The Northern Hemisphere saw record hurricane activity in 2015, but one basin — the Central Pacific — was especially impressive.
A typical hurricane season is relatively quiet in this portion of the Pacific, but this was certainly not the case in 2015. There were a total of 14 named storms, eight hurricanes and five major hurricanes that either formed in the Central Pacific or tracked into the Central Pacific in 2015.
This year shattered all of the records set in previous seasons: 10 named storms (set in 1982 and 1997), five hurricanes (set in 1982 and 1994) and three major hurricanes (set in 1994).
In addition, eight named storms actually formed in the basin, which doubled the previous record of four set in 1982. Lastly, the season’s total activity, measured by accumulated cyclone energy, also reached record high levels in 2015 (an ACE of 127, breaking the old record of 107 set in 1994).
Probably the most iconic satellite imagery of the extraordinarily active Pacific season was generated on Aug. 30, when three Category 4 hurricanes spanned the East and Central Pacific Ocean basins. This was the first time it had occurred in any location on record. Two of these Category 4 hurricanes (Kilo and Ignacio) were in the Central Pacific basin at the time.
What were the physical factors that caused the 2015 hurricane season to be so active in the Central Pacific? One of the primary culprits was the strong El Niño that developed. El Niño brings warmer than normal water to this region, which provides more fuel for strengthening hurricanes.
In addition, a combined atmosphere-ocean phenomena known as the Pacific meridional mode was strongly positive. This mode works through a combination of feedbacks between low-level winds, surface pressures and sea surface temperatures, that when strongly positive, cause waters in this area to be much warmer as well.
For comparison, while the 1997 El Niño was slightly stronger by most measures than the El Niño of 2015, the Pacific meridional mode was strongly positive in 2015 (compared to negative in 1997), leading to warmer sea surface temperatures across most of the Central Pacific this year than in 1997.
The ocean temperatures were way above average from July through October 2015 –record levels in the part of this basin where the strongest hurricanes form. The values recorded in 2015 were about 1.3 degrees warmer than 1994, the prior record-holder for warmth.
One other factor that was probably a big contributor to this record-breaking hurricane season was record low values of vertical wind shear. Too much vertical wind shear, the change in wind direction with height in the atmosphere, tears apart storms as they are trying to form and intensify.
On average, a typical July through October averages 25 miles per hour of vertical wind shear across the basin, which is considered to be quite harsh for hurricane formation. However, in 2015, vertical wind shear was less than 10 miles per hour. Weak vertical wind shear aided storms fueled by warm ocean waters to reach intense levels this year.
Fortunately, despite the incredibly active Central Pacific hurricane season, Hawaii avoided any significant impacts from tropical cyclones. The last tropical cyclone to make landfall in Hawaii was Iselle last year, which made landfall as a tropical storm on the Big Island. The most notorious land-falling storm in recent Hawaiian history was Iniki (1992), which devastated Kauai, causing six fatalities and costing nearly $2 billion dollars in damage.