The Atlantic hurricane season certainly hit the United States hard this year. First came Florence. That storm dropped 91.3 centimeters (35.9 inches) of rain. Then Michael roared ashore as the third most-intense U.S. tropical cyclone to ever hit landfall. But the eastern Pacific had it even worse. As of October 22nd, this region recorded its most energetic cyclone season on record.
That big — and scary — assessment comes from Phil Klotzbach. He used to work for the National Hurricane Center in Miami, Fla. Now he works at Colorado State University in Fort Collins. There, he analyzes tropical storms and hurricanes. He calculates how much energy they release. That energy is expressed as a numerical score known as ACE. It’s an abbreviation for Accumulated Cyclone Energy. These scores have been kept since around 1970.
Klotzbach and other atmospheric scientists use ACE measurements to describe the amount of destructive force unleashed by a storm’s winds over its life. To figure it out, scientists take the maximum sustained wind speed within the eyewall of a storm. It’s measured in knots. Then they square that number. To account for variations in the storm’s strength, this process is repeated every six hours. Once a storm has fallen apart, researchers tally up the energy estimates computed from each six-hour chunk of a storm’s lifetime.
The results are pretty big numbers! To make the digits more manageable, scientists normally divide the total by ten. To see how a hurricane season stacks up, they keep a running sum and compare that year's score to those for other years. The hurricane season is longer in the Pacific Ocean than in the Atlantic. It runs from May 15 to November 30. (In the Atlantic, it runs from June 1 to November 30.) That’s because the waters in the eastern Pacific tend to be much warmer, thanks to the structure of its ocean currents.
By late October most years, the ACE score for storms in the eastern Pacific totals around 125.7 units. So far this year, storms have churned out a staggering 305 units! That’s how scientists know it’s been a really bad year.
These numbers, by the way, come from tallies for storms in the ocean basin that’s east of the International Date Line. (This imaginary north-south line passes through the middle of the Pacific Ocean.) West of there, tropical cyclones are called typhoons. Near the Date Line, there’s a natural lull in storm activity.
There may be more to come
Keep in mind, there is still another month left in the eastern Pacific hurricane season. Despite this, 2018 has already claimed the top spot since recordkeeping began. It’s above the season-long 295 ACE units counted in 1992, and the 297 tally for 2015. In the coming weeks, this year’s ACE total could top 350 or 400! Simply stated, we’re in uncharted territory.
So what churned up so much Accumulated Cyclone Energy? At least 10 major hurricanes developed in that region this year. Each reached Category 3 status or greater. That means they had winds of 179 to 209 kilometers (111 to 130 miles ) per hour. Of these, four climbed to Category 4 status. Another 3 reached the top tier — Category 5. Those potentially lethal storms boast winds stronger than 251 kilometers (156 miles) per hour. For perspective, this year is only the third in recorded history to generate three Category 5 storms in the eastern Pacific.
Such monster storms can wreak havoc wherever they strike. On October 22, Hurricane Willa was the year’s third Category 5 storm. Its winds revved up to 257 kilometers (160 miles) per hour. En route to Mexico, it then weakened to a Category 3 hurricane. Even after making landfall shortly after midnight on October 24, it held its Category 3 intensity.
The two other Category 5 hurricanes — Lane and Walaka — used up most of their destructive energy whirling offshore. Walaka even danced the “Typhoon Tango.” It spun in the eastern Pacific while a typhoon of Category 5 strength spun thousands of kilometers to the west.
Lane, meanwhile, took an unusual track that brought it to Hawaii. It dropped 132.13 centimeters (52.02 inches) of rain in an unprecedented September deluge. This came months after an ordinary springtime rainstorm in April doused Waipa, on the island of Kauai. That storm dropped a mind-boggling 126.21 centimeters (49.69 inches) in less than 24 hours.
But Lane wasn’t a solo event. Two weeks earlier, in late August, Hurricane Hector also clipped the Hawaiian islands with gusty winds, rough surf, high waves and blinding squalls. They say that two’s a party, but three’s a crowd. And sure enough, a third tropical cyclone — Olivia — swung by Hawaii just after Lane’s rampage.
This marks the first year on record when a trio of windstorms has directly affected Hawaii. Normally, the islands’ position is far enough to the northwest of the region’s main danger zone to be largely immune to such cyclone smacks.
Climate lessons from Hawaii?
