science – Swell Lines

El Niño w/Mark Sponsler

*Mark Sponsler is the creator of StormSurf.com, “a website dedicated to delivering the highest quality marine weather data to those who ride waves.” Sponsler’s weekly forecast videos focus on El Nino indicators year round, regardless of media hype.

El Nino can have profound effects on global weather and ocean conditions. Under normal conditions, trade winds over the equatorial Pacific blow from east to west (Peru towards Asia). This causes warm water to sequester near Asia, and cool water to upwell off Peru. This in turn results in high pressure over the cool water off Peru (producing stable atmospheric conditions) while low pressure and tropical precipitation locks down off Asia.

But about once every 7 years, the trade winds over the equator relax if not reverse direction, with the effect being a flow that travels reverse of the normal direction, or from west to east (Asia towards Peru). When the trades relax or weaken, this situation is know as westerly anomalies. That is, compared to normal for the time of year, the winds have a westerly component to them And when trades fully reverse (they start blowing from the west to the east), this is known as a Westerly Wind Burst (WWB).

A Westerly WInd Burst across the equatorial Pacific Ocean. — A Westerly Wind Burst across the equatorial Pacific Ocean.

WWBs can last for 10-20 days and can blow as hard as 20+ kts. This situation typically occur between New Guinea and the International dateline. When a WWB occurs, it sets the oceans surface in motion moving to the east. The result is warm water off Asia starts migrating east across the tropical Pacific towards Peru. WWBs also typically spawn tropical cyclones, sometimes straddling both sides of the equator simultaneously. Typically the westerly anomaly/WWB cycle starts in Spring or early Summer, with a single WWB, followed by 2-4 more through early Fall, with the warmest waters reaching Ecuador around Christmas.

The warm water doesn’t flow from west to east on the surface. It falls to depth, down about 200 meters, forming a pocket or bubble of anomalously warm water (+7 degs C above normal). That pocket is called a Kelvin Wave, and it travels west to east under the equator for 2.5 months before being forced to the surface first as it encounters the Galapagos Islands, then eventually impacts Ecuador. For each WWB that occurs during the EL Nino cycle, a corresponding Kelvin Wave develops. The more WWBs, the more Kelvin Waves, and the greater the warming in the east.

As a Kelvin Wave erupts in the East Pacific, it warms surface water there, and typically pretty dramatically. This in turn has effects on fisheries and wildlife, especially in the Galapagos, Peru, Central American corridor. Where normally cold upwelled nutrient rich waters are present, now a warmer and far less bountiful food chain is present for fish. Fish stocks leave the area. Local economies that survive off fishing turn non-productive.

Eventually these warm waters, as they erupt near the Galapagos start migrating west by prevailing trades over the East Pacific, moving towards the dateline. Note – the trades don’t completely dissipate in the east. As this cycle progresses, a defined warm water ‘tongue’ develops extending from the Galapagos east and within 5 degrees north and south of the equator reaching west to the the dateline. Depending how warm the surface pool gets and how much area it covers compared to normal determines whether the situation will qualify to be labeled as El Nino. The area of concern is from 5N to 5S and from 120W to 170W, the NINO3.4 area. If temps in this region hold at +0.5 deg C above normal for 3 months, it considered a minimal El Nino. At the top end temps of +1.5 result in a classification of a strong El Nino. As of the Sept monthly reading, this years event was at +1.67 degs C. In comparison, the two strongest El Ninos at their peak in Dec/Jan were at +2.32 (’97/98) and +2.21 (’82/83). Theses were considered Super El Ninos. This years event is already the 6th strongest El Nino ever (as of Sept), and building. And it is tracking mid-way on it’s development path between the ’82/83 and the ’97/98 events, making it possible it too could reach Super El Nino status.

As the surface warm pool builds, it starts interacting with the atmosphere above it, enabling greater evaporation and increasing humidity levels, reducing surface air pressure and increasing the odds for rain in locations of the planet typically bone dry. Likewise as water cools over Asia, surface air pressure builds and drought sets in. Precipitation follows the warmer water across the Pacific, resulting in a complete reversal of the Pacific Basin weather patten.

