Center for Operational Oceanographic Products and Services Archives

What’s a seiche?

Licensed under Public domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Standing_wave_2.gif#mediaviewer/File:Standing_wave_2.gif — This animation reveals a standing wave (black) depicted as a sum of two propagating waves touring in reverse instructions (blue and pink). Related in movement to a seesaw, a seiche is a standing wave by which the most important vertical oscillations are at every finish of a physique of water with very small oscillations on the “node,” or heart level, of the wave. Standing waves can kind in any enclosed or semi-enclosed physique of water, from an enormous lake to a small espresso cup.

Seiches and meteotsunamis. What is the distinction?

Seiches and meteotsunamis are sometimes grouped collectively, however they’re two completely different occasions. Winds and atmospheric stress can contribute to the formation of each seiches and meteotsunamis; nonetheless, winds are sometimes extra essential to a seiche movement, whereas stress typically performs a considerable position in meteotsunami formation. Generally a seiche and a meteotsunami may even happen on the identical time. Seiches are standing waves with longer intervals of water-level oscillations (sometimes exceeding intervals of three or extra hours), whereas meteotsunamis are progressive waves restricted to the tsunami frequency band of wave intervals (two minutes to 2 hours). Seiches are often restricted to partially or totally enclosed basins, akin to Lake Erie. Meteotsunamis can happen in such basins however are additionally prevalent on the open coast. A single meteotsunami can journey lengthy distances and affect a really massive vary of the shoreline.

You probably have noticed water sloshing forwards and backwards in a swimming pool, bathtub, or cup of water, you might have witnessed a small-scale seiche (pronounced saysh). On a a lot grander scale, the identical phenomenon happens in massive our bodies of water akin to bays and lakes. A seiche could happen in any semi- or fully-enclosed physique of water.

Seiches are sometimes brought on when robust winds and speedy modifications in atmospheric stress push water from one finish of a physique of water to the opposite. When the wind stops, the water rebounds to the opposite facet of the enclosed space. The water then continues to oscillate forwards and backwards for hours and even days. In a similar way, earthquakes, tsunamis, or extreme storm fronts can also trigger seiches alongside ocean cabinets and ocean harbors.

Lake Erie is understood for seiches, particularly when robust winds blow from southwest to northeast. In 1844, a 22-foot seiche breached a 14-foot-high sea wall killing 78 folks and damming the ice to the extent that Niagara Falls quickly stopped flowing. As just lately as 2008, robust winds created waves 12 to 16 toes excessive in Lake Erie, resulting in flooding close to Buffalo, New York. Lake Pontchartrain, Louisiana, can also be recognized to routinely kind small seiches after the passage of afternoon squall strains throughout summer time months.

In among the Nice Lakes and different massive our bodies of water, the time interval between the “excessive” and “low” of a seiche might be as a lot as 4 to seven hours. That is similar to the time interval between a excessive and low tide within the oceans, and is commonly mistaken as a tide.

How does NOAA monitor water ranges across the nation?

A National Water Level Observation Network station in San Francisco, California — An NWLON station in San Francisco, California.

The Nationwide Water Degree Commentary Community (NWLON) is a everlasting observing system that features 210 repeatedly working water degree stations all through the U.S. and its territories. NWLON is the “go-to” supply for presidency and commercial-sector navigation, recreation, and coastal ecosystem administration.

Correct water degree knowledge is crucial for a lot of coastal actions, from navigation and engineering to restoration and preservation. Along with water ranges, the stations measure different oceanographic and meteorological parameters resembling water temperature and wind pace. NOAA makes use of this info to make sure protected navigation via entry to real-time knowledge 24 hours a day, seven days every week. NOAA nowcast and forecast fashions use the information to offer a glance into current and future oceanographic situations, which might improve tsunami and storm surge warnings. NOAA additionally makes use of this knowledge to publish annual tide predictions.

