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Sunday, November 23, 2008

Atlantic Meridional Overturning Circulation Data

I’ve moved to WordPress.  This post can now be found at Atlantic Meridional Overturning Circulation Data

I believe this is the paper that describes the data set:


In their Climate Indices webpage, the Royal Netherlands Meteorological Institute (KNMI) has an interesting historical data set available for downloading: Atlantic Meridional Overturning Circulation (AMOC).

KNMI website:
Climate Indices webpage:
ORA-S3, MOC [Sv] Meridional Overturning Circulation at 26N webpage:
MOC Data in text form:

The monthly data set begins in January 1961 and ends in December 2005. The units are Sverdrup (Sv), where 1 Sv is equal to a volume flow rate of 10^6 cubic meters per second. The notation “ORA-S3” at the top of the data page indicates the source, which is the European Centre for Medium-Range Weather Forecasts (ECMRF). Their webpage:
Chasing the data set farther, the ECMRF’s ORA-S3 System is described beginning on page 9 of their Autumn 2007 newsletter:

Enough with the background.


Figures 1 and 2 illustrate the Atlantic Meridional Overturning Circulation (AMOC) Data from January 1961 to December 2005. In Figure 1, the data is raw. The data has been smoothed with a 12-month running average filter in Figure 2 to remove the monthly noise. At first glance, the graph of the data set doesn’t appear similar to any other data I’ve run across to date, and the 1972 spike does not to correlate with any of the usual suspects: volcanic aerosols or ENSO.

Figure 1
Figure 2

The representation of AMOC bears no resemblance to the AMO. Refer to Figure 3.
Figure 3

The AMOC curve does not appear to have a basis in the High Latitude North Atlantic SST anomalies, Figure 4. Since that’s the area of AMOC subsidence, it seemed possible there might be a correlation. There’s not.
Figure 4

Maybe we need to change the appearance.


In Figure 5, the AMOC data has been inverted (multiplying it by -1). There are times when the inverted AMOC data may correlate with ENSO.
Figure 5

Figure 6 is a comparative graph of Inverted AMOC and NINO3.4 SST anomalies. I have not scaled the NINO3.4 data. I simply shifted it, changing its temperature range. Prior to 1977, there seems to be no correlation between NINO3.4 and Inverted AMOC. But, from the early 1977 to present, they correlate very well, with the exceptions of in-phase then out-of-phase response to the 1997/98 El Nino. What took place just before 1977? The Great Pacific Climate Shift of 1976.

Keep in mind that the AMOC data has been inverted in Figure 6 and that it illustrates that an increase in NINO3.4 SST anomaly causes a decrease in AMOC flow during most ENSO events from 1977 to present.
Figure 6


The source of the AMOC data is discussed in the Introduction of this post.

The Smith and Reynolds Extended Reconstructed SST (ERSST.v2) data is available through the NOAA National Operational Model Archive & Distribution System (NOMADS).http://nomads.ncdc.noaa.gov/#climatencdc


Carl said...

It's interesting that the circulation data correlates to Pacific Nino/Nina variability and not Atlantic Nino/Nina. It also looks like the 96/97 El Nino had a long-term effect on circulation in the N Atlantic. How would we expect the Atlantic Meridional Overturning Circulation to effect SST in the N Atlantic? How does the circulation effect the AMO?

Bob Tisdale said...

Carl: On decadal scales, an increase in North Atlantic SST should result in an increase in AMOC flow rate, and vice versa. Refer to:
It's not an overly scientific paper but it does a reasonable job. If the link doesn't go though, it's the second pdf when you Google "Atlantic THC SST".
The title: 3.2 Dynamics and Predictability of North Atlantic / European Climate Variability. It's from the IFM - GEOMAR Report 2002-2004.

And yes, you're right. The AMO is a residual, not an SST, so I should've used North Atlantic SST for the comparison, not the AMO in Figure 3. But my primary objective was to show the relationship with ENSO.

The thing to recall about this AMOC data set is that it is not a sensed flow rate; it’s a computer reconstruction. Does that make it wrong? No. It also doesn't make it right. Refer to the link at the start of the thread. The authors discuss the data pretty well in the summary, but they miss the link to ENSO.


climatechange1 said...

Is the apparent long-term response to the '97 El Nino exhibited by the AMOC in your graph what is refered to here:
http://www.nature.com/news/2008/081129/full/news.2008.1262.html ?

If so, they need to be pointed to the relationship with ENSO.


Bob Tisdale said...

Carl: I really can't answer your question. I'm not a subscriber to Nature so I don't have access to the full article. I'd want to see how they present the variance in MOC.


Anthony G Williams said...

Sorry to come late to this one, but I'm trying to find the answer to a question. I am not a scientist but take a layman's interest in climate change issues.

Up to a year or two ago the great concern in NW Europe was that the fresh water released by melting ice in the N Atlantic would block the northerly flow of warm water which keeps temperatures (especially in winter) much warmer than they otherwise would be. The fear was that this could happen quite quickly (over a period of years) and have a rapid chilling effect on the climate which would completely swamp the global warming effect. It was stated that the volume of water moving north had already reduced by about 30% since the 1970s.

Now all of a sudden this concern seems to have gone away, and I never hear it being mentioned any more. What happened to change opinions?

Bob Tisdale said...

Anthony: The catastrophic predictions were based on a study that was incomplete. The 2005 study showed a slowdown in AMOC between 1957 and 2004. Basically, they determined the flow in 1957 and 2004 and found the flow was slower in 2004:

A more complete study in 2007 showed that the slowdown was part of a normal (annual?) cycle of flow rates:

Anthony G Williams said...

Thanks for that, Bob. It's been quite a concern here in the UK.

I have updated my web article on the effects of global warming, intended for SF writers and other laypeople like myself: http://www.quarry.nildram.co.uk/Global%20warming.htm


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