What can we learn about biological production and air-sea carbon flux in the Southern Ocean from 12 years of observations in the Drake Passage? Colm Sweeney David Munro, Taro Takahashi, Tim Newberger, Nikki Lovenduski, Britt Stephens, Janet Sprintall, Teresa Chereskin

Motivation The Southern Ocean is currently a sink for atmospheric CO 2 The Southern Ocean is currently a sink for atmospheric CO 2 There is significant interannual variability in sea-air CO 2 flux There is significant interannual variability in sea-air CO 2 flux Nutrients that upwell around the Southern Ocean support ~75% of the biological production in the global oceans (Sarmiento et al., 2004) Nutrients that upwell around the Southern Ocean support ~75% of the biological production in the global oceans (Sarmiento et al., 2004) Data from Lenton et al. (2013)

Motivation Increase in wind drives an increase in the outgassing of natural CO 2 From Lovenduski et al. (2013)

Drake Passage Measurements o Underway (~18-24 crossing per year) o ADCP (since 1996) o Atmospheric and ocean CO 2, (since 2002) o Ocean O 2 (since 2004) o Atmospheric O 2 (since 2012) o Discrete (~6-8 crossing per year) o XBT/XCTD (since 1996) o TCO 2 measurements (since 2002) o Nutrients o PO 4 (Since 2002) o NO 3 and SiO 4 (Since 2005) o 13 C of TCO 2 (since 2005) o 14 C of TCO 2 (2005 – 2010) o Total water column measurements (2006 and 2009)

Drake Passage time series Subantarctic Front Winter Polar Front (JAS) Summer Polar Front (JFM)

Annual cycle of pCO 2 in Drake Passage  atm Region 1 (N of APF) Region 4 (S of APF) Temp-driven pCO 2 TCO 2 -driven pCO 2 Observed pCO 2 Year day Munro et al., in revision

observed Comparison of Drake Passage pCO 2surf to eight CMIP5 model runs Jiang et al. (2014) -Models have difficulty reproducing the observed seasonal cycle of pCO 2surf

Trends SST Salt TCO 2 pCO 2 pH 1.58± ± ± ± ± Ocean pCO 2 is increasing slower than the atmosphere Consistent with Landschutzer et al. (2014)

Ocean carbon uptake from Landschutzer et al. (2014) -Ocean carbon uptake has increased by nearly 1 Pg C yr-1 -Increase in uptake driven by the Southern Ocean

~70 ‰ Change in Surface water  14 C Comparison of surface water  14 C suggests change of 70 ‰  14 C (‰)

Surface  14 C in the Drake Passage 70 ‰ GEOSECS (1973) Surface data shows 70±14 ‰ change DP ( ) WOCE (1990)

Surface  14 C in the Drake Passage 70 ‰ GEOSECS (1973) MOM4 run also shows a 70 ‰ change WOCE (1990) 70 ‰ model run with increase in winds

Gas Exchange for  14 C ~50 ‰ Courtesy of Majkut MOM4 run with unvarying winds shows only a 50 ‰ change due to decrease in atmospheric 14 CO 2

Surface  14 C in the Drake Passage 70 ‰ GEOSECS (1973) DP ( ) WOCE (1990) 50 ‰ MOM4 run with unvarying winds shows only a 50 ‰ change due to decrease in atmospheric 14 CO 2

Surface  14 C in the Drake Passage 70 ‰ GEOSECS (1973) DP ( ) WOCE (1990) 50 ‰ 20 ‰ change in  14 C could be due to increase in meridional overturning 20 ‰

Budget based approach to estimate NCP Time rate of change Zonal advection Meridional advection Vertical advection NCP Air-Sea Gas Exchange TCO 2 and T 13 CO 2 only Also can include a term for entrainment

NCP No significant increase in NCP over the history of the time series NCP based on C budget is negatively correlated with SAM Munro et al., in revision

Cumulative NCP Budget based approach suggests almost all NCP occurs during spring Munro et al., in revision

Conclusions Models have difficulty reproducing the seasonal cycle of pCO 2surf in Drake Passage Models have difficulty reproducing the seasonal cycle of pCO 2surf in Drake Passage pCO 2surf increasing at a slower rate than pCO 2atm over the past decade pCO 2surf increasing at a slower rate than pCO 2atm over the past decade Long term observations in the Drake Passage suggest that overturning has increased. Long term observations in the Drake Passage suggest that overturning has increased. NCP appears not to have increased over the time series NCP appears not to have increased over the time series Most annual NCP accomplished in the spring Most annual NCP accomplished in the spring

Questions What is the seasonal evolution of NCP? What is the seasonal evolution of NCP? How does physical variability control air-sea CO 2 flux and biological productivity? How does physical variability control air-sea CO 2 flux and biological productivity?