NWLON supplies historic in addition to present-day water degree info. Historic knowledge is used to compute relative native imply sea degree traits and to grasp the patterns of excessive tide occasions. The primary water degree observing stations had been established within the 1850s, and a few have been gathering knowledge ever since!

NWLON is managed by NOAA’s Middle for Operational Oceanographic Merchandise and Companies.

What’s a tide gauge?

The NOAA San Francisco Tide Station, in operation for more than 150 years. — The rise and fall of the tides play an vital position within the pure world and might have a marked impact on maritime-related actions. The picture aboves reveals the NOAA San Francisco Tide Station, in operation for greater than 150 years.

A tide gauge, which is one part of a contemporary water degree monitoring station, is fitted with sensors that constantly document the peak of the encompassing water degree. This knowledge is important for a lot of coastal actions, together with protected navigation, sound engineering, and habitat restoration and preservation.

Earlier than computer systems had been used to document water ranges (particularly tides), particular “tide homes” sheltered everlasting tide gauges. Housed inside was the instrumentation—together with a properly and a mechanical pen-and-ink (analog) recorder—whereas hooked up outdoors was a tide or tidal employees. Basically a large measuring stick, the tide employees allowed scientists to manually observe tidal ranges after which examine them to readings taken each six minutes by the recorder. Tide homes and the information they recorded required month-to-month upkeep, when scientists would acquire the information tapes and mail them to headquarters for handbook processing.

The New

diagram of newer tide stations — Whereas related in design to older tide homes, newer tide station enclosures are designed to guard delicate electronics, transmitting tools, and backup energy and knowledge storage gadgets. The older stilling properly has been changed with an acoustic sounding tube and the tidal employees with a stress sensor. The brand new discipline tools is designed to function with the best degree of accuracy with a minimal of upkeep, transmitting knowledge straight again to NOAA headquarters for evaluation and distribution.

The Previous

diagram of older tide stations — Earlier than computer systems, particular “tide homes” had been constructed to shelter everlasting water degree recorders, defending them from harsh environmental circumstances. On this diagram, we will see how the analog knowledge recorder is located inside the home with the float, and the stilling properly situated straight beneath it. Connected to one of many piers’ pilings is a tidal employees. This machine would enable scientists to manually observe the tidal degree after which examine it to the readings taken by the analog recorder.

The pc age led to tide gauges that use microprocessor-based applied sciences to gather sea-level knowledge. Whereas older tide-measuring stations used mechanical floats and recorders, fashionable monitoring stations use superior acoustics and electronics. At the moment’s recorders ship an audio sign down a half-inch-wide “sounding tube” and measure the time it takes for the mirrored sign to journey again from the water’s floor. Knowledge continues to be collected each six minutes, however whereas the outdated recording stations used mechanical timers to inform them when to take a studying, a Geostationary Operational Environmental Satellite tv for pc (GOES) controls the timing on right this moment’s stations.

Along with measuring tidal heights extra precisely, fashionable water degree stations are able to measuring 11 different oceanographic and meteorological parameters, together with wind pace and path, air and water temperature, and barometric stress. NOAA makes use of this data for a lot of functions, amongst them to make sure protected navigation, document and predict sea-level developments and different oceanographic circumstances through nowcasts and forecasts, and publish annual tide predictions.

What’s a meteotsunami?

a stormy ocean, iStock image — Stormy climate over the ocean can typically gasoline the event of meteotsunamis, a sequence of waves usually a lot smaller and fewer harmful than these related to seismically generated tsunamis.

Seiches and meteotsunamis. What is the distinction?

