June 28th, 2017

North Atlantic Basin Tropical Storm Outlook for 2017: Normal to Above Average: Part II: Factors Influencing 2017 Forecasts

Posted at 1:00 AM ET

El Niño Southern Oscillation

The El Niño Southern Oscillation (ENSO) is a phenomenon of shifting sea-surface temperatures (SSTs) in the Tropical East Pacific. The warm phase, or “El Niño,” is related to elevated wind shear in the Tropical Atlantic Basin, which disrupts tropical cyclone formation, usually with a reduction in hurricane counts for the Atlantic Basin. The cool phase, or “La Niña,” is related to elevated hurricane counts in the Atlantic Basin due to reduced wind shear.

Conditions have relaxed to ENSO-neutral after weak La Niña conditions over the past winter. There is intermittent evidence that a weak El Niño might develop. However, model guidance shows considerable spread, and there is some question on whether a weak El Niño would bring sufficient wind shear in the Atlantic Basin to suppress hurricane development.

The National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC) has adjusted their outlook to account for a lack of anything exciting in the tropical Pacific Basin to indicate a developing El Niño. The CPC also notes that the effects of a developing El Niño on atmospheric circulation (including wind shear) may not be of substance this summer. All else being equal, this development would be associated with near-normal hurricane activity in the Atlantic Basin.

Atlantic Sea-Surface Temperatures

In the Atlantic Main Development Region (MDR), where a majority of Atlantic hurricanes form, observed SSTs have been trending warmer than average while model SSTs for the peak of hurricane season show considerable spread. This pattern could change over the coming months, but with all else equal, ongoing warm SSTs in the MDR would be associated with elevated hurricane activity in the Atlantic Basin. Long-term indicators of the Atlantic Multidecadal Oscillation are still unclear as to whether a transition has occurred from the warm/active phase that began in 1995.


Basin to Landfall Ratio

The basin to landfall ratio is highly volatile on an annual basis; impactful hurricanes have occurred even during quiet years. Hurricane Audrey (1957) rendered severe impacts to the Northern Gulf during a season with only three observed hurricanes in the basin. The impact of Hurricane Andrew (1992) is a matter of historical record. The 2010 season was very active with 12 hurricanes in the basin, yet not a single U.S. landfall occurred. Hurricane landfall is influenced by the location of hurricane development and steering currents in play during the hurricane’s lifecycle; neither of these is reliably predictable on long timescales.


Link to Part I>>

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