The Science of Tropical Cyclones

Tropical cyclones are incredibly powerful and dangerous natural phenomena. Every year, coastlines all across the globe are impacted by these tempests. When the storms move toward land, heavy rain, strong wind, storm surge and tornadoes all contribute to their destructive power. Depending on location and strength, tropical cyclones are called tropical depressions, tropical storms, hurricanes, cyclones, typhoons, super typhoons, etc. Wave-riders have a strange relationship with tropical cyclones. We know their power but enjoy their wave energy. Let’s look at the cyclogenesis of these storms. 

The location and naming conventions for tropical cyclones. Image: NASA
The location and naming conventions for tropical cyclones.        Image: NASA

The sun warms the tropical Ocean and delivers radiant energy to its surface. Wind blows over the water, it evaporates and ascends energetically into the atmosphere.  The rising moisture condenses into towering thunderheads. Air rushes upward and the atmospheric pressure in the center drops. This is a thunderstorm.

saffir-simpson-smAn organized group of thunderstorms that persists for 24 hours is called a tropical disturbance. When winds exceed 30mph, it becomes a tropical depression. The Earth’s rotation (coriolis effect) drives wind around the warm core of the storm. For the storm to continue to strengthen, it must remain over warm water and encounter minimal wind shear. This is when vertical winds slant the storm, dispersing the heat over a larger area, degrading the storm. Without wind shear, the cyclone remains upright and continues to develop.

When winds reach 39mph, the cyclone becomes a tropical storm and meteorologists give it a name. At 74mph, the storm becomes a hurricane. Hurricanes are characterized by a defined eye and strong low pressure in the center. Around the eye is the eye wall, an area of intense thunderstorms and the storm’s strongest wind. The Saffir- Simpson Scale rates hurricanes based on wind speed. From Category 1, 74-95mph to Category 5, >155mph. Category 3, 4 and 5 storms are deemed major hurricanes with the most dangerous conditions.  

A tropical cyclone is a heat engine, fueled by the temperature gradient between the warm Ocean surface and the cooler upper atmosphere. Heat becomes motion as the warm, moist air rushes upward and is replaced by surrounding air.Hurricane-en.svg

There are many meteorological resources devoted to studying and predicting the path that a hurricane will take. There are lives and communities at stake. Wave-riders also take a particular interest in hurricane forecasting because the path of storm is a very important aspect of it’s swell generation. Although, tropical cyclones have strong winds, they lack the size and fetch of their extratropical cousins that generate 15-20+ second groundswell. Hurricane force winds might extend 100 miles from the eye and gale force winds another 300 miles beyond that. For ideal swell impact, a tropical cyclone will slowly move toward land, strengthening as it moves but turn away or dissipate before making landfall.

Origin of our North America's tropical cyclones. Image: NOAA
Origin of our North America’s tropical cyclones.         Image: NOAA

North America is impacted by two distinct tropical cyclone regions: the North Atlantic and Northeast Pacific. Interestingly, it is hypothesized that cyclones in both originate from the same phenomena. African easterly waves are disturbances in jet stream flowing off Saharan Africa. They can develop into tropical storms in the warm Atlantic or continue across the Caribbean and Central America without development. Reaching the tropical Pacific, the disturbance can develop into a tropical cyclone using the warm water off the coast of Mexico and Central America as fuel.

North America hurricane tracks from 1958-2011.  Image: NOAA
North America hurricane tracks from 1958-2011.        Image: NOAA

Sources:
NOAA Hurricane Research Division
Cyclone Center

 

 

Purple Blob Report: Summer 2014

Newport Point- Hurricane Simon
Newport Point- Hurricane Simon

Record breaking Pacific Tropical season, solid Southern Hemisphere energy, early start for the Northern Hemisphere, warm water, moderate winds, clear skies: the 2014 summer surf season was/is phenomenal. An all-timer, a comparison for epic summers of the future, literally one for the record books. Wave-riders are battered and rashed… but smiling in remembrance of summer and excitement for winter.

Velella velella
Velella velella

 Summer has the potential for weak storm tracks and minimal swell. Last summer was terrible. This summer was special. Purple blobs sent swell from various directions. Minimal marine layer allowed for plentiful sunny days.  Four thunderstorms graced San Diego in July and August. The water temperature moved into the 70’s in June and remained mostly trunkable well into October. The appearance of millions of velella velella- aka- ‘by-the-wind sailors’ on our beaches was seen as a positive omen by many water-people. Peter Hamann of San Diego Fishing Adventures says, “This summer was exceptional! The warm water allowed fish that normally stay deeper south to move north and provide us with amazing fishing.”

The 2014 Pacific Hurricane Season is one of the strongest since accurate measuring began in 1971. The Accumulated Cyclone Energy or ACE index, which measures overall storm energy, sees 2014 at 45% above the average as of press time. The season doesn’t officially end until Nov. 30th. SwellWatch forecaster, Nathan Cool remarks, “I’ve never seen so many hurricanes form in the NE Pacific in one year that took a northward trajectory, bringing not just swell to SoCal, but also torrential rain to inland areas.”  With 21 tropical storms, 15 hurricanes and 9 major hurricanes, 2014 compares favorably with the benchmark 1992 season.

