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 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

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)

Thank You