Atlantic versus Eastern Pacific: Lopsided hurricane season to continue

As of August 19, 2014, there have been six times the number of named tropical systems in the Eastern Pacific Basin compared to the Atlantic Basin: 12 versus 2.

While the Atlantic is likely to catch up to some extent over the balance of the season, the difference is likely to remain large.

In the Eastern Pacific, AccuWeather is expecting newly formed Tropical Storm Lowell to be followed by a more intense tropical system this weekend into next week.

According to AccuWeather Hurricane Expert Dan Kottlowski, “Moisture from one or both features in the Eastern Pacific could flow into the southwest U.S. starting later this week and this could lead to some heavier rainfall.”

On the Atlantic side, a cluster of thunderstorms that moves away from the East Coast of the United States will be monitored for development late this week. Another system could form much farther southeast over the Atlantic Basin, from one of three disturbances moving westward.

There is the potential for development in the Caribbean and Gulf of Mexico next week from one of the surviving disturbances. The environment may be more favorable for development in this area by next week.

Since early this spring, AccuWeather has been forecasting a lower-than-average number of tropical storms and hurricanes in the Atlantic, along with an above-average number of the same for the Eastern Pacific

According to Long Range Expert Paul Pastelok, “Despite the low numbers forecast for the Atlantic, there is still a risk of a strike along the central and western Gulf coast and along the southern Atlantic coast during the next couple of months.”

Hurricane season continues until Nov. 30 in the Atlantic and Eastern Pacific. It is not impossible for a tropical storm or hurricane to occur in December.

On average, there are 11 named systems (tropical storms and hurricanes) per year in the Atlantic and 15 in the Eastern Pacific.

While the Eastern Pacific is ahead of schedule, the Atlantic is actually running close to where it should be at this point in the season in terms of the number of tropical storms and hurricanes.

Factors Behind the Hurricane Season So Far

“The greatly lopsided tropical activity may be due, in part, to the very early effects of El Niño,” Kottlowski said.

During an El Niño, waters over the tropical Pacific are warmer than average and can support more tropical systems.

Meanwhile, over the Atlantic during an El Niño pattern, there is often a zone of strong winds aloft along the coast of North America. This wind shear can not only limit the intensity of tropical systems but can also prevent their formation.

“The effects extend beyond what can be explained by El Niño,” Kottlowski said.

“In addition to an unusually large zone of wind shear extending from the Caribbean through the western part of the Atlantic, there has also been a very large and persistent zone of dry air, dust and cool water.”

The dry air is being pumped by a very large zone of high pressure centered over the central Atlantic.

The dry air has been occasionally picking up dust from Africa and Spain. Both of these factors work against drenching showers and storms that initially fuel tropical systems then later allow them to thrive.

“The circulation around that high pressure area has not only been forcing tropical systems unusually far south over the Atlantic, but also causing cool waters from the depths to churn up to the surface,” Kottlowski said.

The cool surface water slows the development of tropical systems and can lead to weakening of an established tropical storm or hurricane.

There are other factors that contribute to the frequency of tropical systems in one ocean basin versus the other.

One is a phenomenon discovered in the early 1970s, known as the Madden-Julian Oscillation (MJO).

This pulse of moisture travels very slowly eastward around the globe in the tropics and can lead to a flare of several tropical systems in a short period of time. Away from the MJO, there can be a lull in other basins.

“This pulse may explain the uptick in tropical systems in the eastern Pacific forecast over the next week, and it is possible it could lead to an uptick in the Atlantic shortly thereafter,” Kottlowski said.

Other impacts on tropical systems include fluctuations in ocean temperatures in the Atlantic and in other parts of the Pacific that may last from months to years to a decade or more.