From a surf perspective, the weather changes associated with El Nino are most pronounced in Fall and Winter months in the Northern Hemisphere. The warm pool feeds more energy into the jetstream, which in turns causes the jet to track further south forming a strong semi-permanent low pressure system just east of the dateline and south of the Aleutians easing into the Gulf of Alaska. The upper level energy feeds development of larger, stronger and more consistent storms tracking across the Norther Pacific, which in turn generates larger, stronger and more consistent surf aimed at breaks in which the jetstream is flowing towards, like Hawaii, California, the Pacific Northwest, Mexico. But because the jetstream is further south, it also offers the specter of much rain and stormy local conditions, making surf conditions less than ideal. During pronounced episodes during strong El Nino years, the jetstream can drive storm energy straight from Siberia clear across the Pacific and directly into the US, often tracking right into normally drier regions of Central and Southern California. This can bring significant rain and snowfall to regions that are typically desert like, having severe economic and ecologic affects. In the Atlantic, the El Nino enhanced jetstream creates upper level shear that suppresses hurricane production.

Sea Surface Temperature Anomalies- Oct. 17, 2015.

So El Nino can be a mixed blessing, depending on where you live.

After El Nino has run it’s course, typically in the early summer following it’s peak, a new pattern emerges: La Nina that has almost the opposite effect. Colder than normal water starts to develop in the eastern equatorial Pacific in the mid summer as the trades start raging from east to west (Peru to Asia). Strong high pressure rebuilds over the eastern equatorial Pacific while low pressure follows the warm waters being blown back towards Asia. By Christmas time the year following El Nino, the North Pacific jetstream is displaced well north, driving up towards the Aleutians into Alaska and northern Canada, and high pressure dominates the NE Pacific pattern. As a result storm and swell production starts to decrease. And the whole cycle is then set to start again.

October, 2015February, 2018

Swell Lines Magazine

The Science of Coastal Geology- Part 1

We’re all beach people. We’re attracted to the coast for the beauty, serenity and the waves. There are many variations to coastlines around the globe and these changes are the factors that create the great variety of waves. In this multiple-part article, we’ll analyze coastal geology to better understand how differences in a coast’s dynamics dictate how waves break in a given area.

Geology is the Science of the rocks that create landforms and the processes that change those rocks.The coastal environment has many dynamic factors. Plate tectonics, wave energy, weather, rivers, and humans all have an impact on coastal geology. The coast is always changing. Some changes happen daily, like the movement of sand. While others, like the uplifting of land from plate movement, take place over the course of hundreds, thousands and millions of years.

If you observe any stretch of coast, you’ll see that waves break differently on varying parts of the beach. This is because the Ocean bottom varies. Bathymetry is the underwater contours of the seafloor. It is often overlooked as a surf variable. We cannot directly see it and much of it is fixed for our lifetime. But a closer investigation reveals the dynamic and vital story of our coastlines.

Swell approaching a coastline will always refract and bend toward shallow water. Reefs, sandbars, points and submarine canyons cause swell energy to focus and shoal. Because there is a variety of depths along a coast, waves break differently everywhere. Sandbars frequently shift and move as the sand is carried by longshore currents. California reefs are uplifted remnants of the coastal bluffs and marine terraces. Points are created by the uneven shape of a dynamic coast. Submarine canyons form when fresh water runs off the land and erodes a chasm in the continental shelf.

California’s coast primarily consists of sedimentary rocks in identifiable layers. All of these layers are associated with ancestral rock formations and their subsequent weathering, erosion and deposition. According to a sign at San Elijo State Beach, “The coastal bluffs were formed by the accumulation of mineral and organic sediments. In more recent times, the Ocean level has receded, leaving the sedimentary deposits exposed in elevated marine terraces. Once exposed, these terraces were eroded along the seaward margins, leaving the steep coastal bluffs present in the park today.”

