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RECENT OBSERVATIONS FROM THE 26ºN RAPID MOORING ARRAY: DROPS, DECLINES AND IMPACTS Gerard McCarthy National Oceanography Centre UK Molly Baringer, Adam.

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Presentation on theme: "RECENT OBSERVATIONS FROM THE 26ºN RAPID MOORING ARRAY: DROPS, DECLINES AND IMPACTS Gerard McCarthy National Oceanography Centre UK Molly Baringer, Adam."— Presentation transcript:

1 RECENT OBSERVATIONS FROM THE 26ºN RAPID MOORING ARRAY: DROPS, DECLINES AND IMPACTS Gerard McCarthy National Oceanography Centre UK 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

2 INTRODUCTION

3 Why 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.

4 The AMOC in a changing climate In a changing climate, the AMOC is predicted to weaken

5 Data available from www.rapid.ac.uk/rapidmocwww.rapid.ac.uk/rapidmoc AMOC timeseries

6 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?

7 EXTREME EVENTS

8 Ekman drop associated with negative Arctic Oscillation/NAO Only explains 3 months of downturn Arctic Oscillation 2  Slowdown in winter 2009/10

9 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 Slowdown in winter 2009/10

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

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

12 MOC 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

13 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

14 MULTI-YEAR DECLINE

15

16 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

17 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

18 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

19 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

20 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

21 CONCLUSIONS Extreme 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 21 st century -Rate of decline indicates a relationship with AMO, accumulated NAO

22 End

23 The research leading to these results has received funding from the European Union 7th Framework Programme (FP7 2007-2013), under grant agreement n.308299 NACLIM www.naclim.euwww.naclim.eu


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