Last year marked the 10th consecutive year without a majorhurricane landfall in the United States. This is a record worthconsidering from a predictive capacity in light of the upcoming2016 Atlantic hurricane season.

Recall that last season was characterized by both below-normallandfall and basin activity, which has occurred four times (in2006, 2009, 2014 and 2015) since the overly productive years of2004 and 2005. The 2015 season was influenced by a full-blown ElNiño — a phenomenon that occurs every two to seven years, duringwhich warm water in the western tropical Pacific extends eastwardto South America — and typically contributes to below-normaltropical cyclone frequency in the North Atlantic and above-normalactivity across the Pacific. Last year, there was record-breakinghigh activity across the Eastern and Central Pacific.

The well-known impact of El Niño — increased vertical wind shearover the Atlantic main development region that is detrimental tohurricane formation — is opposite to that of La Niña, the conditionexpected to emerge this season. It is too early, however, to saywhether one will actually develop. The unusually cold oceanconditions in the equatorial Pacific that occur during La Niñaproduce lower vertical wind shear over the main development region,creating a much more favorable environment for hurricaneformation.

El Niño and La Niña events are defined bythe Oceanic Niño Index, which effectively quantifies the magnitudeof the sea surface temperature anomaly over the equatorialCentral and Eastern Pacific — known as the Niño 3.4 region (Figure1).

Figure 1 (right). Click image toenlarge. A map of the Niño 3.4 region in which El Niño andLa Niña events can occur. (Source: National Oceanic and AtmosphericAdministration)

Positive Oceanic Niño Index values greater than 0.5°Ctypically indicate El Niño events, whereas values less than -0.5°Cindicate La Niña events. Very strong El Niños occurred in 1972-73,1982-83, 1997-98 and 2015-16, during which the Oceanic NiñoIndex exceeded 2°C at times. In these first three El Niñoevents, the transition to La Niña was steady and strong, reachingvalues near or below -1°C by the end of the year.  Thecurrent (January–March) Oceanic Niño Index is 1.9°C,decreasing from a peak of 2.2°C.

Related: One early forecast calls for below-averagehurricane season

From an Atlantic hurricane standpoint, it is important to notethat tropical cyclone activity was typically higher once thesethree historically strong El Niño events transitioned to La Niñas.The same pattern can be seen even if we consider three other pairedyears in which the El Niño portion was not very strong and wasfollowed by a transition to a moderate-to-strong La Niña; the LaNiña phase usually exhibited more activity in the NorthAtlantic.

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Table 1 (left). Clickimage to enlarge. Tropical cyclone activity and peakOceanic Niño Index for six pairs of historical years thattransitioned to moderate-to-strong La Niña events from El Niñoevents in the North Atlantic basin. (Source: AIR; data source:NOAA)

Table 1 shows the tropical cyclone activity and Oceanic NiñoIndex for these six pairs of historical years in the NorthAtlantic basin. Pairings in green indicate an increase in tropicalcyclone activity as conditions transitioned from El Niño to LaNiña, which has occurred only two-thirds of the time since 1951.Pairings in white indicate a decrease in activity or little changeas conditions transitioned from El Niño to La Niña, and reveal alesser change in Oceanic Niño Index for those pairings. Ingeneral, it may not be until the end of June — when a criticalpoint is reached in the expected La Niña evolution — that thesignificance of any event's impact on tropical cyclone activitywill be known.

Another significant climate signal, the Atlantic MultidecadalOscillation, must also be examined when considering the upcomingAtlantic hurricane season. A mode of variability in the NorthAtlantic Ocean that influences water temperatures over a timescaleof multiple decades, the Atlantic MultidecadalOscillation is classified in phases.

Positive Atlantic MultidecadalOscillation phases, such as those seen from 1878-1899,1926-1969 and since 1995 (Figure 2, right),are characterized by above-average far North and tropicalAtlantic sea surface temperatures, below-average tropical Atlanticsea level pressures and reduced levels of tropical Atlanticvertical wind shear — conditions known to create a more favorableenvironment for Atlantic hurricane formation andintensification.

Figure 2 (above). Clickimage to enlarge. The Atlantic Multidecadal Oscillationindex showing positive (red) and negative (blue) phases, indicatinga decrease in mean  sea surface temperatures from2012-present. (Source: AIR; data source: NOAA)

Recently, the Atlantic Multidecadal Oscillation hasbeen decreasing, as is shown in Figure 2. Since 2012, seasurface temperatures in the tropical and far NorthAtlantic have been lower on average and SLPs throughout most of theAtlantic have increased, creating conditions associated with aweaker thermohaline circulation akin to those seen in negativeAtlantic Multidecadal Oscillation phases such as 1900-1925and 1970-1994.

A closer look at the indices reveals many instances in which theAtlantic Multidecadal Oscillation has been negative for ashort time before returning to positive, and a review of thehistorical record shows that the Atlantic MultidecadalOscillation has gotten a slight positive boost every timethe atmosphere transitioned to La Niña, even when it had beennegative. If that happens again this year, it is more likely thatthe Atlantic Multidecadal Oscillation will remain positive andAtlantic sea surface temperatures may be slightlyhigher than they were last year. Coupled with lower vertical windshear from a developing La Niña, these conditions provide a morefavorable environment for tropical cyclone genesis overthe main development region of the NorthAtlantic.

Related: Get ready for what could be a busy hurricane seasonthis year

Although the Atlantic Multidecadal Oscillation and theOceanic Niño Index provide some insight, there is more toforecasting hurricane activity for the upcoming season.Many other factors are at play, including Sahelian drought, Saharandust, tropical mid-level moisture, easterly waves and other seasurface temperature anomalies over other portions of thebasin, to name a few. Organizations that provide preseasonhurricane outlooks currently project 11-16 named storms during theseason proper (not including Hurricane Alex in January). Some ofthe lower projections are likely influenced by the recent trend ofthe Atlantic Multidecadal Oscillation Index.

The historical record suggests we will know more about La Niñain just a couple of months, at which point forecasts are likely tobe more reliable. Even though the official hurricane season willhave begun, there will be many more months of potential activity tocome.

Landfall activity is a much lesser known quantity and,therefore, much more difficult to project. It is important tohighlight the uncorrelated behavior between active basin years andlandfall activity. The complexity of this relationship wasdemonstrated in 1992. While only six named storms formed, one ofthem was Category 5 Hurricane Andrew, which devastated southernFlorida and is one of the most destructive landfalling hurricanesin U.S. history.

A similar scenario unfolded in 1983, which had only four namedstorms and one of them was Category 3 Hurricane Alicia that hit theHouston-Galveston area and caused almost as many direct fatalitiesthere as Andrew. At the other end of the spectrum, 2010 had 19named storms and 12 hurricanes, but not a single U.S. landfall.

If Colin — the third name on the 2016 tropical cyclone list —forms this year and makes landfall even as a tropical storm, itwill be the first time since 2012 that we have gone that far intothe alphabet to name a U.S. landfalling storm. Whether or not LaNiña comes to fruition, which may increase the odds of meetingColin, remains to be seen.

Peter Sousounis is assistant vice president anddirector of meteorology for research at Boston-based catastrophemodeling firm AIR Worldwide.

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