Helmuth Thomas 1, Friederike Prowe 1,4, Ivan D. Lima 2, Scott C. Doney 2, Rik Wanninkhof 3, Richard Greatbach 1,4, Antoine Corbière 5 & Ute Schuster 6 1: Dalhousie University, Halifax, Canada. 2: Woods Hole Oceanographic Institution, Woods Hole MA, USA. 3: NOAA, Miami, USA. 4: now at IfM-Geomar, Kiel Germany. 6: Laboratoire d'Océanographie et du Climat: Expérimentations et Approches Numériques, Pairs France. 6:University of East Anglia, Norwich, UK Temporal variability of the CO 2 system in the North Atlantic Ocean

Observations in the North Atlantic during /4 BATS, Bermuda ΔpCO 2 constant Bates, 2001/7 North Sea ΔpCO 2 declining VOS line UK – Caribbean ΔpCO 2 declining Schuster and Watson 2007 Sub Polar North Atlantic ΔpCO 2 declining Omar and Olsen, 2006 South of Greenland ΔpCO 2 declining Corbiere et al., 2007 North Atlantic Drift Region ΔpCO2 declining Lefèvre et al., 2004 NW Atlantic ΔpCO 2 increasing Lueger et al., 2006 Thomas et al., 2008, GBC

Simulations from Community Climate System Model, POP coarse resolution Ecosystem model coupled to modified OCMIP-II biogeochemistry NCEP/NCAR reanalysis forcing Further reading: Doney et al., 2004, Moore et al. 2004, Yeager et al., 2006 pCO 2 decomposition (DIC norm, A Tnorm, T, S) Trend regression analysis Anthropogenic and preindustrial runs Simulations for the North Atlantic Ocean What is the driver for the observed CO 2 flux variability in the North Atlantic Ocean? Thomas et al., 2008, GBC

Simulations vs. Observations for the North Atlantic Ocean Corbière et al. Model water+2.5 ppm yr ppm yr -1 Atmos.+1.6 ppm yr ppm yr -1 temp.- pCO ppm yr -1 DIC+0.6 μmol (kg yr) -1 ATAT -0.3 μmol (kg yr) -1 Thomas et al., 2008, GBC

Period Atm1.9 µatm/yr2.1 µatm/yr Ocean3.6 µatm/yr5-7 µatm/yr Why ?Warming+DIC/-TA Full story presented by Corbière et al. (Poster, CT1) Ocean CO 2 trends in the North Atlantic Subpolar Gyre : winter / (based on DIC/TA Suratlant data) Skògafoss Nuka Arctica

Schuster and Watson, 2007 JGR mean CO 2 uptake in 1994/5 and 2002/4 [mol CO 2 m -2 yr -1 ] The overall sink (14-65 degrees N): 1995: 0.40 PgC (Takahashi climatology) 2005: 0.24 Pg C for % decline in CO 2 uptake? UK-Caribbean route

Simulations vs. Observations for the North Atlantic Ocean One conclusion: We need to maintain an observing system. Tool: Trend regression analysis instead of differences.

Simulations vs. Observations for the North Atlantic Ocean Tool: Trend regression analysis instead of differences

Simulations vs. Observations for the North Atlantic Ocean neutr./neg. NAO Most of the observations cover the period ΔpCO 2 trends long term pos. NAO Note change in scale ! Thomas et al. 2008, GBC

Outline 1: Fundamental control 2: Adjustments 3: Overlaying perturbance Real WorldOcean Building the house You move in Your in-laws move in

NAO & the subpolar and subtropical gyres Positive NAO: Strong westerlies Strong NAC Negative NAO: weaker westerlies weaker NAC ↑ ↑ 1: Fundamental control

North Atlantic Oscillation during the past two decades 1: Fundamental control Thomas et al., 2008, GBC

positive NAO ( average) Response of surface ocean to NAO forcing Surface velocity (each 2 nd grid point shown) x x 1: Fundamental control Subpolar gyre Subtropical gyre Labrador Current Thomas et al., 2008, GBC

