riley x. brady€¦ · riley x. brady . university of colorado at boulder . coauthors: ryan...

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

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

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

The California Current was split into three distinct regions based on coastal geography.


Nor

th

Cent

ral

Sout

h

These regions display distinct seasonal cycles in both vertical velocity and alongshore wind stress.


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

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.


Long-term changes in upwelling and wind stress vary by season and location.


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


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

)


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

)


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)


Run

5 Ru

n 23

Total Natural Forced

Chan

ge in

Ver

tical

Vel

ocity

(cm

/s x

10-3

)

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.


Supplemental Slides

Frac

tiona

l Cha

nge

Rela

tive

to H

ist. M

ean

Frac

tiona

l Cha

nge

Rela

tive

to H

ist. M

ean

Supplemental Slides

riley x. brady€¦ · riley x. brady . university of colorado at boulder . coauthors: ryan...

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