Ocean Structure and MotionOcean Structure and Motion

TidesTides

Essential QuestionsEssential Questions

What are tides?What are tides? What are the different types of tides?What are the different types of tides? How do we measure and predict tides?How do we measure and predict tides? What are the consequences of tides?What are the consequences of tides?

What are tides?What are tides?

DefinitionDefinition

Tides are the regular rising and falling of the sea. Tides are the regular rising and falling of the sea. Tides are very long-period waves that move Tides are very long-period waves that move through the oceans in response to the forces through the oceans in response to the forces exerted by the moon and sunexerted by the moon and sun

When it is HIGH tide, the water has come a long When it is HIGH tide, the water has come a long way up the beach and at LOW tide you will see way up the beach and at LOW tide you will see lots of the shore because the sea has gone a long lots of the shore because the sea has gone a long way out. way out.

There are about TWO high tides and TWO low There are about TWO high tides and TWO low tides a day. tides a day.

Cause?Cause? Tides are due to the Tides are due to the gravitygravity of the of the MoonMoon trying trying

to pull the part of the sea closest to it towards it.to pull the part of the sea closest to it towards it. TheThe Sun Sun also has a gravitational effect on the also has a gravitational effect on the

sea. Although the Sun is larger than the Moon, it sea. Although the Sun is larger than the Moon, it is is furtherfurther away from the Earth, which means away from the Earth, which means that it has that it has lessless effect on our tides. effect on our tides.

High Tide

Low Tide

The Earth rotates once every 24 hours The Earth rotates once every 24 hours The Moon rotates around the Earth once every 27 The Moon rotates around the Earth once every 27

daysdays– The result is a tidal period which is 24 hours and 50 The result is a tidal period which is 24 hours and 50

minutesminutes– Most places on Earth have 2 high and 2 low tides each Most places on Earth have 2 high and 2 low tides each

separated by 12 hours and 25 minutesseparated by 12 hours and 25 minutes– High tide and low tide are separated by 6 hours and High tide and low tide are separated by 6 hours and

12.5 minutes12.5 minutes Twice a month, during the Twice a month, during the new moonnew moon and the and the full full

moonmoon, the Moon and the Sun are in line with the , the Moon and the Sun are in line with the Earth and so they pull together. This causes very Earth and so they pull together. This causes very high high tides and very low low tides high high tides and very low low tides

What causes the 2 BulgesWhat causes the 2 Bulges

On the side of the Earth facing the moon On the side of the Earth facing the moon (the near-side) gravity pulls the water (the near-side) gravity pulls the water towards the Moontowards the Moon

On the side of the Earth away from the On the side of the Earth away from the moon (the far-side) the Moon’s gravity is not moon (the far-side) the Moon’s gravity is not strong enough to cause a bulge. strong enough to cause a bulge. – Here inertia causes the water to continue to Here inertia causes the water to continue to

travel away from Earth, resulting in the far-side travel away from Earth, resulting in the far-side bulgebulge

What affects Tide heightsWhat affects Tide heights

Tidal heights are also strongly influenced by the shape of Tidal heights are also strongly influenced by the shape of the shorelinethe shoreline – When oceanic tidal bulges hit wide continental margins, the height When oceanic tidal bulges hit wide continental margins, the height

of the tides can be magnified. of the tides can be magnified. – Conversely, mid-oceanic islands not near continental margins Conversely, mid-oceanic islands not near continental margins

typically experience very small tides of 1 meter or lesstypically experience very small tides of 1 meter or less The shape of bays and estuaries also can magnify the The shape of bays and estuaries also can magnify the

intensity of tides.intensity of tides. – Funnel-shaped bays in particular can dramatically alter tidal Funnel-shaped bays in particular can dramatically alter tidal

magnitudemagnitude – Narrow inlets and shallow water also tend to dissipate incoming Narrow inlets and shallow water also tend to dissipate incoming

tidestides

Weather and windsWeather and winds– Strong offshore winds can move water away from Strong offshore winds can move water away from

coastlines, exaggerating low tide exposures. coastlines, exaggerating low tide exposures. – Onshore winds may act to pile up water onto the Onshore winds may act to pile up water onto the

shoreline, virtually eliminating low tide exposuresshoreline, virtually eliminating low tide exposures – High-pressure systems can depress sea levels, leading High-pressure systems can depress sea levels, leading

to clear sunny days with exceptionally low tides. to clear sunny days with exceptionally low tides. – Conversely, low-pressure systems that contribute to Conversely, low-pressure systems that contribute to

cloudy, rainy conditions typically are associated with cloudy, rainy conditions typically are associated with tides than are much higher than predictedtides than are much higher than predicted

What are the different types of tides?What are the different types of tides?

