1. Low latitude production and its high latitude nutrient sources Jennifer Ayers 1,2 and Peter Strutton 1,2 1 Institute for Marine and Antarctic Studies (IMAS), University of Tasmania 2 ARC Centre of Excellence for Climate System Sciences (Interannual variability in SAMW nutrients) Liège, 17 May 2013

2. Low latitude production: fueled by Subantarctic Mode Water (SAMW) nutrients SAMWs primary source of nutrients to the global thermocline [Sarmiento et al., 2004] 33-75% of tropical export production supported by SAMW nutrients [Palter et al. 2010] Image: dimes.ucsd.edu Palter et al., Biogeosci. 2010 Fraction of thermocline nutrients from preformed SAMW pool, SAMW Research questions: - Variability in SAMW nutrients? - Forcing? - Implications for downstream productivity

3. Observed nutrient variability range/mean = % Pacific: 0.25/1.5 = 16% SR03+P11A: 0.27/1.1 = 25% range/mean = % Pacific: 3.1/21.5 = 14% SR03+P11A: 2.6/16.1 = 16% range/mean = % Pacific: 4.4/8.7 = 50% SR03+P11A: 1.3/4.8 = 27% range mean Pacific: 2.0°C 6.5°C SR03+P11A: 0.6°C 8.7°C Pacific sector Australian sector Indian sector

4. Other potential drivers of variability not considered: Variation in max winter mixed layer depth (vertical entrainment) Upstream lateral induction SAM, MOC as drivers of variability MLD Motivated by Sarmiento et al. (2004) and Lovenduski and Gruber (2005) Mean MOC ΔMOC, +SAM ΔNutrients, +SAM

5. ** * * 0.41 < R 2 < 0.59 ΔSAMW nutrients (%) per 1 std. dev. change in -WSC ΔSAMW temp (°C) per 1 std. dev. change in -WSC Pacific sector SAMW nutrient response to MOC Correlations significant at p < 0.05. * indicates significance only at p < 0.10. Support for: Increase in SAMW nutrients with increase in Meridional Overturning Circulation (MOC) Decreasing lag times with increasing proximity to formation region Insufficient biological response to consume extra nutrients Greater change in SAMW Si relative to N &P Increased upwelling Increased SAMW nuts (t=1yr) Increased upwelling Increased SAMW nuts (t<1yr) Increased downwelling Increased SAMW nuts (t<<1yr) Climatological Ekman transport time: 1.2 yrs Ayers and Strutton (2013, submitted)

6. Australian sector SAMW nutrient response to ENSO Correlations significant at p < 0.05. Lag time: 1 year 0.34 < R 2 < 0.56 El Niño (+MEI Index) associated with decreased SAMW nutrients Ridgway and Hill, 2009 El Niño  stronger EAC  decreased SAMW nutrients Ayers and Strutton (2013, submitted)

7. Why the greatest change in Si? Nutrient Supply (Fe + macronutrients) Nutrient export in SAMWs High N, P, Low Si Fe-limited conditions: Si:N uptake is ~5:1 * Biological Nutrient Uptake 1 stdev ΔWSC: ΔSi: 15% ΔN,P: 5% Fe-limitation eases a little … nutrient replete conditions (which they still aren’t) Si:N uptake ~1:1 * N, P + 5%, Si + 15% When the MOC increases, nutrient supply increases Mean conditions * Brzezinski, 1985 & 2003 Diatoms could decouple Si from N and P

8. Impact on low latitude productivity Global Export Production ~10±3 PgC/yr 33-75% of tropical export fueled by SAMW nuts [Palter et al., 2010] ~1.1-2.5 PgC/yr Tropical Export Production is about 1/3 of that ~3.3±1 PgC/yr Dunne et. al, 2007 ≈ Global C emissions [CDIAC] 2.58 PgC in 2011 ≈ Δ Tropical export fueled by SAMW nuts N,P: Δ(55 - 250) TgC/yr Si: Δ(165 – 375) TgC/yr 1/2 China’s C emissions (1/2)x 677TgC in 2011 1 stdev change in WSC ΔSAMW nutrient concentrations: ~5-10% N,P ~15% Si ≈ 6-13% of mean tropical C export > Equatorial Pacific export production (15°N-15°S) 1.09 PgC/yr Dunne et. al, 2007

9. Significant interannual variability in SAMW nutrients (16-50% of the mean) In sum: 40-60% of Pacific Sector variability driven by strength of MOC Consequences for low-latitude productivity (1 std. dev. ΔWSC impacts annual tropical C export by 6-13%) Australian Sector variability correlated with ENSO, attributed to its impact on East Australian Current Ayers and Strutton (2013, submitted)

10. Thank You