation left: This visualization shows the warm waters that Hurricane Katrina tracked through. The data is from August 23 through 30, 2005. The colors on the ocean represent the sea surface temperatures, and satellite images of the hurricane clouds are laid over the temperatures to clearly show the hurricane positions. Orange and red depict sea surface temperatures 82 degrees Fahrenheit and warmer, warm enough for hurricanes to form. Hurricane winds are sustained by the heat energy of the ocean, so the ocean is cooled as the hurricane passes and the energy is extracted to power the winds. The sea surface temperatures are 3-day moving averages based on the AMSR-E instrument on the Aqua satellite, while the cloud images were taken by the GOES-12 satellite. Credit: NASA/SVS

David Adamec, an oceanographer at NASA's Goddard Space Flight Center, Greenbelt, Md. said that "the current temperature signal at the end of April is near normal and the ocean surface temperature has not yet caused the atmosphere to respond in a La Nina-like way." Adamec used what is called a NASA coupled atmosphere-ocean land computer model. This model, developed at Goddard, is used for experimental forecasts of the ocean, land and atmosphere for periods 3-12 months in the future. The data used came from 2 NASA satellites: Jason and QuikSCAT. Jason provided sea-surface height information, and QuikSCAT provided surface wind data.

Adamec said that in order for La Nina to have an effect on the Atlantic Ocean hurricane season, it would have to exist for a much longer time, especially into peak hurricane season which is August and September.

Further, he said, another factor associated with La Nina is the Southern Oscillation Index, is also normal. The Southern Oscillation Index is an atmospheric pressure indicator of the large scale surface winds. "La Nina is already a memory," said Adamec.

Image right: La Niña is characterized by unusually cold ocean temperatures in the central equatorial Pacific. The colder than normal water is depicted in this image in blue. During a La Niña stronger than normal trade winds bring cold water up to the surface of the ocean. Credit: NASA

According to 12 major ocean-atmosphere computer models, the equatorial Pacific will be neutral to warm in August, when it really matters for hurricanes. August and September are the peak season for hurricane formation in the Atlantic Ocean. According to scientists, the atmosphere takes about two weeks to "react" to a change in ocean surface temperature.

Forecasters and other scientists still expect a greater than average number of Atlantic Ocean hurricanes this year, but La Nina will not be a factor in that. The more active season is expected because of other environmental conditions favorable to hurricanes, such as the location of the Bermuda high removing much of the wind shear in the western Atlantic that thwarts hurricanes, warm sea surface temperatures in the Gulf of Mexico.

Image left: This image shows both the El Nino and La Nina conditions in the central equatorial Pacific Ocean, as seen by NASA's TOPEX/Poseidon and Jason-1 satellites. El Nino's warmer waters are indicated in red in this 1997 image, and La Nina's cooler waters are indicated in blue in this 1999 image. Credit: NASA

La Niña also influences where Atlantic hurricanes form. During La Niña more hurricanes form in the deep Tropics from African easterly waves. Easterly waves are "long waves" in the atmosphere that occur between 5-15 degrees North that start in Africa and move across the Atlantic Ocean. About 60% of the Atlantic tropical storms and minor hurricanes originate from easterly waves.

According to NOAA, these systems have a much greater likelihood of becoming major hurricanes and of eventually threatening the U.S. and Caribbean Islands.

Bill Patzert, oceanographer at NASA's Jet Propulsion Laboratory in Pasadena, Calif. noted that, "The recent increased frequency of the hurricanes is thought to be part of a larger decades-long cycle of alternating increases and decreases of hurricane activity. The current busy hurricane cycle began in 1995 and could continue for another 10 to 25 years. For the U.S. East and Gulf coasts, the fading La Nina is a real good thing, but Atlantic sea surface temperatures are still very toasty. It's the summer conditions that will dictate the fall hurricane activity, and I suspect those forecasts will be modified."


Image above:These are images showing sea surface temperature (left) and sea surface heights (right). The dots indicate the location and intensity of Hurricane Katrina (circle data from the National Hurricane Center). In a) intensification of the storm is not related to sea surface temperature (from POES high-resolution infrared data). In b) the storm intensification agrees well with the highs in the sea surface height. Image credit: NOAA

Related Link:

+ El Niño/La Niña influence on Atlantic and Pacific hurricane seasons

 

 

Rob Gutro
Goddard Space Flight Center