A couple of recent papers in Science this week relate to discussions we have had on RealClimate recently. The first by Curry and Mauritzen relates to the Gulf Stream Slowdown? post and describes the amount of fresh water that has been added to the North Atlantic over the last few decades (calculated from a database of salinity measurements) and what impact that has had on density and overturning circulations. There is a press release available at WHOI which is quite informative (hat tip to Joseph O’Sullivan for the link).
The second is related to the Storms and Climate Change post and is a perspective by Kevin Trenberth on the potential for a hurricanes and global warming link. The NCAR press release is available here.
These will undoubtedly not be the last word on the subject, and so we will probably be revisiting these topics at some point soon…
those were good articles
Another story that updates a previous RC thread:
“Scripps Studies Provide New Details About Antarctic Iceberg Detachment”
http://www.sciencedaily.com/releases/2005/06/050614002105.htm
Interesting findings by Curry and Mauritzen. Unexpected is the timing of the largest freshening (10,000 km3) of the Nordic Seas in the late 60s / early 70s. That occured at the moment that the whole NH was (slightly) cooling and midst of a negative NAO, AO and PDO. What was the origin of that extra water?
Probably not from precipitation, as the discharge of the 6 largest rivers in the Arctic increased with only 7% (or 128 km3) in the past 60+ years. Neither from a change in precipitation in the whole Arctic, which is estimated some 500 km3 extra in recent decades.
Probably not from Arctic sea ice cover reduction either, as the ice cover in the late 60s, begin 70s doesn’t show a decline (as far as reliable in the pre-satellite era). Neither from land ice, as the retreat of Greenland glaciers was lowest in the 1950-1975 period.
Neither from ice export, as that is positively related to the NAO index, thus with a negative NAO period, the ice export would be lower.
Thus what caused the fresh water pulse into the (sub)Arctic in the late sixties?
I suppose that it would be fair to point to an alternative view on hurricanes and global climate change by Roger Pielke, Chris Landsea e.a., accepted for publication by the Bulletin of the American Meteorological Society.
Is the “freshening” a result of additional fresh water, or a reduction in salt / salt water?
Perhaps salt was removed through accellerated deep water formation? Lower temp might mean deep water would have formed and sank at a higher rate, depleting surface salt levels faster than usual. The surface water would thus have freshened until a new equilibrium was reached.
[Response: Nope. It’s the addition of freshwater. Since salt is (for these purposes) a conserved variable in the ocean, mean changes in salinity can only occur through fresh water addition. The only issue is where this freshwater comes from – arctic sea ice, groundwater discharge, increasing rainfall or melting glaciers. -gavin]
I see no areas of significant substantive disagreement between Trenberth (2005) and Pielke et al. (in press). Read the two papers, Trenberth’s paper is narrower (e.g., focuses on Atlantic and does not consider societal impacts) but where the two papers overlap there is a clear consensus. For a more detailed perspective on this overlap, see:
http://sciencepolicy.colorado.edu/prometheus/archives/climate_change/000465consensus_on_hurrica.html
I have asked Ruth Curry about the origin of the large pulse of fresh water in the late sixties. Here follows here answer:
“It is believed that in the late 1950s and early 1960s, freshwater and sea ice accumulated in the Beaufort Gyre as a consequence of the prevailing Arctic atmospheric circulation patterns (anti-cyclonic around an Arctic high pressure center.) Those atmospheric patterns shifted to cyclonic in the early- to mid-1960s driving the accumulated freshwater and sea ice through Fram Strait — the main source of the Great Salinity Anomaly in the late 1960s.
It is doubtful that the fresh water flow actually “reversed”. The magnitude and sense of net fresh water flow between the Arctic and Nordic Seas is determined by the difference between poleward flowing saline Atlantic waters and southward flowing fresh waters. By volume, the fresh water always wins; but both flows exhibit significant fluctuations with time. Hence in some periods, the net freshwater flow out of the Arctic may be enhanced or reduced.”
The latter answer was not on what I asked, as that was meant if a lower inflow of salty water (via the Gulf Stream in negative NAO conditions), may be the cause (with relative constant fresh water inflow / precipitation). But it seems that the outflow of fresh water/ice increased, thus that doesn’t point to less inflow from the GS.
There’s an important new study on aerosols, another topic recently covered on RC: http://davidappell.com/archives/00000865.htm . I have a Nature sub but somehow missed this. I haven’t had a chance to read the paper itself as yet, but the abstract and discussion are to the effect that we could be looking at 6 to 10 degrees C by 2100. Interestingly, this is the same range of temperature increase that climate prediction.net got slapped down for postulating earlier this year. Is this new study more credible?
[Response: See my latest post. -gavin]