Dramatischer Meeresspiegelanstieg 2009-2010 an der nordamerikanischen Ostküste zwischen New York und Neufundland: Verantwortlich war eine seltene Konstellation atlantischer Ozeanzyklen

In den Jahren 2009 und 2010 ist der Meeresspiegel an der nordamerikanischen Ostküste zwischen New York und Neufundland um unglaubliche 12,8 cm nach oben geschnellt. Ein Resultat des menschengemachten Klimawandels? War dies der Anfang der von Klimaapokalyptikern prophezeiten Sintflut? Ein Forscherteam um Paul Goddard nahm sich der Sache an und untersuchte den Fall. Zunächst stellten die Wissenschaftler fest, dass sich der Anstieg auf die Jahre 2009 und 2010 beschränkte. Dies machte sie hellhörig, denn es passte so gar nicht zur eher langfristigen Wirkung des Klimawandels.

Im Zuge der Überprüfung der verschiedenen natürlichen Ozeanparameter stießen Goddard und seine Kollegen auch auf atlantische Ozeanzyklen, die genau zu dieser Zeit auffällige Entwicklungen durchmachten. Die sogenannte AMOC („Atlantic meridional overturning circulation”) fiel zu dieser Zeit stark ab und die NAO („North Atlantic Oscillation”) nahm außerordentlich negative Werte an. Diese seltene Kombination führte schließlich zum beobachteten starken Meeresspiegelanstieg. Als die Zyklen-Konstellation dann wieder vorbei war, sank der Meeresspiegel im betrachteten Küstengebiet wieder nahezu auf Normalwerte. Im Folgenden die Kurzfassung der Arbeit, die im Februar 2015 in Nature Communications publiziert wurde:

An extreme event of sea-level rise along the Northeast coast of North America in 2009–2010
The coastal sea levels along the Northeast Coast of North America show significant year-to-year fluctuations in a general upward trend. The analysis of long-term tide gauge records identified an extreme sea-level rise (SLR) event during 2009–10. Within this 2-year period, the coastal sea level north of New York City jumped by 128 mm. This magnitude of interannual SLR is unprecedented (a 1-in-850 year event) during the entire history of the tide gauge records. Here we show that this extreme SLR event is a combined effect of two factors: an observed 30% downturn of the Atlantic meridional overturning circulation during 2009–10, and a significant negative North Atlantic Oscillation index. The extreme nature of the 2009–10 SLR event suggests that such a significant downturn of the Atlantic overturning circulation is very unusual. During the twenty-first century, climate models project an increase in magnitude and frequency of extreme interannual SLR events along this densely populated coast.

In einer Pressemitteilung der University of Arizona vom 24. Februar 2015 erläuterten die Autoren ihre Resultate näher (Auszüge):

Sea Level Spiked for Two Years Along NE North America
[…] The paper is also the first to show that the unusual spike in sea level was a result of changes in ocean circulation. Co-author Jianjun Yin, UA assistant professor of geosciences, said, “We are the first to establish the extreme sea level rise event and its connection with ocean circulation.” Goddard detected the two-year-long spike in sea level by reviewing monthly tide-gauge records, some of which went back to the early 1900s, for the entire Eastern Seaboard. No other two-year period from those records showed such a marked increase. The team linked the spike to a change in the ocean’s Atlantic Meridional Overturning Circulation and also a change in part of the climate system known as the North Atlantic Oscillation. [...]

Yin’s previous work on climate models suggests that weakening of the Atlantic Meridional Overturning Circulation could cause sea levels to rise faster along the northeast coast of North America. Yin wondered whether such sea level rise had actually been observed, so he asked Goddard to compile the tide-gauge records for the east coast of North America. The 40 gauges, spanning the coast from Key West, Florida, north to Newfoundland, have been recording sea levels as far back as the 1920s. Goddard’s work revealed a surprise — that during 2009 and 2010, sea level between New York and Newfoundland rose an average of four inches. Sea level from Cape Hatteras to New York also had a notable spike, though not as dramatic. “The sea level rise of 2009-10 sticks out like a sore thumb for the Northeast,” Goddard said. […]

In addition to the weakening AMOC, during 2009-10 the region’s atmosphere was in a very negative phase of the climate mode called the North Atlantic Oscillation. The NAO flip-flops between negative and positive phases. “The negative North Atlantic Oscillation changes the wind patterns along the northeast coast, so during the negative NAO the winds push water onto the northeast coast,” Goddard said. Although the NAO has resumed flipping between positive and negative states, observations show that the AMOC, while somewhat stronger, has still not recovered its previous strength. Even now, sea level is still higher than before 2009, Yin said. He’s not surprised, because most of the climate models predict a weakening of the AMOC over the 21st century. Yin said that at the current rate of increase in greenhouse gases, most climate models predict a weakening of the AMOC over the 21st century. Therefore, such extreme sea level rise events and coastal flooding are quite likely to occur along the densely populated northeast coast of North America more often.