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RECENT OBSERVATIONS FROM THE 26ºN RAPID MOORING ARRAY: DROPS, DECLINES AND IMPACTS. Gerard McCarthy National Oceanography Centre UK. - PowerPoint PPT Presentation
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RECENT OBSERVATIONS FROM THE 26ºN RAPID MOORING ARRAY:
DROPS, DECLINES AND IMPACTS
Gerard McCarthy
National Oceanography CentreUK
Molly Baringer, Adam Blaker, Harry Bryden, Stuart Cunningham, Aurélie Duchez, Eleanor Frajka-Williams, Joel Hirschi, Will Hobbs, Bill Johns, Chris Meinen, Matt Palmer, Darren Rayner, Chris
Roberts and David Smeed
INTRODUCTION
INTRODUCTIONWhy we study the AMOC:• Impact on climate• Evidence of major shifts in the past associated with major climate events
from Rahmstorf, S. and A. Ganopolski, Long-term global warming scenarios computed with an efficient coupled climate model. Climatic Change, 1999. 43: p. 353-367.
The AMOC in a changing climate
In a changing climate, the AMOC is predicted to weaken
• Large downturn in 2009/10 driven by short term Ekman variability (3 months) and long term strengthening of the gyre/UMO (18 months)
• Double dip in winter 2010/11• Evidence of a decline in the AMOC from the latest timeseries
AMOC timeseries to Oct. 2012
Major Downturn
Double Dip
Continued downturn?
EXTREME EVENTS
Ekman drop associated with negative Arctic Oscillation/NAOOnly explains 3 months of downturn
Arctic Oscillation
2 s
Slowdown in winter 2009/10
McCarthy, G., et al. (2012), Observed Interannual Variability of the Atlantic Meridional Overturning Circulation at 26.5N, Geo. Res. Lett.
*Seasonal cycle was removed, and data smoothed with 180-day filter
Longer time scale changes: 18 month weakening of MOC
Anomalously southward UMO: shift from overturning to gyre circulation
Slowdown in winter 2009/10
Cunningham—next talk: The North Atlantic cooled following the downturn
the thick pink curve (total predicted heat content change) would match the black curve (observed heat content change)
Cunningham et al., Atlantic Meridional Overturning Circulation slowdown cooled the subtropical ocean submitted to GRL
Implications for Heat Content
Double Dip: Winter 2010/11
Taws SL, Marsh R, Wells NC, Hirschi JJM (2011) Re-emerging ocean temperature anomalies in late-2010 associated with a repeat negative NAO. GRL
Second large dip in AMOC transport in winter 2010/11 following Arctic oscillation low
This is largely explained by Ekman contributions
Ocean re-emergence of SST links the two events
Double Dip: Winter 2010/11
Arctic Oscillation
AMOC
Double Dip: Winter 2010/11MOC
Ekman
In an ensemble of NEMO runs, double dips of MOC have occurred previously in 1969/70 and 1978/79
Extreme negative Arctic Oscillation (AO) correspond with double dip analogues
Corresponds with Ekman lows Blaker et al.; Historical analogues of the recent extreme minima observed in the Atlantic meridional overturning circulation at 26◦N. submitted to Clim. Dyn.
Historical Analogues of Double Dip
AO
Double Dip: Winter 2010/11
Winters of 2010/11 (green), 1969/70 (blue) and 1978/79 (red). Black shows mean (1960-2011) with 1, 2 std envelopes
• Reemerging SSTs are observed in 1969/70 as well as in 2010/11
• These were conducive to the development of the negative NAO in winter 2010
Double Dip: Implications
Maidens et al. in prep, The Influence of Surface Forcings on Prediction of the North Atlantic Oscillation Regime of Winter 2010-11. submitted to Monthly Weather Review
Re-emerging SSTs
Buchan et al., North Atlantic SST anomalies and the cold north European weather events of winter 2009/10 and December 2010. accepted in Monthly Weather Review
• Evidence that this second negative is predictable due to the ocean
MULTI-YEAR DECLINE
MULTI-YEAR DECLINE
Anomalies (positive is northward) relative to mean annual cycle. A 45-day low-pass filter was applied to each time series. Top: MOC - trend = -0.54 ± 0.45 Sv/yr Bottom: Ekman - trend = 0.00 ± 0.32 Sv/yr
MULTI-YEAR DECLINE
Anomalies (positive is northward) relative to mean annual cycle. A 45-day low-pass filter was applied to each time series. Top: Gulf Stream - trend = -0.11 ± 0.32 Sv/yr Bottom: UMO - trend = -0.41 ± 0.34 Sv/yr
MULTI-YEAR DECLINE
Anomalies (positive is northward) relative to mean annual cycle. A 45-day low-pass filter was applied to each time series. Top: UNADW - trend = -0.00 ± 0.26 Sv/yr Bottom: LNADW - trend = 0.46 ± 0.30 Sv/yr
MULTI-YEAR DECLINE
Annual average AMOC from the 26°N array (red, Sv), estimates of the AMOC from 6 hydrographic sections (black, Sv), time series of annual average values of the AMO (blue, °C) and accumulated NAO index (green, arbitrary units).
MULTI-YEAR DECLINE
Conclusions:• From April 2008 to March 2012 AMOC was 2.7 Sv weaker than from April 2004 to March 2008.
• 0.2 Sv of change was due to Gulf Stream, 0.5 Sv was change in Ekman but majority (2.0 Sv) was in upper mid-ocean.
• Removing the anomalous year of 2009 the reduction was 1.6 Sv.
• The increase in southward flow above the thermocline was compensated by a reduction in the flow of LNADW. There was no significant change in UNADW.
• The rate of slowing of LNADW was 7% per year.
Observed decline of the Atlantic Meridional Overturning Circulation 2004 to 2012
Smeed, D. A., et al. Ocean Sci. Discuss., 10, 1619-645
MULTI-YEAR DECLINE
CONCLUSIONSExtreme Events:- A decline of 30% over 18 months in the AMOC cooled the North Atlantic in winter 2009/10
- A second dip occurred in winter 2010/11 with reemerging SSTs linking the winters
- When oceanic conditions are right, two cold winters in NW Europe occur together and the second one may be predictable
- Sea Level: Boston, Atlantic City, Norfolk all had peaks in sea level following extreme AMOC
Multi-Year Decline- At least a 2 Sv decline over observed period (11% in a decade)- IPCC predictions range from 11% to 34% over 21st century- Rate of decline indicates a relationship with AMO, accumulated NAO
End
The research leading to these results has received funding from the European Union 7th Framework Programme (FP7 2007-2013), under grant agreement n.308299NACLIM www.naclim.eu