Hawaii may find itself pummeled by hurricanes even more in coming years. The reason is climate change, according to scientists at the University of Hawaii at Mānoa. They work at its Department of Meteorology and the International Pacific Research Center in Honolulu.
Bin Wang works at the Honolulu center. He warns that his team’s models indicate “a substantial increase in the likelihood of tropical cyclone frequency … with a northwestward shifting of the tropical cyclone track.” That would put the southern Hawaiian islands directly in a path that future storms are more likely to take.
Hawaii’s not the only place this season to be impacted by “out of bounds” Pacific storms. Both Rosa and Sergio entered the United States' desert southwest as remnant tropical rainstorms earlier this year. The former brought 8 to 12 centimeters (3.1 to 4.7 inches) of rain to Arizona. The resulting flash floods triggered mudslides and debris flows.
Such drenching in the Grand Canyon State may be a symptom of a rapidly changing climate. The Intergovernmental Panel on Climate Change advises that flooding is more likely across the southwestern United States as tropical moisture is able to stream farther northwards. And that’s thanks to our warming globe.
So what made 2018 so wild? Klotzbach points to two things. One is low amounts of wind shear in the upper atmosphere. The other is far higher-than-normal sea-surface temperatures.
“The northeast Pacific was quite toasty this year,” he says. “Sea-surface-temperature anomalies were 1 to 1.5 degrees Celsius [1.8 to 2.7 degrees Fahrenheit] warmer on average across the basin.” That’s a primary factor, he explains, behind the growth of cyclones.
But there was something else.
Weaker-than-normal winds in the upper atmosphere led to a drop in wind shear. That’s a turning of the winds with height. If the winds veer too much, they can tear apart an infant storm before it can grow into a towering monster. But Klotzbach points out that wasn’t an issue this season. “We had reduced shear throughout the [eastern Pacific] basin for most of this year.”
This year's spike in hurricane activity, he notes, is surprising in the absence of an El Niño. That term describes a period when winds reverse directions near the equator in the Pacific. During El Niño years, the normally easterly trade winds become westerly. This leads warm waters to pool off of the west coast of Central America. That’s a prime site for the formation of Pacific hurricanes. Even without that El Niño, hurricanes erupted anyway. Blame the region’s super-warm seas.
“It doesn’t always take an El Niño event to increase northeast Pacific hurricane activity,” explains Klotzbach. There was no El Niño this year. Nor was there one in 1992, he notes. But both years still emerged as “the most active seasons as measured by ACE.” There may be no overall increase in the number of future hurricanes. But climate scientists expect storms that do arise to become increasingly strong and more destructive as global warming continues to raise the temperature of sea-surface waters.
And that’s not the only fingerprint of climate change showing up in big storms. More and more hurricanes erupt into monsters within a day or less. That fast change is called rapid intensification. The pattern marks cyclones whose winds pick up by at least 55.5 kilometers (34.5 miles) per hour within 24 hours. Hurricane Florence did this in 2018. So did Michael; it revved up from a weak tropical storm to a Category 4 cyclone in less than two days. And it continued to gain strength right up to its landfall in the Big Bend of Florida on October 10. By then, it had winds as strong as a tornado’s.
In the Pacific, more than four in every five tropical cyclones this year underwent such a rapid strengthening. That’s according to Brian McNoldy. He’s a hurricane researcher at the University of Miami in Florida. Back in late August, he notes, Hurricane Norman vigorously strengthened by 129 kilometers (80 miles) per hour in fewer than 24 hours.
Meteorologists warn that this rapid intensification of cyclones will become more common in the years ahead. It is but one symptom of a warming planet.
Accumulated Cyclone Energy, or ACE A calculation that can be used to approximate the energy expended by a tropical cyclone’s winds. It takes into account wind speed and how long the maximum winds of a cyclone last.
Atlantic One of the world’s five oceans, it is second in size only to the Pacific. It separates Europe and Africa to the east from North and South America to the west.
atmosphere The envelope of gases surrounding Earth or another planet.
average (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.
basin (in geology) A low-lying area, often below sea level. It collects water, which then deposits fine silt and other sediment on its bottom. Because it collects these materials, it’s sometimes referred to as a catchment or a drainage basin.
climate The weather conditions that typically exist in one area, in general, or over a long period.
climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
cyclone A strong, rotating vortex, usually made of wind. Notable examples include a tornado or hurricane.
debris Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.
digit A structure, like a finger or toe, at the end of the limbs of many vertebrates.
