Mike Mann on Defining the Gulf Stream

March 26, 2015

Michael Mann, along with lead author Stefan Rahmstorf, Glaciologist Jason Box, and others, have published new findings on a slowdown in the Atlantic Meridional Overturning Circulation (AMOC).  If you have not seen the post on that, go there now.
For most folks, and in many media accounts, this circulation sounds very much like what we think of as “the Gulf Stream”. But there are some distinctions.  In our wide ranging conversation, I asked Mike to flesh that out.

Below, video from a Dutch group that has been modeling the impact on changes in temperature and salinity in the north atlantic, due to melting in Greenland and elsewhere. Those changes can impact weather around the globe, but, maybe not as intuitively, sea level rise.  Recent published research has shown that changes in the flow of the AMOC can cause abrupt impacts on sea level on the eastern coast of North America.

Salenger et al, Nature Climate Change 2012:

Climate warming does not force sea-level rise (SLR) at the same rate everywhere. Rather, there are spatial variations of SLR superimposed on a global average rise. These variationsare forced by dynamic processes, arising from circulation and variations in temperature and/or salinity, and by static equilibrium processes, arising from mass redistributions changing gravity and the Earth’s rotation and shape.
_

Our analyses support a recent acceleration of SLR on 1,000km of the east coast of North America north of Cape
Hatteras. This hotspot is consistent with SLR associated with a slowdown of AMOC.
BBC:

Sea levels along the northeast coast of the US rose by record levels during 2009-2010, a study has found.

Sea levels north of New York City rose by 128mm (4-5 inches) in two years, according to a report in the journal, Nature Communications.

Coastal areas will need to prepare for short term and extreme sea level events, say US scientists.

Climate models suggest extreme sea level rises will become more common this century.

“The extreme sea level rise event during 2009-10 along the northeast coast of North America is unprecedented during the past century,” Prof Jianjun Yin of the University of Arizona told BBC News.

“Statistical analysis indicates that it is a 1-in-850 year event.”

Dr Dan Hodson, also from the University of Reading, said the analysis underlined the importance of understanding the connections between surges in sea levels and ocean currents.

“Sea level change is a complex phenomenon, especially on the regional scale, where changes to the global ocean circulation can play a major role,” he said.

“The east coast of North America is quite close to an area of active, fast ocean currents, and so is quite sensitive to changing ocean circulation.”

He said the Atlantic Meridional Overturning Circulation (AMOC), a major current in the Atlantic Ocean, had implications for Europe and Africa as well as the US.

UA geoscientist Paul Goddard and his colleagues determined that sea level rose four inches from New York to Newfoundland (red dots) in 2009 and 2010. Gauges from New York south to Cape Hatteras (pink dots) showed a smaller spike in sea level for the same time period. No sea level spike was recorded on the gauges (white dots) south of Cape Hatteras. (Paul Goddard/UA Department of Geosciences)

UA geoscientist Paul Goddard and his colleagues determined that sea level rose four inches from New York to Newfoundland (red dots) in 2009 and 2010. Gauges from New York south to Cape Hatteras (pink dots) showed a smaller spike in sea level for the same time period. No sea level spike was recorded on the gauges (white dots) south of Cape Hatteras. (Paul Goddard/UA Department of Geosciences)

Goddard and Yin et al, Nature Communications, Feb 2015

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.

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