Tide Types Tide Types

Based on pattern of Based on pattern of appearanceappearance– DiurnalDiurnal– Semi-diurnalSemi-diurnal– MixedMixed

Based on tidal rangeBased on tidal range– SpringSpring– NeapNeap

Tides based on pattern of Tides based on pattern of appearanceappearance

DiurnalDiurnal– When there is only 1 high and 1 low tide per dayWhen there is only 1 high and 1 low tide per day

Semi-diurnalSemi-diurnal– When the 2 high tides and 2 low tides are about When the 2 high tides and 2 low tides are about

the same heightthe same height

MixedMixed– When the 2 high tides and 2 low tides are When the 2 high tides and 2 low tides are

different heightsdifferent heights

Types of tides based on tidal rangeTypes of tides based on tidal range

Neap TideNeap Tide– Tides with a lower high Tides with a lower high

tide and a higher low tide and a higher low tide, results in a less tide, results in a less than average tidal than average tidal rangerange

– Occur during the first Occur during the first and third quarters of the and third quarters of the moonmoon

– A result of the A result of the gravitational forces of gravitational forces of the Sun and Moon the Sun and Moon being cancelled outbeing cancelled out

Spring TideSpring Tide– Tides with the highest high tides and the lowest low Tides with the highest high tides and the lowest low

tides, results in a larger tidal rangetides, results in a larger tidal range– Occur during the full or new moonsOccur during the full or new moons– A result of the combined gravitational force of the Sun A result of the combined gravitational force of the Sun

and Moonand Moon

How do we measure and predict How do we measure and predict tides?tides?

Tidal TermsTidal Terms

Tidal rangeTidal range– The difference in height between the high tide The difference in height between the high tide

and the low tideand the low tide

Flood currentFlood current– incoming tide along the coast and into the bays incoming tide along the coast and into the bays

and estuariesand estuaries

Ebb currentEbb current– the outgoing tidethe outgoing tide

Tidal Terms continuedTidal Terms continued National Tidal Datum Epoch – National Tidal Datum Epoch –

– The specific 19-year period adopted by the National The specific 19-year period adopted by the National Ocean Service as the official time segment over which Ocean Service as the official time segment over which tide observations are taken and reduced to obtain mean tide observations are taken and reduced to obtain mean values for tidal datums. values for tidal datums.

Mean Higher High Water (MHHW) Mean Higher High Water (MHHW) – The average of the higher high water height of each The average of the higher high water height of each

tidal day observed over the National Tidal Datum tidal day observed over the National Tidal Datum Epoch. Epoch.

Mean High Water (MHW) Mean High Water (MHW) – The average of all of the high water heights of each tidal The average of all of the high water heights of each tidal

day observed over the NTDE. day observed over the NTDE. Mean Sea Level (MSL) Mean Sea Level (MSL)

– The arithmetic mean of hourly heights observed over The arithmetic mean of hourly heights observed over the NTDE. the NTDE.

Tidal Terms continuedTidal Terms continued

Mean Tide Level (MTL) Mean Tide Level (MTL) – The arithmetic mean of mean high water and The arithmetic mean of mean high water and

mean low water. mean low water. Mean Low Water (MLW) Mean Low Water (MLW)

– The average of all of the low water heights of The average of all of the low water heights of each tidal day observed over the NTDE. each tidal day observed over the NTDE.

Mean Lower Low Water (MLLW) Mean Lower Low Water (MLLW) – The average of the lower low water height of The average of the lower low water height of

each tidal day observed over the NTDE. each tidal day observed over the NTDE.

Today's recorders send an audio signal Today's recorders send an audio signal down a half-inch-wide sounding tube down a half-inch-wide sounding tube and measure the time it takes for the and measure the time it takes for the reflected signal to travel back from the reflected signal to travel back from the water's surface. The sounding tube is water's surface. The sounding tube is mounted inside a 6-inch diameter mounted inside a 6-inch diameter protective wellprotective well – This system also records 11 different This system also records 11 different

oceanographic and meteorological oceanographic and meteorological parameters. These include wind speed and parameters. These include wind speed and direction, water current speed and direction, direction, water current speed and direction, air and water temperature, and barometric air and water temperature, and barometric pressure.pressure.

Stations collect data every six minutesStations collect data every six minutes Timing is controlled on stations by a Geostationary Timing is controlled on stations by a Geostationary

Operational Environmental Satellite (GOES). Operational Environmental Satellite (GOES). The stations also use these satellites to transmit The stations also use these satellites to transmit

their data hourly to NOAA headquarters. their data hourly to NOAA headquarters. – In the event of a storm, the stations can be programmed In the event of a storm, the stations can be programmed

to transmit their data every six minutes.to transmit their data every six minutes. Field teams can quickly check and maintain the Field teams can quickly check and maintain the

systems using laptop computers. systems using laptop computers. In addition, all of the raw and processed data are In addition, all of the raw and processed data are

available over the Internet.available over the Internet.

Who uses tidal informationWho uses tidal information

Commercial and recreational fishermen use their Commercial and recreational fishermen use their knowledge of the tides and tidal currents to help knowledge of the tides and tidal currents to help them improve their catchesthem improve their catches

Navigation of ships into portsNavigation of ships into ports Coastal zone engineering projects, including the Coastal zone engineering projects, including the

construction of bridges, docks, etc.construction of bridges, docks, etc. Scientists are concerned with tides, water levels Scientists are concerned with tides, water levels

and tidal currentsand tidal currents – tidal mixing, pollutants, ocean – tidal mixing, pollutants, ocean circulation, climate, movement of floating species circulation, climate, movement of floating species

Recreational beachgoers and surfersRecreational beachgoers and surfers

What are the consequences of What are the consequences of tides?tides?

Erodes the shoreline and the near shore sea bottom.

Deposits or removes items from the shore. Brings food and removes waste from tidal

communities.