Mediterranean Sea Level
Monday, March 7, 2011 4:06:15 PM
“At the end of the 20th century, the rise in sea level of the Mediterranean sea was lower than in the rest of the world due to atmospheric pressure, but since the start of the 21st century the levels of the Mediterranean have regained pace and seem to be accelerating.”
To begin with, here is how I see the sea level regime in the Mediterranean. It is to a large extent determined by rain and river runoff on the plus side and evaporation on the minus side. Evaporation greatly exceeds precipitation and river runoff. Without the connection to the Atlantic Ocean via the Strait of Gibraltar, the Mediterranean Sea would slowly dry out and become an enormous salt pan with salt lakes at the deepest points. There is plenty of evidence that this has happened in the past. Through the strait of Gibraltar, water generally flows more or less continually in both an eastward and a westward direction. A smaller amount of deeper saltier and therefore denser waters continually work their way westwards (the Mediterranean outflow), while a larger amount of surface waters with lower salinity and density continually work their way eastwards (the Mediterranean inflow). On the whole and over time, the balance of the water flow is eastwards, exactly due to the high evaporation rate within the Mediterranean basin. Relatively cool, low-salinity water from the Atlantic flows across the basin. It warms and becomes saltier as it travels east, then sinks in the region of the Eastern Mediterranean Sea and circulates westward, to spill over the Strait of Gibraltar. The following map shows the resulting surface currents in the Mediterranean.
And next a few word about terms used in connection with sea-level rise:
When we talk about sea-level rise in connection with global warming, we are normally thinking about the eustatic sea level. The eustatic sea level is a measure of the distance between the sea-surface and the centre of the Earth.
Relative sea-level is on the other hand the distance between the sea surface and a local point, for example the top of the basement rocks in a sedimentary basin, or a local point on the sea shore.
Global warming lead to sea level rise in two ways:
1) Melting of glaciers and ice caps adding to the total global amount of sea water.
2) Thermal expansion of the sea water.
Global changes in sea level due to thermal expansion and salinity variations is covered by the term steric sea-level change.
As to more long term sea-level changes I refer to my post on
Long Term Sea-Level Changes.
According to a paper from 2002 on Sea level in the Mediterranean Sea: The contribution of temperature and salinity changes
"Temperature variations cause most of the overall steric sea level change in the upper 400 m. Between 1960 and the 1990s cooling of the upper waters of the Eastern Mediterranean caused reduction in the steric heights while after 1993 warming caused sea level to rise. The steric sea level changes in the upper waters of the Adriatic and the Aegean Sea are correlated with the North Atlantic Oscillation. ... the diverse behaviour of the Eastern Mediterranean at sub-basin scales questions the meaningfulness of climatic basin averages."
And now back to the EurekAlert news item, which a.o. states that "the rise in sea level of the Mediterranean sea was lower than in the rest of the world due to atmospheric pressure". Now, I wonder why that would be, but as I said I have not read the book. At a local level higher pressure will normally lead to drier weather, and a lower pressure lead to wetter weather, or greater rainfall, if you like - wind patterns are by the way also dependent on atmospheric pressure. Apart from the amount of water, rainfall will also influence the salinity. If that is what they are hinting at, why don't they write so, and if not it might have been a good idea to spell it out. Further the sea level in the Mediterranean is said to have risen by between 1 and 1.5 mm each year since 1943, but it now seems that the speed at which it rises is accelerating.
According to the book the changes which occurred in the temperatures are not only due to the effects of climate change, but also to natural and "normal" atmospheric (there we go again!) changes - is the author just talking about the weather - or what?
See also: http://my.opera.com/nielsol/blog/2008/03/07/long-term-sea-level-changes