The 2014 Atlantic hurricane season officially began on Sunday, June 1, and is set to run through Sunday, November 30, 2014. The National Oceanic and Atmospheric Administration (NOAA) just released its seasonal outlook, which forecasts a near-normal or below-normal season for the Atlantic hurricane region, which includes the North Atlantic, Caribbean, and Gulf of Mexico.

According to the report, the “high-activity pattern [of the last few years] is expected to be offset in 2014 by the impacts of El Niño.”

This sounds like good news . . . but what is El Niño and how does it influence hurricane activity in the Atlantic Ocean?

In the Pacific Ocean there exists a naturally occurring, large-scale ocean- atmosphere climate interaction called the “El Niño-Southern Oscillation” (ENSO) — an irregular, inter- annual (every few years) cycle of warming and cooling sea surface temperatures in the central and east equatorial Pacific, coupled with reversing surface air pressure between the eastern and western Pacific.

These changes in sea surface temperature and surface air pressure are, for the most part, simultaneous. During periods of high surface air pressure in the eastern Pacific, surface waters are pushed westward, allowing colder ocean waters to flow up from the deep. When the surface air pressure decreases, the warmer surface waters flow back to the east again.

As a result, ENSO fluctuates between two phases: warmer than average sea surface temperatures in the central and eastern Pacific (known as “El Niño”) and cooler than average sea surface temperatures in the central and eastern Pacific (known as “La Niña”).

Each phase typically lasts 9-12 months (developing in the summer, peaking in the fall, and decaying the following spring), but some may last for multiple years, and an El Niño or La Niña event usually occurs about every three to five years, with El Niño phases being more frequent.

And, although ENSO refers to a phenomenon in the Pacific Ocean basin, it is one of the primary controls on regional and global climate patterns.

In the United States, for example, El Niño years have increased winter precipitation and cooler winter temperatures in the southeast, warmer than average winter temperatures in the northwest, and a decrease in the number of tropical storms that develop in the Atlantic, Caribbean, and Gulf of Mexico.

La Niña years are often the opposite, with drier and warmer winters in the southeast, cooler than average winter temperatures in the northeast, and more favourable conditions for hurricane development in the Atlantic, Caribbean, and Gulf of Mexico.

But how does an El Niño event originating in the Pacific impact storm formation in the Atlantic?

Because ENSO is a phenomenon that involves both the ocean and the atmosphere, changes that occur in one ocean basin (the Pacific) will influence global atmospheric circulation patterns. During El Niño years, there is an increase in westerly winds in the upper atmosphere, creating high horizontal wind shear.

This wind shear effectively spreads the storms out over a larger area, removing the heat and moisture they need from the area around their core and preventing many tropical disturbances from developing into more significant storms.

During La Niña years there are reduced winds from the west. This creates conditions of low horizontal wind shear, which allows a storm to concentrate heat and moisture near its core and intensify.

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For more information on hurricane formation and the 2014 Atlantic Hurricane season, visit the Bermuda Weather Service or the NOAA National Hurricane Center.