equatorial pacific primary productivity: spatial and temporal variability and links to carbon...
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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
20
22
24
26
28
30
32
Mar-98 Apr-98 May-98 Jun-98 Jul-98 Aug-98 Sep-98 Oct-98
SST [°C]
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Chlorophyll [
μ g l
-1]
SSTChl
Strutton et al., GRL, 2001.
0.4
0.5
0.6
0.7
0.8
0.9Growth
0.1
0.2
0.3
5 Day21 Day
4005006007008009001000110012001300
130
140
150
160
170
180
190 0.03
0.04
0.05
0.06
0.07
0.08
Depth
f
Date
12/1/96 1/1/97 2/1/97 3/1/97 4/1/97 5/1/973.0
3.5
4.0
4.5
5.0
25
26
27
28
29
NO3
SST (oC)
A
B
C
chl [
mg
m-3]
dept
h [m
]SS
T [°
C]
1 p
rod
[mgC
m-2 d
-1]
grow
th [d
-1]
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