El Niño's Odds to Return By Late Summer or Fall Increasing

Jonathan Belles and Brian Donegan
Published: March 23,2017
The odds of El Niño's development by the late summer or early fall have increased, according to the latest output from forecast model guidance.
NOAA's Climate Prediction Center (CPC) officially declared La Niña's end in early February as sea temperatures have steadily warmed in the equatorial region of the central and eastern Pacific, and we're now in the neutral phase of the oscillation. Neutral means that neither La Niña or El Niño conditions exist.
As shown below, models currently suggest we'll be in the neutral category through the spring and into the early summer months (April-May-June, or AMJ), but after that, sea temperatures could be warm enough for El Niño conditions to take over.
(MORE: When Do Temperatures Typically Warm Up?)

The chance for various phases of El Niño, according to IRI's mid-March model-based probabilistic forecast. Red bars show the probability of El Niño's development during each three-month period.
(International Research Institute for Climate and Society)

In the heart of hurricane season – August, September and October (ASO) – the chance for El Niño climbs to 67 percent, according to the International Research Institute for Climate and Society's (IRI) model-based probabilistic forecast.
The ECMWF (European) computer model currently has about 70 percent of its ensemble members suggesting a moderate or strong El Niño will develop by September.
"Since 1870, we haven't seen a second strong El Niño in such quick succession, so if 2017 turned out to be one, it would be unprecedented," Ben Noll of New Zealand's National Institute of Water and Atmospheric Research (NIWA) told weather.com.
NOAA's CPC currently forecasts a 53-percent chance of El Niño's development sometime September through November.
(MORE: Could Leftover Heat from the Last El Niño Fuel a New One?)
However, El Niño/La Niña model forecasts this time of year are very uncertain, as NOAA's CPC cautioned in a blog, due to the "Spring Predictability Barrier." This is because spring is a transitional time of year, which makes it difficult to forecast a change to a new phase.
El Niño is defined as a season-long, or longer, warming of the eastern central Pacific waters close to the equator by at least 0.5 degrees Celsius. La Niña, like its counterpart El Niño, often has major effects on weather patterns around the world. El Niño and La Niña occur naturally in periods of two to seven years, with varying predictable effects around the globe – including hurricane activity.
These effects are generally predictable, but not all El Niños are created equally, especially when it comes to the atmosphere. El Niño, as mentioned before, is a change in the ocean, not a straightforward atmospheric phenomenon.
The ocean and our atmosphere respond to one other in a loose relationship, which can result in slightly different effects from one El Niño to the next. For example, last year's strong El Niño failed to result in significant rainfall across Southern California, where it's typically expected in the winter during that phase.
(MORE: Does a Warm Winter Lead to an Early Severe Peak?)

Sea temperatures in the 2015 El Niño, where red signifies warmer-than-average temperatures.
(NASA)

History suggests most El Niños inhibit hurricane growth through a couple of different ways.
Due to the warmer waters in the eastern Pacific, air rises above that water, just like it would over a warm bowl of soup. That air cannot just go out to space.
"What goes up must come back down" is a good way to explain what happens in parts of the Caribbean and western Atlantic. That air cools and descends on the northern side of Central America, stunting thunderstorm growth and stabilizing the weather in the region.

The effects of El Niño in the eastern Pacific, Caribbean and western Atlantic Ocean.

The winds that cross Central America are faster than without El Niño. This enhanced wind takes the tops of the tropical clouds eastward faster than at the surface, which limits vertical thunderstorm growth.
(MORE: Arctic Sea Ice at a Record Low; What Happens When It All Melts?)
The low-level trade winds are also enhanced. These winds blow toward the west, pushing showers and storms westward faster than usual. This, combined with enhanced upper-level winds, tilts storms eastward, further hampering storm growth.
These effects aggregate into a less-active hurricane season, but this is what typically occurs, rather than a hard rule.

Hurricane Activity by ENSO Phase (1900-2009, unless otherwise stated)
From Klotzbach, 2010

	Named Storms	Hurricanes	Hurricanes ≥ Category 3
Average (1966-2015, AOML)	11.7*	6.3*	2.4*
El Niño	7.5	4.0	1.5
Neutral	9.5	5.4	2.1
La Niña	11.6	6.5	3.2

*Note that these averages are slightly higher in recent decades and "a more realistic estimate of the long-term climatology may be closer to 13 tropical storms," according to the Hurricane Research Division.

It's too early to know whether the latter half of the 2017 hurricane season will be impacted by any developing El Niño. Also unknown is how much of the aforementioned atmospheric response would occur before the end of the season, even if El Niño did develop. In the months ahead, we should have a clearer picture of the ultimate outcome.
Hurricane season in the Atlantic Basin runs from June 1 to Nov. 30, but tropical cyclones – including hurricanes and tropical storms – can occur any time of the year.