Three prominent layers include:

The Del Mar Formation– the bottom and oldest layer of the San Diego coast sedimentary rocks. It is often greenish or gray mudstone, containing many fossils, laid down in muddy lagoons 45-50 million years ago. The Del Mar Formation now forms many of the reef wave setups we have in San Diego.

Torrey Sandstone– a large layer of light colored sandstone that is the main constituent in San Diego’s coastal bluffs. It is roughly the same age as the Del Mar Formation and was laid down as a sandbar and beach deposit.

Monterey Formation– an oil rich layer that is responsible for the tar on beaches and the offshore oil rigs towards Santa Barbara. It is 6-16 million years old and comprises the remains of billions of microorganisms that once swam in a shallow sea. When they died, they sank to the bottom and were covered by sand and silt. With pressure, heat and time, the organisms became hydrocarbons: the source of our much beloved oil. The Monterey Formation is seen throughout the Central Coast and into the Coastal Range Mountains. It makes up many of Central California’s reef breaks.

-KS

Sources:
Mark Bordelon- Irvine Valley College
PBS Coastal Geological Processes

July, 2015February, 2018

Swell Lines Magazine

Purple Blob Report: Winter 14/15 and Spring 2015

The winter swell season 14/15 began with a series of solid NW swells in early December.

Dec. 10, 2014

Dec. 11, 2014

Dec, 13, 2014

Dec. 14, 2014

December dropped an exciting 6+ inches of rain on San Diego county. The North Pacific swell and precip engine was alive and well.

A very cold storm brought the snow level down to 1000' on the night of Dec. 30, 2014. — A very cold storm brought the snow level down to 1000′ on the night of Dec. 30, 2014.

To begin the new year, high pressure settled over the eastern Pacific, effectively shutting off the precipitation. Sunny skies, hot days, light wind and warm water pervaded much of January and February.

From our perspective, Sunday, January 25th was the best day of the winter. Classic long period, NW swell started filling in on Saturday. The next morning dawned 6-10ft, glassy and pumping.

Jan. 25, 2015

Complex low pressure system in the North Pacific. Jan. 20, 2015. Photo: StormSurf — Complex low pressure system in the North Pacific- Jan. 20, 2015. Image: StormSurf

The rest of winter passed with consistently fun but not epic surf. Spring is acting rather strange. May 2015 is one of the wettest Mays in recorded history with well over an inch of rain. That brings our season total in San Diego to 9+ inches; only an inch below average but not enough to quench our thirst.

In March, Tropical Cyclone Pam devastated the South Pacific island nation of Vanuatu. The storm, moving east, then went extra-tropical and strengthened. A solid SSW swell reached California beginning on Sat. March 28th and peaking on Sunday the 29th. NW windswell provided just enough cross up to create stellar left-hand bowl sections.

March, 28, 2014

March 29, 2015

Total Lunar Eclipse on the morning of April 4th.

May, 2015March, 2016

Swell Lines Magazine

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 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.

An 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.

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

Sources:
NOAA Hurricane Research Division
Cyclone Center

April, 2015

Swell Lines Magazine

Breeze and Gale: The Science of Wind

Simply look at a flag and you can reckon one of the most important surf factors. If it’s pointed toward the Ocean or hanging slack, quicken your pace towards the sea. It has an enormous impact on our daily surf conditions, and in fact it is mostly responsible surf’s existence. Moving air. We call it wind. The creator and sometimes destroyer of our waves.

The Earth’s atmosphere is roughly 60 miles of gases that surround the planet. If Earth were the size of a classroom globe, the atmosphere would only amount to a couple layers of paint. Composed mostly of nitrogen and oxygen, the atmosphere is held in place by Earth’s gravity. We live and surf in the very base known as the troposphere.