Notice the "Record Warmest" off the coast of Central America where storms form and off Baja where storms enter the SoCal swell window.  Photo: NOAA
Notice the “Record Warmest” off the coast of Central America where storms form and off Baja where storms enter the SoCal swell window. Image: NOAA

Very warm water in the entire Eastern Pacific and low wind shear in the upper atmosphere fed a healthy storm track moving westward off the coast of Central America. The season started fast as Hurricane Amanda formed on May 24th. June and July combined for 5 tropical storms but nothing substantial. Then August exploded.

Hurricane Genevieve started in the East Pacific, then passed through all three North Pacific basins. She strengthened into a Category 4 hurricane south of Hawaii and was reclassified as Super Typhoon Genevieve after passing over the International Dateline. She then moved north and met her demise over cool water.  Hurricane Iselle followed, moving west and reaching Category 4 status. On Aug. 7th, she (now a tropical storm) was the strongest tropical cyclone to ever make landfall on the Big Island of Hawaii.  Next, Hurricane Julio strengthened into a Category 3 as he moved to the NE of Hawaii, delivering fun surf to places unaccustomed to such tropical swell. Hurricanes Karina and Lowell reached category 1 status as they moved into the California swell window. Lowell delivered a fun sized pulse of tropical swell that was a hint of things to come.

Aug. 7th- Four Hurricanes in the North Pacific  Photo: Global Wind Map
Aug. 7th- Four Hurricanes in the North Pacific. Image: Global Wind Map

Beginning on Aug. 10th, the National Hurricane Center monitored a tropical wave as it moved off the coast of Africa and across the Atlantic Ocean. Nine days later, a low pressure system crossed over Panama into the Pacific. Above average water temperatures and low wind shear allowed for rapid intensification and on Aug. 22nd, the NHC classified her Tropical Storm Thirteen-E.  Just six hours later, she became Marie. Moving into the SoCal swell window on a N-WNW track, she strengthened into a Category 5 hurricane with 160mph winds on Aug. 24th.  

 

On the evening of Tuesday the 26th, abnormally long, 18” tropical forerunners began to show at south facing beaches that could handle the SSE direction. First light, Big Wednesday, the 27th: 25ft. Wedge freight trains off the Jetty, perfect 12ft. Malibu reeling past the Pier, 15ft. Newport Point as good as California gets. The wind stayed down for the rest of the day and everyone near the coast was treated to a decadal spectacle of Ocean power and beauty.

 

Hurricane Odile approaching Baja. Photo NASA
Hurricane Odile approaching Baja. Photo NASA

Two weeks later, Hurricane Odile made landfall on the Baja Peninsula as the strongest storm ever to do so, with 125mph winds on Sep.15th. The widespread damage to Cabo San Lucas is a terrible reminder that these storms aren’t just fun swell producers but also extremely powerful and dangerous.  Hurricane Simon strengthened and took a favorable swell producing path during early October.  Tuesday the 7th saw another round of solid, clean hurricane swell lighting up Newport Point and Wedge. 

The Southern Hemisphere filled in the gaps nicely between hurricanes with consistent SW swell. There were no real standout southern hemi swells but it wasn’t dormant either. With a bit of patience, fun sets were always on their way… eventually.  Locally fun swell for southern exposures meant the surf rarely dropped below waist-high for weeks at a time. A glorious, fire-free Santa Ana wind event with pulsing SW swell lit up the California coast in early October. Even the North Pacific pitched in with a series of early season swells derived from typhoon remnants off the coast of Japan. 

Santa Ana dreams.
Santa Ana dreams.

Here are reviews from two Newport locals that have been in tune with summer surf for more than 50 years combined:

-Wedge Historian Mel Thoman says, “I’ve been recording “Wedge Calendars” since 1978 and that year remains #1 for size/shape/number of swells/conditions etc. 2014 made my Top 10 by the end of August and then vaulted into the Top 3 by October. It’s 2 or 3 for sure. The whole Crew charged hard with great vibes in and out of the water.”

Newport Summer 2014 Photo: Ron Romanosky
Newport Summer 2014 Photo: Ron Romanosky


-Long-time Wedge observer and pundit, Ron Romanosky, gave this review of 2014: ”This last summer’s swells rank among the top 5 in the last 3 or 4 decades.  Fantastic weather kept the waves rideable from first to last light far more than usual.  And let’s not forget the warm water!  By their performance in big waves, several of the young guns of Wedge bodysurfing now reside at the top of the list of Wedge’s top dogs. They/we know who they are.”

Purple Blob Review: Summer 2014
Consistency4/5
-No major Southern Hemis but plenty of swell around even between the epic tropical pulses.
Conditions5/5
-Sunny skies, warm water, minimal on-shore flow…yes please.
Intensity5/5
-One of the largest, most intense summer swells in California surf history and a series of other very solid swells. Not much else to ask for out of a summer season.
Overall- 5/5
-An all-time classic.

Winter 2014/2015 Forecast

To El Niño or not El Niño?  Eastern Pacific sea surface temperatures have fluctuated over the past few months and with them the predictions for El Niño. From very strong to moderate back to strong. My prediction: This is the beginning of a 3 year El Niño cycle. Starting moderate this year with low pressure dominating the Gulf of Alaska; bringing abundant NW winter swells to California and Hawaii. Along with reservoir-filling precipitation and feet of powder to all slopes. Then another strong tropical summer in 2015 and an epically strong El Niño winter next year. Pumping swell for years to come!
-KS

Sources
National Hurricane Center
Surfline- Hurricane Marie
Wikipedia- 2014 Pacific Hurricane Season