Low latitude production and its high latitude nutrient sources
Jennifer Ayers1,2 and Peter Strutton1,2
1Institute for Marine and Antarctic Studies (IMAS), University of Tasmania2ARC Centre of Excellence for Climate System Sciences
(Interannual variability in SAMW nutrients)
Liège, 17 May 2013
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
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 meanPacific: 2.0°C 6.5°CSR03+P11A: 0.6°C 8.7°C
Pacific sectorAustralian sector
Indian sector
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 MOCΔMOC, +SAMΔNutrients, +SAM
* *
*
*
0.41 < R2 < 0.59
ΔSAM
W n
utrie
nts
(%)
per 1
std.
dev
. cha
nge
in -W
SC
ΔSAM
W te
mp
(°C)
per 1
std.
dev
. cha
nge
in -W
SC
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)
Australian sector SAMW nutrient response to ENSO
Correlations significant at p < 0.05.
Lag time: 1 year0.34 < R2 < 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)
Why the greatest change in Si?
Nutrient Supply(Fe + macronutrients)
Nutrient export in SAMWsHigh N, P, Low Si
Fe-limited conditions:Si:N uptake is ~5:1*
Biological Nutrient Uptake1 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
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 WSCΔSAMW nutrient concentrations:~5-10% N,P~15% Si
≈6-13% of mean
tropical C export
>Equatorial Pacificexport production
(15°N-15°S)
1.09 PgC/yrDunne et. al, 2007
• 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)