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Emergence of Anthropogenic Trends in California Current Upwelling in the Presence of Natural Climate Variability
Riley X. Brady University of Colorado at Boulder
Coauthors: Ryan Rykaczewski (U. South Carolina)
Michael Alexander (NOAA ESRL)
CESM Workshop June 23rd, 2016
Breckenridge, CO
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A mechanism exists whereby global greenhouse warming could . . . lead to acceleration of coastal upwelling. (Bakun, 1990)
Source: Rykaczewski & Checkley 2008 Upwelling Large Ensemble Regions Historical Upwelling SNR Deltas Natural Variability Summary
“. . . we still cannot attribute coastal wind intensification in EBUS to global warming because we cannot discount the role of multi-decadal climate variability in the observed trends.”
Garcia-Reyes, et al. 2015
“Summertime winds near poleward boundaries of climatological upwelling zones are projected to intensify, while winds near equatorward boundaries are projected to weaken."
Rykaczewski, et al. 2015
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By holding the model and emissions scenario constant, we can attribute disparities entirely to internally-generated climate variability.
Modified from Kay et al. 2015
1850 Control Member 1
Observations Members 2-35
Generated ensemble with 10-14K air
temperature differences
Forced with RCP8.5 emissions scenario
Upwelling Large Ensemble Regions Historical Upwelling SNR Deltas Natural Variability Summary
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The California Current was split into three distinct regions based on coastal geography.
Upwelling Large Ensemble Regions Historical Upwelling SNR Deltas Natural Variability Summary
Nor
th
Cent
ral
Sout
h
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These regions display distinct seasonal cycles in both vertical velocity and alongshore wind stress.
Upwelling Large Ensemble Regions Historical Upwelling SNR Deltas Natural Variability Summary
Vert
ical
Vel
ocity
at 5
0m D
epth
(cm
/s x
10-3
)
Area
-Wei
ghte
d Al
ongs
hore
Win
d St
ress
(dyn
e/cm
2 )
North
Central
South
Ensemble Historical Coastal Upwelling (1920 – 2005)
Ensemble Historical Alongshore Wind Stress (1920 – 2005)
Model Data Observational Data
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Coastal Grid-Cell
A Signal-to-Noise Ratio quantifies the relative strength of anthropogenic climate change.
Signal (S) = ∆𝒘𝒘 Noise (N) = 𝝈𝝈∆𝒘𝒘
N = 35
1∘
1∘
∆𝑤𝑤𝜎𝜎∆𝑤𝑤
≡ 𝑆𝑆𝑁𝑁
> 𝟐𝟐 is considered a significant emergence of the signal.
Upwelling Large Ensemble Regions Historical Upwelling SNR Deltas Natural Variability Summary
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Long-term changes in upwelling and wind stress vary by season and location.
Upwelling Large Ensemble Regions Historical Upwelling SNR Deltas Natural Variability Summary
Forced Change in Alongshore Wind Stress (dyne/cm2)
Forced Change in Vertical Velocity (cm/s x 10-3)
RCP
8.5
(2
041-
2100
– 1
941-
2000
) SNR >= 1
SNR >= 2
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Upwelling Large Ensemble Regions Historical Upwelling SNR Deltas Natural Variability Summary
Chan
ge in
Ver
tical
Vel
ocity
(cm
/s x
10-3
)
July
Tot
al C
hang
e in
Ver
tical
Vel
ocity
(2
041-
2100
– 1
941-
2000
)
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Upwelling Large Ensemble Regions Historical Upwelling SNR Deltas Natural Variability Summary
Chan
ge in
Ver
tical
Vel
ocity
(cm
/s x
10-3
)
Ensemble Mean (Forced Signal)
July
Tot
al C
hang
e in
Ver
tical
Vel
ocity
(2
041-
2100
– 1
941-
2000
)
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Upwelling Large Ensemble Regions Historical Upwelling SNR Deltas Natural Variability Summary
Chan
ge in
Ver
tical
Vel
ocity
(cm
/s x
10-3
)
July
Nat
ural
Com
pone
nt o
f Cha
nge
in V
ertic
al V
eloc
ity
(204
1-21
00 –
194
1-20
00)
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Upwelling Large Ensemble Regions Historical Upwelling SNR Deltas Natural Variability Summary
Run
5 Ru
n 23
Total Natural Forced
Chan
ge in
Ver
tical
Vel
ocity
(cm
/s x
10-3
)
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Summary
Spring intensification and summer weakening could be driven by different mechanisms.
The sign of change in upwelling may vary meridionally and intra-annually.
Natural variability can largely impact the magnitude of change in California Current Upwelling.
Upwelling Large Ensemble Regions Historical Upwelling SNR Deltas Natural Variability Summary
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Supplemental Slides
Frac
tiona
l Cha
nge
Rela
tive
to H
ist. M
ean
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Frac
tiona
l Cha
nge
Rela
tive
to H
ist. M
ean
Supplemental Slides