Atmospheric pressure is the measure of how much air is pressing down on a given region on Earth’s surface. The movement of air from areas of high pressure to areas of low pressure is the primary cause of Earth’s surface winds. The Sun heats the Earth unevenly, so the atmosphere always contains different temperature gradients. As air molecules lose energy, they cool down, becoming more dense and sink through the atmosphere. As air molecules gain energy, they heat up, becoming less dense and rise in the atmosphere. This flow is called convection and its happening all around all the time.

Strong low pressure with surrounding areas of high pressure over the North Pacific. Image: NWS — Strong low pressure surrounded by areas of high pressure over the North Pacific. Image: NWS

Atmospheric convection creates the high and low pressure that we associate with our weather. As the warm air rises, the pressure on the surface drops. When cool air descends, the pressure on the surface increases. High pressure generally brings clear skies, light wind and stable weather. Low pressure generates precipitation, strong winds and unstable weather.

When the pressure drops, air rushes in and wind speed increases as the atmosphere attempts to find equilibrium. The rotation of the Earth causes the wind to also move in a circular pattern. Counter clockwise in the Northern and clockwise in the Southern Hemisphere: this is called the Coriolis effect.

When air moves over water, the water’s surface is disturbed. A phenomenon known as capillary waves are formed. These ripples begin the swell generating process. Gravity pulls the ripples downward but they also provide extra surface area for the wind to transfer energy into the water.

As stated before: the stronger the wind, the bigger the area (fetch) and the longer they blow- the bigger the swell. Wind speed is measured by the Beaufort Scale: from calm- breeze (4-30mph)- gale (31-63mph)- storm (64-72mph)- hurricane (72mph+). With extreme low pressure at their core, hurricanes and tornadoes contain the strongest winds on Earth, sometimes measured between 200-300mph! The planet Neptune has the strongest known winds in the solar system at 1,300mph+. Imagine the swell that would generate!

Prevailing winds are the predominant day to day winds of an area. In general, westerly winds blow across the mid-latitudes and easterly trade winds blow across the tropics. Hawaiian trade winds act like a big air conditioner for the whole island chain. Blowing from the northeast, they blow gently offshore to many famous beaches on the north coasts of the Islands. Kona winds develop from the south when the trades slow down. They often lead to muggy hot weather and volcanic fog (vog) blowing in from Kilauea on the Big Island.

Sea Breeze and land breeze Image: ncsu.edu — Sea Breeze and land breeze. Image: ncsu.edu

Diurnal temperature variation is responsible for the daily land and sea breeze cycle that impacts many coastlines across the globe. Offshore at first light, perfectly glassy at 9am, onshore- blown out by noon and glassing off as the sun sets. The Ocean remains a more consistent temperature than the land during the day-night cycle. The gradient between land and sea temperatures decreases at night as the land cools down. Land breezes, blowing from the land to the sea, accompany many mornings with favorable surf conditions. As the sun rises and heats the land more than the water, the temp rises and pressure decreases over the land. The sea breeze picks up as air moves from the water to the land, frequently blowing out and negatively impacting surf conditions. As the sun sets, the land cools down and the evening glassoff can occur.

As coastlines vary, so too the wind’s impact on different surf spots. Nicaragua is known for all-day offshore winds because Lake Nicaragua sits just 10 miles inland and keeps the land breeze blowing most of the day. Regions with bending coastlines can be a blown out mess at one spot while around the corner is offshore. Protected coves can shelter surf spots from wind. Areas with large kelp beds are less impacted by afternoon onshores because the kelp cuts the wind and smooths the sea surface outside the lineup. Katabatic winds, meaning “downhill,” like Southern California’s Santa Ana winds can change the pattern for days at a time. When fire-free, Santa Ana, offshore winds meet swell at the coastline, SoCal waveriders rejoice.

