By Alex Sosnowski | Senior Expert Meteorologist for AccuWeather.com
AccuWeather reports recent indications continue to point toward a building El Nino, a pattern that can greatly impact the second half of the hurricane season.
El Nino is part of the El Nino Southern Oscillation (ENSO), which represents a cyclical variation in sea-surface temperatures over the tropical Pacific Ocean.
During the El Nino phase, water is warmer than average in the tropical Pacific. During the La Nina phase, like the past two winters, water is colder than average over the same area.
Both phases of ENSO, along with other factors, can have profound effects on weather patterns around the globe. Although the numbers of both are nearly equal, there are slightly more La Nina than El Nino patterns on record. The strength of both can vary significantly from one similar phase to another.
Simply put, during an El Nino, air is generally rising over the tropical Pacific and generally sinking over the tropical Atlantic.
More technically, wind shear is generally lower on the Pacific side and often higher on the Atlantic side, based on the setup of strong steering winds high in the atmosphere known as the jet stream.
Rising air and low wind shear favors tropical storm and hurricane development, while sinking air and wind shear inhibits it.
Depending on how quickly El Nino develops, there could be a quick shutdown of tropical systems during the latter part of the Atlantic season and tropical cyclones galore and an extended season on the Pacific side.
According to Tropical Weather Expert Dan Kottlowski, “We continue in a lull of activity in the Atlantic now, but not necessarily from a developing El Nino.
“During much of July, we usually see a rather routine separation of the main jet stream with the Atlantic, which often results in a quiet period in terms of tropical cyclones. We currently have disrupting dry air, wind shear and cool waters over the tropical Atlantic.
“Since the jet stream has departed to the north, there are no old frontal zones and upper-level disturbances left in the region from which tropical systems typically form earlier in the season.”
These features are deposited by the jet stream.
The main driver of tropical systems during the second half of the hurricane season is the flow of disturbances coming off of Africa, which pass near the Cape Verde Islands in the Atlantic.
This part of the season is also called the Cape Verde season.
The disturbances pick up moisture and often intensify as they move westward over the warm tropical Atlantic waters.
The Cape Verde season ramps up during the second half of the summer and reaches a peak in the early autumn. It often results in long-tracking, powerful hurricanes with Andrew, Hugo and Gloria to name a few.
If the pattern continues with the development of El Nino late in the summer and fall, a number of disturbances could tiptoe along across the Atlantic, only to ramp up near the East and Gulf coast of the United States, where waters are generally much warmer than average.
There is the possibility that the Atlantic season may be truncated somewhat earlier than average this year due to a moderate El Nino forecast by AccuWeather.
However, there could be a pack of formidable storms over several weeks spanning August into September, before the full effects of El Nino come into play.
Only if neutral conditions were to persist, or El Nino only reaches a weak status late in the game, then there would be less truncation and perhaps a more typical length of the Atlantic and Pacific hurricane season.
If El Nino were to crank up strongly early on, it could cut into overall numbers of tropical storms and hurricanes, despite already four (tropical storms) through June.
Experts expect the number of tropical systems to be near-normal by the end of the season factoring in the early-season and likelihood of the mid-August to mid-September spike.
With wind shear, dry air and dust issues now and later over the central Atlantic, there is still the danger of near-U.S. coast hurricane formation and impact mid- to late-season.
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