A Hyperactive 2018 Hurricane Season in the Tropical Pacific

The accumulated cyclone energy (ACE) index is a parameter that is used to quantify overall cyclonic activity. It’s calculation is intuitive – ACE equals the squared sum of the maximum single-minute sustained wind speed at six-hourly intervals (00, 06, 12, and 18 UTC) multiplied by 10^-4 (or 1/10,000). The squared velocity is an analog to the kinetic energy generated by the storm and accumulating that energy over the six-hourly intervals yields a value for ACE. Dimensional analysis reveals that the units can also be expressed as m2 / s2, which describes the energy per unit mass. In climatology, the average ACE in the eastern Pacific through the 31st of August is about 72 (assuming the unit is kt^2). However, the 2018 season has so far seen a value for ACE around 163 which is more than twice of climatology to the date (compared to the quieter Atlantic with current ACE of around 18). This indicates that tropical cyclone activity in the Pacific is characterized by hyperactivity.

So what factor(s) are responsible for the frequent development of cyclones that we’ve witnessed this season in the Pacific? One of the key forcing mechanisms for the active Pacific season lies within the warming of the Pacific ocean related to the transition between a La-Niña pattern to more of a weak El-Niño pattern. A consequence with the onset of an El-Niño pattern is the deepening of the ocean thermocline in the Pacific, or depth at which ocean temperatures drop considerably, meaning that warmer waters are available at greater depth for storms to potentially use. Also at the global scale, the Madden-Julian Oscillation (MJO) is in a favorable phase for growth of tropical cyclones in the Pacific Ocean. Briefly, the MJO consists of eight phases with each individual phase focusing elevated levels of convection between the Indian and Pacific Oceans. Over the last month, the MJO signal was most prominent in phases six and seven (signal over the Western Pacific), although right now the signal is rather weak and focused over the Maritime Continent. Still, the presence of an MJO signal in the Western Pacific was enough to hint at enhanced convection over much of the Pacific basin. A synopsis at greater detail on the MJO signals can be found in this presentation.

Another contributing factor is the lack of vertical wind shear. Without significant wind shear, thunderstorms surrounding the center of low pressure are free to grow unabated and vertically aligned while rotating, keeping the storm intact or perhaps intensifying. A host of scientific journal articles delve into the shear-intensity problem. An interesting paper that describes this effect at a high level can be accessed below:

Paterson, L. A., B. N. Hanstrum, N. E. Davidson, and H. C. Weber, 2005: Influence of Environmental Vertical Wind Shear on the Intensity of Hurricane-Strength Tropical Cyclones in the Australian Region. Mon. Wea. Rev., 133, 3644-3660.

With three months still outstanding in the 2018 Pacific hurricane season, the high development frequency of tropical cyclones serves as a message to always remain vigilant for any activity that could form and threaten land. As an aside, it is always important to stay alert to upcoming tropical development, especially as the Atlantic basin approaches peak development during the months of September and October.

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© 2018 Meteorologist Brian Matilla