[Coral-List] Changes in the Gulf Stream

[Thomas Goreau]

Unfortunately the article quoted below by Melissa Keyes about the  
Gulf Stream weakening is incorrect.

Although the newspaper article said that the researchers had detected  
a 30% decrease in the Gulf Stream, this is not in fact what they  
reported. Harry Bryden and colleagues were using vertical  
oceanographic density profile cross sections for calculating changes  
in the flow of the Gulf Stream between Florida and the Bahamas, and  
the degree to which this flow then bifurcated between the northern  
branch that flows to the British Isles and Scandinavia, and the  
southern branch that flows towards Portugal, Morocco, and then  
circles around back to the Bahamas in the North Atlantic gyre. The  
researchers reported no significant change in the flow of the Gulf  
Stream itself off Florida (despite what the newspapers claimed), but  
that more was heading to the southern branch than the northern  
branch. This was based on measurements at a few time intervals. Later  
work, including the same authors, then showed that this difference  
was much less than the normal year to year fluctuations, and so no  
meaningful temporal trends could be demonstrated.

It should be realized that the Gulf Stream is a consequence of the  
rotation of the Earth, and so is basically impossible to stop unless  
the earth stops spinning. But that does not mean it cannot shift  
direction or flow rates. But the direct oceanographic data is still  
too sparse in time and space to give very good handles on long term  
changes. There is an indirect, or "poor man's" way to get at this  
issue, namely to look at the long term trends in sea surface  
temperature anomalies. The entire area affected by the Gulf Stream  
and by the Kuro Shio is warming well above the average rate of the  
ocean as a whole. This implies that the surface heat flow out of the  
tropics in these currents is increasing, but says little about the  
integrated vertical structure of the flows, which is what the  
oceanographic density sections are used to calculate.  While changes  
in thermocline depth could be involved (but are unlikely as this  
would need a shallowing thermocline, while there is clear evidence  
that the global average thermocline is getting deeper). Above average  
rise of SST is especially common in the bulk of the ocean where wind  
speeds are decreasing, and these are largely areas with decreasing  
surface chlorophyll.  The opposite is happening in those areas where  
wind speed is increasing, which are primarily in the middle of the  
big ocean gyres in the SE Paciific, NE Pacific, SE Indian Ocean,  
North Atlantic near Iceland, SE Atlantic,and all around Antarctica. I  
these areas SST is rising more slowly than average, and  chlorophyll  
is increasing since more nutrients are coming up from wind driven  
upwelling. but these areas are much smaller and do not compensate for  
the larger areas of reduced wind speed and above average temperature  
rise and decreasing primary production.

For more discussion of these issues see:
T.J. Goreau R.L. Hayes, & D. McAllister, 2005, Regional patterns of  
sea surface temperature rise: implications for global ocean  
circulation change and the future of coral reefs and fisheries, WORLD  
RESOURCE REVIEW, 17: 350-374


Thomas J. Goreau, PhD
President
Global Coral Reef Alliance
37 Pleasant Street, Cambridge MA 02139
617-864-4226
goreau at bestweb.net
http://www.globalcoral.org


> Date: Mon, 14 Jan 2008 02:29:07 -0800 (PST)
> From: Melissa Keyes <mekvinga at yahoo.com>
> Subject: [Coral-List] The Gulf Stream
> To: Coral-List at coral.aoml.noaa.gov
> Message-ID: <789199.24268.qm at web50106.mail.re2.yahoo.com>
> Content-Type: text/plain; charset=iso-8859-1
>
> Hello, Listers,
>
>   30% weakening of the Gulf Stream detected.  The following is  
> blatantly copied off the internet:
>
>   THE Gulf Stream currents that give Britain its mild climate have  
> weakened dramatically, offering the first firm scientific evidence  
> of a slowdown that threatens the country with temperatures as cold  
> as Canada?s.
>
>   The rest of the article at:  http://www.timesonline.co.uk/article/ 
> 0,,2-1898493,00.html
>   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   And another from North Carolina, U.S.A.:
>
>   A consequence of the theory of global warming is that the polar  
> ice caps will melt, and scientists postulated that this would have  
> a dramatic affect on the Gulf Stream.
>   With global warming, extra freshwater from the melting ice caps  
> and glaciers reduces the salinity of the Arctic waters, stopping it  
> from sinking, and breaking the circuit of the Gulf Stream.
>
>   Scientists have always predicted that the melting of the ice caps  
> could disrupt the Gulf Stream, but new research suggests this  
> process is already in play. In fact, they have concluded that the  
> strength of the Gulf Stream has weakened by 30 percent in just the  
> past 12 years.
>
>   Furthermore, these studies point to a cooling of 1C (1.8F) over  
> the next decade or two for Britain's climate, with an even deeper  
> freeze predicted if the Gulf Stream system were to shut down  
> completely.
>
>   Though most oceanographers think it is very unlikely that the  
> Gulf Stream will stop altogether, if it did happen, it could reduce  
> average temperatures by between 4C (7.2F) and 6C (10.8F) in as  
> little as 20 years, far outweighing any increase in temperatures  
> predicted across the globe as a result of global warming.  
> Essentially, as the rest of the world heats up, Britain and the  
> British Isles would get much colder.
>
>   Over the same period of time, the flow of warm water that  
> branches off the Gulf Stream near the North Carolina coast and  
> heads east towards Africa has increased significantly, contributing  
> in the decline of warm waters being carried to Europe.
>
>   A project is currently underway to determine if these findings  
> were an indication of a long term, progressing problem for the Gulf  
> Stream, or simply a disruption that may change seasonally or  
> annually. The currents of the Gulf Stream will be monitored  
> continuously for a four year period to find the answer to this  
> question.
>   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   The rest of the article at:  http://www.ncbeaches.com/Features/ 
> Weather/GulfStream/
>
>   Cheers,
>
>   Melissa E. Keyes
>   St. Croix, USVI, Caribbean
>