-KS

Sources:
National Center for Atmospheric Research
United Kingdom Meteorological Office
NOAA

February, 2015February, 2018

Swell Lines Magazine

The Science of North Pacific Swell

Summer of 2014 saw an incredible run of swell. But any surfer worth their saltwater, knows that winter is the best time. Swell producing storms are more frequent and much closer to our coastline. Regardless of the colder water and shorter days, winter produces the best swells. The North Pacific Ocean (NPAC) generates some of the largest and most powerful storms on Earth. Let’s investigate the genesis of these storms and track their energy across the Ocean to our coastlines.

mangled-jet-stream-may-14 — The Polar Jet Stream Image: NOAA GFS

The Polar Jet Stream: a major factor in the production of our winter surf. What is it? More importantly, where is it? It’s a belt of strong, upper-level winds that blow across the mid-latitude atmosphere. Here, the gradient between the cold/dry polar air and the wet/warm tropical air is strongest. Known as the Polar Front, this region funnels the Polar Jet Stream from west to east across the North Pacific Ocean, North America to Europe and beyond. At between 23,000 and 39,000ft. above sea level and 100mph+, this enigmatic force impacts global weather.

As the Earth orbits the Sun over the course of 365.25 days, tilted on its axis, different parts of the planet receive more solar energy than others. The Northern Hemisphere is pointed toward the Sun in the summer. The temperature gradient between the North Pole and equator is weaker because the solar energy reaches farther north. Hence, the Polar Jet Stream is positioned close to the Arctic and the North Pacific is quiet.

In our winter, the Northern Hemisphere leans away from the sun, strengthening the gradient and moving the Polar Jet Stream south over the Pacific Ocean. This is the fuel that feeds energy into the storms that generate our waves. The Jet Stream doesn’ t flow in a straight line, instead troughs and ridges form along its length. Rossby Waves meander the jet, unsettling the atmosphere. As the jet stream flows along these bends, it speeds up, taking air from surface-level high pressure upward. This creates low pressure that begins to intensify as it spins with the Coriolis Effect. The pressure gradients tighten, wind speed increases and kinetic energy is transferred into the water below.

On the surface, the warm Kuroshiro Ocean Current brings heat from the equator up the coast of Japan and into the Northwestern Pacific. The cold Oyashio Current flows south from the Arctic Ocean. The Westerlies blow cold, dry wind across Asia and out to sea near Japan. In the winter, these forces collide at the Polar Front. This is the genesis of many North Pacific swells. The Polar Jet Stream moves these systems to the east, first near the Kamchatka Peninsula, then past the Aleutian Islands and into the Gulf of Alaska. Purple, black, yellow and white blobs marching across the NPAC: the active storm track that we all love to see.

The direction the storm takes is of utmost importance to swell production. Sometimes they track north into the Bering Sea. This is poor for California and Hawaii swell production because the energy is moving away from our coastlines. Ideally, the storm stays to the south, intensifying as it passes north of Hawaii. Moving east, the storm slams into a giant ridge of high pressure off the California coast. The system and it’s inclement weather are spun north towards Canada. But the goods are delivered. Swell trains march across the Pacific, slamming the sunny California coast with perfect surf…ideally.

NPAC Blob Image: StormSurf

NPAC energy on the North Shore of Oahu

North Shore

If the high pressure ridge is weakened off the coast of California, the storms can stay south and deliver (much needed) precipitation. These storms can also deliver very intense, shorter period, stormy groundswell. La Nina conditions represent a weaker and more northerly Polar Jet Stream that supports high pressure in the Gulf of Alaska with a less active NPAC storm track. El Nino brings the jet to the south, setting up low pressure in the Gulf and a more active storm track.

Low Pressure near the California Coast

Large, intense swell generated by close proximity storm.

There are innumerable variables that dictate the production and quality of waves at any location. From the size, strength and track of a storm to the local winds and bathymetry. Forecasting accuracy continues to improve as computer and satellite technology advance. Along with our understanding of the large-scale connections between atmospheric phenomenon. So when the Polar Jet Stream troughs, we can rearrange our schedules to enjoy the energy.
-KS

Sources:
NOAA- Jet Stream
NC State- Climate
Geog-Online