Seiches and meteotsunamis are sometimes grouped collectively, however they’re two totally different occasions. Winds and atmospheric stress can contribute to the formation of each seiches and meteotsunamis; nevertheless, winds are usually extra necessary to a seiche movement, whereas stress typically performs a considerable function in meteotsunami formation. Generally a seiche and a meteotsunami may even happen on the similar time. Seiches are standing waves with longer durations of water-level oscillations (usually exceeding durations of three or extra hours), whereas meteotsunamis are progressive waves restricted to the tsunami frequency band of wave durations (two minutes to 2 hours). Seiches are often restricted to partially or absolutely enclosed basins, resembling Lake Erie. Meteotsunamis can happen in such basins however are additionally prevalent on the open coast. A single meteotsunami can journey lengthy distances and affect a really massive vary of the shoreline.

You’ve heard of tsunamis—these large oceanic waves triggered primarily by earthquakes that may roll ashore, inflicting lack of life and catastrophe. However have you ever heard of meteotsunamis?

Meteotsunamis are massive waves that scientists are simply starting to raised perceive. In contrast to tsunamis triggered by seismic exercise, meteotsunamis are pushed by air-pressure disturbances typically related to fast-moving climate occasions, resembling extreme thunderstorms, squalls, and different storm fronts. The storm generates a wave that strikes in direction of the shore, and is amplified by a shallow continental shelf and inlet, bay, or different coastal function.

Meteotsunamis have been noticed to succeed in heights of 6 ft or extra. They happen in lots of locations all over the world, together with the Nice Lakes, Gulf of Mexico, Atlantic Coast, and the Mediterranean and Adriatic Seas.

Figuring out a meteotsunami is a problem as a result of its traits are nearly indistinguishable from a seismic tsunami. It may also be confused with wind-driven storm surge or a seiche. These uncertainties make it tough to foretell a meteotsunami and warn the general public of a possible occasion. Nonetheless, NOAA scientists have recognized atmospheric circumstances which are more likely to generate a meteotsunami and proceed to work on methods to forecast them.

Are tides larger when the moon is instantly overhead?

Earth viewed from the surface of the moon — This NASA picture from the Apollo 8 mission exhibits the Earth seen over the horizon of the moon. Whereas the moon and solar trigger tides on our planet, the gravitational pull of those celestial our bodies doesn’t dictate when excessive or low tides happen.

Tides originate within the ocean and progress towards the coastlines, the place they seem because the common rise and fall of the ocean floor. Because of Sir Isaac Newton’s 1687 discovery, we all know that tides are very long-period waves that transfer by the ocean in response to forces exerted by the moon and solar. Nevertheless, these gravitational forces don’t management when excessive or low tide occasions happen. Different forces, extra regional than the moon or solar, management the tides. Lots of them need to do with the geography and form of the Earth.

The form of our planet has loads to do with variations in gravitational pull at numerous places. If Earth was a real sphere lined by an ocean of fixed depth, then it might be true {that a} excessive tide occasion would happen on the location with the moon overhead. The tidal “bulge” would transfer across the Earth with the moon, however this isn’t the case with our planet. The Earth just isn’t a real sphere, however bulges barely on the Equator. Additionally it is dotted with massive land lots (continents). Areas the place the Earth’s floor is larger, akin to mountains, have a stronger gravitational power than do locations the place the floor is decrease, akin to a valley or cavern. On the similar time, the depth of the world ocean varies vastly. All of those components play into the peak of the tides.

One other factor to think about when enthusiastic about tides is the dimension of time. There are lags in time when an observer experiences excessive or low tide, and the lag varies from one location to a different. For instance, the rising slope of the ocean ground approaching the U.S. East (often known as the continental rise, continental slope, and continental shelf) slows down the tidal “bulge” or “wave” of water. The lag may be hours and even near a complete day in some elements of the world. Consider the wave or bulge that you simply see if you throw a rock right into a pond. The wave travels outward in a hoop. If there’s a tree or different obstruction within the pond, the ring of waves would journey and wrap round it. The identical holds true for the tides.

NOAA’s Heart for Operational Oceanographic Merchandise and Providers makes use of superior acoustic and digital tools to measure and monitor the tides alongside our coasts. NOAA has water remark stations in additional than 3,000 places across the U.S. to look at water ranges and generate tidal predictions.