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Author Topic: Why does the rate of sea level change vary by location?  (Read 7793 times)

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Why does the rate of sea level change vary by location?
« on: March 17, 2013, 02:07:20 AM »
At last a forum, where I can ask this question! (and probably get an answer)

So, Why does the rate of sea level change vary by location? (after correcting for subsidence/uplift and barometric pressure anomalies)

For bonus points, why specifically does it vary in different locations around Australia?

For background, the Bureau of Meteorology has been monitoring sea level change for about 20 years. On page 36 of this report: http://www.bom.gov.au/ntc/IDO60202/IDO60202.2011.pdf rates over the last 20 years vary from 2.9 to 9.0 mm a year around the Australian coast.

The report does refer to cyclical variations in La Nina/El Nino, which has a strong effect, but short cycles, and the Pacific Decadal Oscillation which has long cycles, but no estimate is made of it's effect on sea level - is this perhaps enough on it's own to explain the variation?

Also the average across all the coasts seems to be higher than estimates of global average sea level rise I've seen (ca 3.5mm/yr)

Thanks in advance....


RaenorShine

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Re: Why does the rate of sea level change vary by location?
« Reply #2 on: March 18, 2013, 07:17:14 PM »
I saw an article on this last month

http://www.climatecentral.org/news/ice-melt-means-uneven-sea-level-rise-around-the-world-15640

While mainly about modelling future sea level rise it goes into some of the reasons for regional differences they took into account.

1.  Ground rising and falling due to tectonics and response to the last ice age.
2.  The ocean is not warming uniformly.
3.  Changes due to the gravitational pull of large ice sheets in Greenland and Antarctica.

Hope this helps.

Glenn Tamblyn

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Re: Why does the rate of sea level change vary by location?
« Reply #3 on: March 25, 2013, 09:13:59 AM »
Jerry Mitovica's talk is good, apart from the fact that he keeps wandering away from the mike.

Drivers of sea level, and thus potentially sea level change, in rough order of scale, and particularly as observed relative to the coastlines:

Gravitational attraction to the mass of the continents raises sea levels near continents by large amounts - potentially 100's of meters. So this elevation difference varies depending on distance from the continent and the size of the continent.

Ice masses on the continents contribute further gravitational elevation near continents. Loss of this ice causes large scale sea level falls near those continents. Loss of the Greenland ice sheet would lower sea level at the Greenland coast by around 100 meters. This means that each separate region of ice loss has a specific 'fingerprint' wrt how much it effects sea level at different points on the Earth.

The distribution of densities inside the Earth's Mantle - it isn't equal density.  Generally sea levels follows the lines of equal gravitational strength - the Geoid - not a single even height. Measured from satellites, sea levels are more bowl shaped, higher near the land, lower in the center of the oceans, but also perturbed by mass distributions inside the Earth.

Land Ice Sheets melting due to the glacial cycle

Isostatic Adjustment as the sea floor sinks due to the extra weight of water in the ocean after the glacial melt. The ocean gets deeper under this extra weight so relative to the coast, sea level falls

Isostatic Adjustment of those continents previously supporting ice sheets that have melted the ice retreated causing those affected continents to rise.

Areas of subduction/mountain building that can result in localized uplift or subsidence

The West Antarctic Ice Sheet is a special case. It is a frozen sea with a large weight of ice sitting on top of it. So if it melts, the 'sea floor' of the West Antarctic 'sea' will rise where as the rest of the oceans sea floor sinks.

Groundwater/Oil/Gas extraction that can result in localized subsidence of the land. Particularly an issue near major cities that rely on groundwater.

Redistribution of water in the oceans as ice melts, with more of the melt water moving towards the equator causing extra elevation there due to bulging at the equator caused by the the Earths rotation.

Seasonal or occasional patterns that change the amounts of rain falling on land. With a lot of rain, storage on land of water increases - dams, lakes, but particularly ground water.  Two effects; less water in the ocean, and a small increase in the gravitational attraction the continents exert due to the extra mass of the water. Dry periods have the opposite effect.

Similarly the normal seasonal fall of rain and snow fall and melt affects global sea levels - Northern hemisphere has more land so in it's winter sea levels are lower than during the NH summer because there is more snow and water on land. Also the change in gravitational effects due to the uneven distribution of this results in uneven distribution of sea level changes over the seasons.

Ocean currents producing localized elevation. Circular currents tend to elevate water levels at their center. Longitudinal Currents can also push water ahead of them producing elevation. The Gulf Stream for example elevates water levels along the US East coast.

Winds producing elevation differences due to pushing water. Storm surges are dramatic short term changes, but prevailing winds such as the trade winds have a more consistent effect.

Differences in water temperature and salinity causing differences in density. Less dense water will have a slightly lower gravitational attraction to other water around it. Also, when water has a lower density this can partly change the pressure profile below it. As a result regions with less dense water in the water column will tend to sit a little higher.

Changes in air pressure above the ocean change water level below - water tends to flow away from under high pressure system.

The monthly lunar cycle as the moon is nearer / farther from the Earth.

The normal tides.

Maybe there is something I have missed.

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Re: Why does the rate of sea level change vary by location?
« Reply #4 on: March 27, 2013, 06:53:12 AM »
Thank you for the detailed response.

I don't if that was a comprehensive reply, but it sure looked like it to me.

Stephen

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Re: Why does the rate of sea level change vary by location?
« Reply #5 on: March 29, 2013, 01:31:11 PM »
I read somewhere ( a New Scientists article probably) that the world's oceans are like a series of bathtubs connected by small channels.  So if the North Atlantic were to mysteriously and immediately rise by 1 metre, it might take up to 30 years for that extra water to spread evenly around the globe.  Is that correct?

I suspect that the Greenland fingerprint effect (gravitational attraction) also means that those rich North Atlantic nations most responsible will not be as badly affected as poorer equatorial nations who have contributed much less to the current CO2 gas levels.  As the GIS melts, its mass will reduce, it will attract less water, so sea levels in the immediate vicinity (not sure how far out) will reduce to counter the effect of the SLR.  I have no idea how all this will balance out.
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The ice was all around:
It crack'd and growl'd, and roar'd and howl'd,   
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Glenn Tamblyn

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Re: Why does the rate of sea level change vary by location?
« Reply #6 on: April 01, 2013, 10:57:18 AM »
Yep, loss of the GIC will have a lesser impact in America/Europe than further south. But loss of the WAIS will have a greater impact in the Northern hemisphere, so it evens out. And if eventually we see all of Antarctica gone - much further away - the Northern Hemisphere will feel that more as well.

Neven

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Re: Why does the rate of sea level change vary by location?
« Reply #7 on: April 01, 2013, 01:45:32 PM »
A great article on SLR by Rob Painting was published a couple of days ago on Skeptical Science. I hadn't followed the SLR stuff for a while, but look at this graph:



I guess that trend line is going to stop rising real soon now, right? Right?
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Neven

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Re: Why does the rate of sea level change vary by location?
« Reply #8 on: April 01, 2013, 01:51:24 PM »
BTW, I'm moving this thread to the Consequences board.
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