Why is the Arctic sea level dropping while global sea levels are rising?

[dimensionless]

Arctic dips as global waters rise
By Jonathan Amos
Science reporter, BBC News

Arctic sea level has been falling by a little over 2mm a year - a movement that sets the region against the global trend of rising waters.

A Dutch-UK team made the discovery after analysing radar altimetry data gathered by Europe's ERS-2 satellite.

It is well known that the world's oceans do not share a uniform height; but even so, the scientists are somewhat puzzled by their results.

Global sea level is expected to keep on climbing as the Earth's climate warms.

To find the Arctic out of step, even temporarily, emphasises the great need for more research in the region, the team says.


I think it's a true statement to say the Arctic Ocean is the least well understood body of water out there
Dr Seymour Laxon, UCL
"We have high confidence in the results; it's now down to the geophysics community to explain them," said Dr Remko Scharroo, from consultants Altimetrics LLC, who led the study.

...
http://news.bbc.co.uk/2/hi/science/nature/5076322.stm

I was quite surprised by all this. With global warming ice is melting, and sea levels in general are going up. Why should the Arctic be any different

 

[Pengwuino]

As an aside, can someon explain why levels aren't uniform in the first place?

 

[dimensionless]

The Earth bulges around the equator. I believe it is due to gravitational and centripetal forces.

 

[Bystander]

I was quite surprised by all this. With global warming ice is melting, and sea levels in general are going up. Why should the Arctic be any different

Probably the "Arctic High" noted in the link, 2mm in sea level is 0.2 mbar, barely above the noise level in atmospheric pressure measurements, but if sustained by an increase in the activity of the north polar atmospheric circulation cell for time exceeding the sea level response time, the sea level will respond --- what sustains that increased activity is another question.

 

[Mk]

Controversy concerning the significance of the present rate of global sea level rise, and how it may relate to the greenhouse effect and anthropogenic global warming hypotheses (AGW collectively) is very much in the public sphere due to it having "gone politicized". As the news media often does, highly complicated matters are over-simplified to give viewers and readers easy-to-digest meals that do not require much thinking—this is where the problem inlies in understanding of sea level.
Mean sea level (MSL) is the average (mean) height of the sea, with reference to a suitable reference surface. Defining the reference level (Permanent Service for Mean Sea Level), however, involves complex measurement, and accurately determining MSL can prove difficult. Finding the MSL change involves comparing the local height of the mean sea surface with a "level" reference surface or datum, called the geoid. In a state of rest with absence of external forces (totally stagnant water), the mean sea level would be the same at every point on the Earth. The geoid would only deviate from the perfect sphere in this theoretical model with local differences in MSL from local deviations in the Earth's gravitational field. In reality, due to currents, air pressure variations, temperature variations, salinity variations, etc., this does not occur, and prevents certain verifiable long term averages from being calculated. The location-dependent, but persistent in time, separation between mean sea level and the geoid is referred to as "stationary sea surface topography," which varies globally by ±2 meters, further offsetting the MSL.

Traditionally, one must have had to process sea-level measurements to take into account the effect of the 228-month Metonic cycle and the 223-month eclipse cycle on the tides (both having to do with the moon's effect on sea level). MSL never remains constant over the surface of the entire earth. For instance, MSL at the Pacific end of the Panama Canal stands approximately 20 centimeters (0.6 ft) higher than at the Atlantic end.

Despite the difficulties, aviators flying under instrument flight rules (IFR) must have accurate and reliable measurements of their altitudes above (or below, for airports such as in the Netherlands) local MSL, and the altitude of the airports where they intend to land.

Several terms are used to describe the changing relationships between sea level and dry land. When the term "relative" is used, it connotes change that is not attributed to any specific cause. The term "eustatic" refers to changes in the amount of water in the oceans, usually due to climatic changes. The melting of glaciers at the end of ice ages is an example of eustatic sea level rise. The term "isostatic" refers to changes in the land level, of land masses due to thermal buoyancy or tectonic effects and implies no real change in the amount of water in the oceans, although isostatic changes change the MSL because it is relative to the land. The subsidence of land due to the withdrawal of groundwater is an isostatic cause of relative sea level rise. Paleoclimatologists can track sea level by examining the rocks deposited along coasts that are very tectonically stable, like the east coast of North America. Areas like volcanic islands often experience relative sea level rise as a result of isostatic cooling of the rock which causes the land to sink.

 

[Andre]

See also:

http://earth.esa.int/workshops/venice06/participants/607/pres_scharroo607.pps#23

or the whole slide show:

http://earth.esa.int/workshops/venice06/participants/607/pres_scharroo607.pps

The paper:

http://earth.esa.int/workshops/venice06/participants/607/paper_607_scharroo.pdf

Could be just noise, changes in the oscillation patterns (especially the North Atlantic Oscilation) or perhaps it has to do with micro gravity changes due to the changing shape of the Earth:

http://www.space.com/scienceastronomy/equator_bulge_020801.html

 

[dimensionless]

Probably the "Arctic High" noted in the link, 2mm in sea level is 0.2 mbar, barely above the noise level in atmospheric pressure measurements, but if sustained by an increase in the activity of the north polar atmospheric circulation cell for time exceeding the sea level response time, the sea level will respond --- what sustains that increased activity is another question.

So in other words it's caused by wind?

 

[Bystander]

Not the wind, the high pressure cell associated with the wind.

 

[eternalenemy]

This is amazing about the discussion about global warming, every1 has something to say, but the conclusion is that we won't be able to stop it now.

 

[Mk]

I didn't get that from the conversation.

 

[jasc15]

What I get from this is that we don't know the cause, only that it is happening.