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What will the 2016 Atlantic hurricane season bring?





 

Last year marked the 10th consecutive year without a major hurricane landfall in the United States. This is a record worth considering from a predictive capacity in light of the upcoming 2016 Atlantic hurricane season.

Recall that last season was characterized by both below-normal landfall and basin activity, which has occurred four times (in 2006, 2009, 2014 and 2015) since the overly productive years of 2004 and 2005. The 2015 season was influenced by a full-blown El Niño — a phenomenon that occurs every two to seven years, during which warm water in the western tropical Pacific extends eastward to South America — and typically contributes to below-normal tropical cyclone frequency in the North Atlantic and above-normal activity across the Pacific. Last year, there was record-breaking high activity across the Eastern and Central Pacific.

The well-known impact of El Niño — increased vertical wind shear over the Atlantic main development region that is detrimental to hurricane formation — is opposite to that of La Niña, the condition expected to emerge this season. It is too early, however, to say whether one will actually develop. The unusually cold ocean conditions in the equatorial Pacific that occur during La Niña produce lower vertical wind shear over the main development region, creating a much more favorable environment for hurricane formation.

El Niño and La Niña events are defined by the Oceanic Niño Index, which effectively quantifies the magnitude of the sea surface temperature anomaly over the equatorial Central and Eastern Pacific — known as the Niño 3.4 region (Figure 1).


Figure 1 (left).  A map of the Niño 3.4 region in which El Niño and La Niña events can occur. (Source: National Oceanic and Atmospheric Administration)

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









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

 

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







Table 1 shows the tropical cyclone activity and Oceanic Niño Index for these six pairs of historical years in the North Atlantic basin. Pairings in green indicate an increase in tropical cyclone activity as conditions transitioned from El Niño to La Niña, which has occurred only two-thirds of the time since 1951. Pairings in white indicate a decrease in activity or little change as conditions transitioned from El Niño to La Niña, and reveal a lesser change in Oceanic Niño Index for those pairings. In general, it may not be until the end of June — when a critical point is reached in the expected La Niña evolution — that the significance of any event’s impact on tropical cyclone activity will be known.

Another significant climate signal, the Atlantic Multidecadal Oscillation, must also be examined when considering the upcoming Atlantic hurricane season. A mode of variability in the North Atlantic Ocean that influences water temperatures over a timescale of multiple decades, the Atlantic Multidecadal Oscillation is classified in phases.

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


   

Figure 2 (left). The Atlantic Multidecadal Oscillation index showing positive (red) and negative (blue) phases, indicating a decrease in mean  sea surface temperatures from 2012-present. (Source: AIR; data source: NOAA)

Recently, the Atlantic Multidecadal Oscillation has been decreasing, as is shown in Figure 2. Since 2012, sea surface temperatures in the tropical and far North Atlantic have been lower on average and SLPs throughout most of the Atlantic have increased, creating conditions associated with a weaker thermohaline circulation akin to those seen in negative Atlantic Multidecadal Oscillation phases such as 1900-1925 and 1970-1994.






A closer look at the indices reveals many instances in which the Atlantic Multidecadal Oscillation has been negative for a short time before returning to positive, and a review of the historical record shows that the Atlantic Multidecadal Oscillation has gotten a slight positive boost every time the atmosphere transitioned to La Niña, even when it had been negative. If that happens again this year, it is more likely that the Atlantic Multidecadal Oscillation will remain positive and Atlantic sea surface temperatures may be slightly higher than they were last year. Coupled with lower vertical wind shear from a developing La Niña, these conditions provide a more favorable environment for tropical cyclone genesis over the main development region of the North Atlantic.

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

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

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

A similar scenario unfolded in 1983, which had only four named storms and one of them was Category 3 Hurricane Alicia that hit the Houston-Galveston area and caused almost as many direct fatalities there as Andrew. At the other end of the spectrum, 2010 had 19 named 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, it will be the first time since 2012 that we have gone that far into the alphabet to name a U.S. landfalling storm. Whether or not La Niña comes to fruition, which may increase the odds of meeting Colin, remains to be seen.

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