Neutral - positive NAO (1996/2004av.) - (1989/1995av.) Response of surface ocean to NAO forcing Surface velocity difference Subpolar gyre (each 2 nd grid point shown) Subtropical gyre 1: Fundamental control Labrador Current Thomas et al., 2008, GBC

Response of surface ocean to NAO forcing 1: Fundamental control Thomas et al., 2008, GBC

Annual salinity anomalies: 1: Fundamental control (one more detail) Expansion of subtropical gyre during positive NAO Relevance western subpolar gyre: Labrador Current cannot move south, it is diverted into the subpolar gyre Thomas et al., 2008, GBC

Response of surface ocean to NAO forcing Pos. NAO Neutr./neg. NAO 1: Fundamental control

DIC norm and Salinity in the North Atlantic 1: Fundamental control Pos. NAO Neutr./neg. NAO North Atlantic Current is fed by low DIC norm / high salinity water: Northward transport of a CO 2 deficiency Thomas et al., 2008, GBC

Main driver of variability: DIC norm (A T,norm exerts minor control) Example: 2 stations at the eastern and western subpolar gyre WEST 1: Fundamental control EAST Note the phase lag between west and east Thomas et al., 2008, GBC

Large scale control: NAC and its low DIC norm Pos. NAO: Fast NAC High supply of low DIC norm High CO 2 uptake Neut. / Neg NAO: Slow NAC Low supply of low DIC norm low CO 2 uptake Long term: no significant trend Corrected for anthropogenic CO 2 !! NAO+ NAO-/  1: Fundamental control Long term Thomas et al., 2008, GBC

Conclusions 1 Key process: –NAC exports CO 2 sink from the tropics northward Pos. NAO: –Fast NAC –High supply of low DIC norm –Higher CO 2 uptake Neut. / Neg NAO: –Slow NAC –Low supply of low DIC norm –lower CO 2 uptake Long term: –no significant trend Temporal aspect: –Observed response depends on distance to NAC source region 1: Fundamental control however:

2: Adjustments Effects of atmospheric temperature Pos. NAO neutr./neg. NAO Long term Atm. Temperature effects: Fast response Spatially not uniform across the basin Particularly dominant in the NW Atlantic Note change in scale! Thomas et al., 2008, GBC

Perturbing process: Great Salinity anomalies Annual salinity anomalies: Annual DIC norm anomalies: 3. Overlaying perturbance Recall:1. Fundamental control: Pos. NAO = enhanced northward flow of low DIC norm Thomas et al., 2008, GBC

Do we still expect correlations?(??) >60ºN Annual SST anomalies: + NAO neutr./neg. NAO p<0.05 Thomas et al., 2008, GBC

Outline 1: Fundamental control 2: Adjustments 3: Overlaying perturbance Real WorldOcean Building the house Response of surface circulation to NAO You move in Atmospheric Temperature Your in-laws move in Great Salinity Anomaly

Conclusions Key process: –NAC exports CO 2 sink from the tropics northward Pos. NAO: –Fast NAC –High supply of low DIC norm –Higher CO 2 uptake –Strong cooling in NW Atlantic Neut. / Neg NAO: –Slow NAC –Low supply of low DIC norm –lower CO 2 uptake –Strong warming in NW Atlantic Alternative view: –Partial redistribution of North Atlantic CO 2 sink between subpolar and subtropical gyres Long term: –no significant trend other than (global) warming and rising atmos. CO 2. Perturbation: –Great Salinity Anomalies Consequences are time variant across the basin because of water mass travel time. High correlations unlikely.

Reference: Thomas, H., F. Prowe, I.D. Lima, S.C. Doney, R. Wanninkhof, R.J. Greatbatch, A. Corbière and U. Schuster (2008). Changes in the North Atlantic Oscillation influence CO2 uptake in the North Atlantic over the past two decades Global Biogeochemical Cycles, in press.