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Greenland Rising/�Kalaallit Nunaat Qaffappoq�

Lesson # 4: Sea Level: Oceans & Tides:

What can monitoring tides tell us about changes in sea level?

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WHAT CAUSES TIDES? �Gravity! The attraction between the Earth and the Moon

Earth and Moon: Gravity creates tidal bulges

Tide bulges form both in the side closest to the moon and on the far side

(Credit: NASA)

�The Moon and Earth exert a gravitational pull on each other. On Earth, the Moon’s gravitational pull causes the oceans to bulge out on both the side closest to the Moon and the side farthest from the Moon. These bulges create high tides. The low points are where low tides occur.The Moon’s gravitational pull on Earth, combined with other, tangential forces, causes Earth’s water to be redistributed, ultimately creating bulges of water on the side closest to the Moon and the side farthest from the Moon.Rising and ebbing tides happen as Earth’s landmasses rotate through the tidal bulges created by the Moon’s gravitational pull. Our observer sees the tides rise when passing through the bulges, and fall when passing through the low points. Of course, in reality the Earth isn’t a smooth ball, so tides are also affected by the presence of continents, the shape of the Earth, the depth of the ocean in different locations, and more. The timing and heights of the tide near you will be affected by those additional elements.Twice a month, when the Earth, Sun, and Moon line up, their gravitational power combines to make exceptionally high tides, called spring tides, as well as very low tides where the water has been displaced. When the Sun is at a right angle to the Moon, moderate tides, called neap tides, result. From our view on Earth, these tides coincide with certain lunar phases since they occur when the Moon reaches specific positions in its orbit.Earth’s tidal bulges don’t line up exactly with the Moon’s position. Because the Moon is orbiting in the same direction as the Earth rotates, it takes extra time for any point on our planet to rotate and reach exactly below the Moon. This means that the high tide bulges are never directly lined up with the Moon, but a little behind it.

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Daily 2 High and 2 Low Tides

Tidal Ranges Change Throughout the Month.

“Spring” and “Neap” High Tides

WHAT CAUSES TIDES?

Daily and Monthly

(Credit: NASA)

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High and Low Tides

Land masses interfere with tidal pull.

‘Polar Tides’

TIDES ARE DIFFERENT IN DIFFERENT LOCATIONS

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How Can Tides Tell Us About Sea Level?

Sea level is the mean tide level between high and low tide in a location. This can change over time.

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�Weather can affect the tide

Top Image:Circuitbasics.com; bottom Swellnet.com

Sun and clouds affect air pressure which affect water level and the tide.

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�Weather events also affect the tide

Mean Sea Level

Astronomical Tide

Storm Surge

Image of Sea level, Astronomical tide, Storm Surge

Image: Maritime Review

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�Example of weather event affecting tides from New York, but this happens everywhere.

Height in meters (NAVD)

3.8

3.0

2.3

1.5

.76

0.0

-.75

3.8

3.0

2.3

1.5

.76

0.0

-.75

DEC file photo

This graph of water levels at NOAA’s Battery tide gauge for the same time period shows Sandy’s destructive storm surge in NY Harbor. The blue plot shows the predicted water levels of the tides, based on the astronomy involved in producing tides. The green plot shows the actual water levels that resulted from Sandy’s visit. Note that the graph shows the same water level peak observed at Albany. Unlike flooding peaks due to runoff, which are generally only observed near Albany (and recorded at the Port of Albany and Schodack Island HRECOS stations), storm surge peaks – generated in the ocean – usually travel all the way up the estuary and are recorded at all the monitoring stations along the way. Click to see vessel grounded by Sandy’s surge in NY Harbor, and Click sailboats toppled over by the same surge at a marina in Rockland County, about 40 miles north of the harbor.

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Tide gauges measure tides and are installed around the world to collect data.

Tide gauges measure sea level continuously - hourly, daily, monthly, yearly. This is used to measure changes over time.

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A map is of long-term tide gauges around the world, many have collected data for >100 years & tell us about sea level change. What do you notice?

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Changes in sea level around Greenland are predicted using the few measurements we have, but there is a lot of uncertainty in the predictions.

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The Greenland Rising Project Installed Tide Gauges in Nuuk, Aasiaat and Kullorsuaq

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Students in Nuuk helped install a tide gauge

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Students in Aasiaat checking a tide gauge

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Tidal Forecast:�What causes this tidal change over the month?

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Moon phase affects the tides!

Which moon phases do you think match the more extreme tides?

meters

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Tide gauge Reading from Aasiaat

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Nuuk tide gauge data merged

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Thule tide gauge data

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(mm/yr)

Look again at other tide gauges around Greenland’s coast. None have been in as long as Nuuk’s and none of the others are seeing change the same way.

Do you remember possible causes for this?

If had data for Assiaat what do you think it would be doing? Why?

Thule 2004

Nuuk 1960

Qatortoq

2008

Ittoqqortoormiit

2008

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Change in relative sea-level

Adhikari, S., Ivins, E. R., Frederiksen, T., Landerer, F. W., and Caron, L.: Sea-level fingerprints emergent from GRACE mission data, Earth Syst. Sci. Data, 11, 629–646, https://doi.org/10.5194/essd-11-629-2019, 2019.

In this lesson we talk about the role of the ocean in local sea level change. For Greenland the loss of land ice has a much larger influence than the ocean. Water melting from Greenland will be added to the ocean where it will affect areas far from Greenland.

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Overall, around Greenland sea level fall �

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Tide Gauges are important for understanding Greenland’s sea level change, providing water depth changes felt by communities.