Equatorial Pacific primary productivity:Spatial and temporal variability and links to carbon cycling

Pete StruttonCollege of Oceanic and Atmospheric Sciences

Oregon State University

Outline

• Physical processes that impact productivity– El Niño– Tropical instability waves (TIWs)– Kelvin waves (MJO)

• How might these processes change as climate changes?• What would be the resulting impact on carbon budgets?• What do the models say?• Variability at longer time-scales: PDO

Physical and biological setting: 1997-2004

Comparison of satellite and in situ data

The in situ database documents large-scale trends but misses ephemeral events

Phytoplankton community composition

• Elevated nutrients in the upwelling zone relative to warm pool• Moderately high chlorophyll concentrations

– 0.2 to 0.3 mg m-3 cf 0.05 to 0.1 mg m-3 in the warm pool & gyres• Dominated by small planktonic species

– Prochlorococcus and Synechococcus– Competitive advantage in low nutrient environment because of large

surface area : volume– Survive on recycled N (as NH4) and Fe

• An additional diatom component, reliant on Fe and Si inputs• Export flux driven largely by the diatoms

Variability of chlorophyll and nutrients: 1997-2004

El Niño-La Niña mixed layer chlorophyll variability

Species composition as a function of nutrients

Figure adapted from Dugdale et al., 2002

El Niño-La Niña mixed layer nutrient variability

El Niño-La Niña mixed layer nutrient variability

El Niño-La Niña source nutrient variability

El Niño-La Niña source nutrient variability

No change in source nutrients as a function of El Nino

Probably not true for iron (source is the EUC)

Increased dominance of El Niño/La Niña?

Almost no change in source nutrients.

La Nina Normal Mild El Nino Strong El Nino

chl [mg m-3] 0.19 0.20 0.18 0.12

NO3 [uM] 4.97 4.10 2.82 0.91

SiO4 [uM] 3.60 3.16 2.65 1.68

PO4 [uM] 0.56 0.54 0.52 0.31

N:Si 1.34 1.25 1.02 0.38

Impact of tropical instability waves (TIWs)

SST and chlorophyll at 2°N 110°W: March to October 1998

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Mar-98 Apr-98 May-98 Jun-98 Jul-98 Aug-98 Sep-98 Oct-98

SST [°C]

0.0

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Chlorophyll [

μ g l

-1]

SSTChl

Strutton et al., GRL, 2001.

0.4

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0.9Growth

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5 Day21 Day

4005006007008009001000110012001300

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190 0.03

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Depth

f

Date

12/1/96 1/1/97 2/1/97 3/1/97 4/1/97 5/1/973.0

3.5

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NO3

SST (oC)

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chl [

mg

m-3]

dept

h [m

]SS

T [°

C]

1 p

rod

[mgC

m-2 d

-1]

grow

th [d

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f

NO3 [μM

]

Impact of TIWs and Kelvin waves

• TIWs– Enhanced chlorophyll at the equator– Averaged over Wyrtki Box, essentially no difference– Evidence for enhanced diatom production and export– TIWs should become less dominant in an ‘El Niño climate’

• Kelvin waves– Small decrease in chlorophyll– Evidence for reduced diatom production and export– Enhanced or diminished in an ‘El Niño climate’?– Impact possibly diminished for a deeper thermocline

Life in a more El Niño- or La Niña-like world

La Nina Mild El Nino Strong El Nino

chl no change no change -40%

NO3 +20% -30% -80%

SiO4 +15% -15% -50%

TIWs more fewer absent

Kelvin waves absent present ?

Export increase small decrease large decrease

Satellites can provide chl, but we need satellites + models to quantify changes in export

• The system has been modeled as a chemostat - limiting nutrient(s) fed in via upwelling

• Can reproduce the general surface chlorophyll patterns well• Ability to reproduce processes and the subsurface structure

heavily dependent on the physics and available data• Controls on new production

– Depends on the type of physical forcing– Nutricline variability: El Nino and Kelvin waves– Variability in upwelling velocity: TIWs and short-term wind events

• Barely enough export data to know if they are getting it right

What do the models say?

Global importance of equatorial Pacific productivity

Figure courtesy of Mike Behrenfeld, OSU

Global importance of equatorial Pacific productivity

From Behrenfeld et al., Science, 2001.

Dec98-Feb99

Jun99-Aug99

98/99-97/98

1999-1998

• El Niño to La Niña transition, 1997-2000• Increase in ocean NPP from ~50 to 53 PgC year -1

• Largely due to increases in:– Equatorial Pacific and Atlantic– Coastal upwelling regions (Canary, Arabian Sea)– Patagonian shelf and regions downstream

• Terrestrial productivity:– approximately constant, globally– regionally variable (Amazonia)

Global importance of equatorial Pacific productivity

The PDO’s impact on the equatorial Pacific

From Chavez et al., Science, 2003.

• TAO array and satellites provide excellent synoptic view of broad physics and surface chlorophyll

• But, to predict the future we need better models• For this we need more data:

– Iron!– Phytoplankton community composition in response to nutrient fluxes– Spatial and temporal variability of export (TIWs)– Mixing and upwelling vs thermocline variability for fueling

productivity• Also need a better understanding of feedbacks

Productivity and export: Knowns and unknowns