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For decades NASA researchers

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have helped refine our understanding of hurricanes and tropical cyclones.

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Among their tools are computer models

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that can help distinguish the roles played by sea surface temperature, pressure and wind speed.

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These model studies show that what makes a busy or slow Atlantic hurricane season

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largely relies on three ocean patterns.

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These three factors contribute to either

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a strong or a weak hurricane season, each acting a little like an on/off switch.

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First, the El Niño Southern Oscillation’s two phases:

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El Niño and La Niña affect sea surface temperature in the Pacific Ocean,

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which then alters wind strength in the Atlantic.

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During El Niño years, Pacific sea surface temperatures near the equator

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are generally warmer than average, which drive winds that shear the tops off hurricanes,

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making it harder for them to form. Cooler Pacific water during La Niña years

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creates more favorable hurricane wind conditions.

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The North Atlantic Oscillation’s two phases: positive and negative

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describe the difference in pressure between two poles of a pressure system

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– low pressure near Iceland and high pressure near the Azores Islands.

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When the pressure difference between them is lower than average, hurricanes are more likely to form.

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When the two pressure systems are more intense than average,

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wind conditions are not conducive to hurricane formation.

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Further south, the two phases of the Atlantic Meriodonal Mode describe sea surface temperatures:

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either warmer or cooler. When the area north of the equator is warmer than average,

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hurricanes are more likely to form.

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When the same area is cooler, hurricanes are less likely to form.

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Together, these factors can make or break a strong hurricane season,

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and understanding how they interact can help better predict the number and strength

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of hurricanes seen throughout the season.

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For example, in 2005, the Pacific sea surface temperatures were average, so the El Niño Southern Oscillation played very little role.

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The North Atlantic Oscillation was mildly negative,

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which influenced winds more favorably for hurricanes.

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The Atlantic Meridional Mode was the strongest influencer this season,

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with very warm Atlantic water temperatures leading to a busy and powerful hurricane season,

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which included Hurricanes Katrina, Rita and Wilma.

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In 2013, the El Niño Southern Oscillation favored hurricane formation at the beginning of the season,

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with a weak La Niña contributing cool water in the Pacific,

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a neutral Atlantic Meridional Mode and a negative North Atlantic Oscillation

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Oscillation setting up favorable hurricane conditions.

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However, in mid-July, the North Atlantic Oscillation suddenly changed to a strongly positive intensity,

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creating unfavorable wind conditions and limiting hurricane formation in the Atlantic.

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(beeping)