El Niño Extended periods when the surface water around the equator in the eastern and central Pacific warms. Scientists declare the arrival of an El Niño when that water warms by at least 0.4 degree Celsius (0.72 degree Fahrenheit) above average for five or more months in a row. El Niños can bring heavy rainfall and flooding to the West Coast of South America. Meanwhile, Australia and Southeast Asia may face a drought and high risk of wildfires. In North America, scientists have linked the arrival of El Niños to unusual weather events — including ice storms, droughts and mudslides.
equator An imaginary line around Earth that divides Earth into the Northern and Southern Hemispheres.
eyewall Also known as a wall cloud, it’s an organized band or ring of cumulonimbus clouds that surround the eye, or light-wind center, of a tropical cyclone.
factor Something that plays a role in a particular condition or event; a contributor.
force Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
frequency The number of times some periodic phenomenon occurs within a specified time interval.
global warming The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.
Grand Canyon A natural canyon in northwest Arizona that formed as the Colorado River cut through the rock here over the past 5 million to 6 million years. This is one of many canyons on the river, which drains water from seven states. The Grand Canyon is 446 kilometers (277 miles) long. Its depth varies. At its deepest point, the river is 1,829 vertical meters (6,000 feet) below the upper rim. From rim to rim, the canyon’s width also varies — from about 16 kilometers (10 miles) to 29 kilometers. Of this impressive national land formation, more than a million acres (4,931 square kilometers, to be exact) was turned into a U.S. national park in 1919.
Hawaii This central Pacific island chain became the 50th U.S. state on Aug. 21, 1959. Moving from west to east, its eight major islands are Niihau, Kauai, Oahu, Molokai, Lanai, Kahoolawe, Maui, and Hawaii (also known as the Big Island). The entire crescent-shaped island chain spans some 2,400 kilometers (1,500 miles). Each of the state’s islands was created from one or more volcanoes that long ago sprang up from the ocean floor. The chain sits some 3,857 kilometers (2,397 miles) west of San Francisco, Calif., and 8,516 kilometers east of Manila, the Philippines.
hurricane A tropical cyclone that occurs in the Atlantic Ocean and has winds of 119 kilometers (74 miles) per hour or greater. When such a storm occurs in the Pacific Ocean, people refer to it as a typhoon.
Intergovernmental Panel on Climate Change (or IPCC) This international group keeps tabs on the newest published research on climate and on how ecosystems are responding to it. The United Nations Environment Programme and the World Meteorological Organization jointly created the IPCC in 1988. Their aim was to provide the world with a clear scientific view on the current state of knowledge in climate change and its potential environmental and social impacts.
knot (in metrology) A unit of measurement for speed equal to 1 nautical mile per hour, or 1.15 statute (land-based) mile per hour. This unit was initially developed by 17th-century sailors to measure the speed of ships at sea. Researchers today also apply it to wind speeds.
meteorology (adj. meteorological) The study of weather as it pertains to future projects or an understanding of long-term trends (climate). People who work in this field are called meteorologists.
model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes. Or an individual that is meant to display how something would work in or look on others.
Pacific The largest of the world’s five oceans. It separates Asia and Australia to the west from North and South America to the east.
remnant Something that is leftover — from another piece of something, from another time or even some features from an earlier species.
sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
squall A sudden, violent gale of wind and usually rain (or other precipitation).
square (In mathematics) A number multiplied by itself, or the verb meaning to multiply a number by itself. The square of 2 is 4; the square of 10 is 100.
tornado A violently rotating column of air extending from the ground to a thunderstorm above.
tropical cyclone A strong, rotating storm. These usually form over tropical areas around the equator where the water is warm. Tropical cyclones have strong winds of more than 119 kilometers (74 miles) per hour and usually have heavy rain. Large ones in the Atlantic are known as hurricanes. Those in the Pacific are termed typhoons.
typhoon A tropical cyclone that occurs in the Pacific or Indian oceans and has winds of 119 kilometers (74 miles) per hour or greater. In the Atlantic Ocean, such storm are referred to as hurricanes.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.
Explainer: Accumulated Cyclone Energy (ACE). World Meteorological Organization.
Article: Matthew Cappucci. The eastern Pacific Ocean has seen its most active hurricane season on record. The Washington Post. October 23, 2018.
Journal: H. Murakami et al. Projected increase in tropical cyclones near Hawaii. Nature Climate Change. Vol. 3, May 5, 2013, p. 749.