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Was the 2015 subpolar North Atlantic cold blob predictable?
Elizabeth Maroon, Steve Yeager, Gokhan Danabasoglu
NCAR, CGD, Oceanography Section
NCAR is sponsored by the
National Science Foundation
Anomalously cold subpolar North Atlantic SST in 2015
Was the cold blob predicted by the CESM Decadal Prediction Large Ensemble (DPLE)?
CESM DPLE (Yeager et al. 2018):
• 10-year-long, 40-member hindcastensembles initialized each November from 1954-2016
• Full field initial conditions from a Forced Ocean Sea Ice (FOSI) simulation driven by CORE* forcing
Using the DPLE hindcast ensemble initialized in November 2014 to assess if the peak cold anomaly in summer 2015 was predictable
First month of integration looks good…
2014 Nov subpolar SST anomaly (K)
None of the ensemble members capture the magnitude of the cold anomaly by summer 2015
Observed and DPLE SST trend in opposite directions in 2015
1. Does CESM capture subpolar anomalies of the same magnitude as observations?
2. Was this a rare event? Do we need more than 40 ensemble members?
Does the CESM Large Ensemble produce subpolar cold blobs of similar magnitude? Yes.
CESM Large
Ensemble (Kay et al.
2015) ensemble mean
(1920-2018)
Observed distribution
(1920-2018)
Spread of internal
variability from all
ensemble members
None of the DPLE members reproduce the observed persistently positive NAO in winter 2015
NA
O in
dex
time (monthly means)
Should we have expected 6 consecutive months of positive NAO conditions in any of the 40 ensemble members?
NAO index
frequency
NAO index
frequency
# of consecutive months with NAO >+1
# o
f events
Should we have expected 6 consecutive months of positive NAO conditions in any of the 40 ensemble members?
Add 20 more ensemble members? 40 ensemble members
NA
O index
# o
f D
PLE
mem
bers
subpolar North Atlantic SST anomaly (K)
JJA 2015
Add 20 more ensemble members?60 ensemble members
No new ensemble members capture cold blob or persistently
positive NAO behaviorN
AO
index
# o
f D
PLE
mem
bers
subpolar North Atlantic SST anomaly (K)
JJA 2015
Additional ensemble members from Nan Rosenbloom
• None of the CESM DPLE ensemble members initialized in 11/2014 capture the 2015 subpolar cold anomalies
• Highly positive NAO conditions contributed to the formation of the 2015 cold blob
• This period also had a once-in-the-historic record 6-consecutive months of NAO positive conditions
• Adding more 11/2014 DPLE ensemble members did not produce a cold blob or persistently positive NAO conditions
Acknowledgments: NSF OPP #1737377, OCE #1243015
Summary
ERA-Interim DPLE
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Surface heat flux
Blue: heat loss by ocean
Heat flux out of the subpolar ocean during December, January, and March was critical for the amplification of the cold blob (Duchez et al. 2016)
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ERA-Interim
DPLE ensemble mean
Surface heat flux
Blue: heat loss by ocean
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ERA-Interim
Was upper ocean heat content better predicted? No.
upper 295m ocean heat content
Both surface heat fluxes and advection contribute to negative heat tendency in early 2015
ocean heating
ocean cooling
FOSI upper 295m ocean heat budget terms
FOSI tendency has two negative anomalies in January and March 2015
upper 295m ocean heat tendency
ocean heating
ocean cooling
No DPLE members replicate the FOSI’s surface heat flux and advection anomalies in January and March.
tendency due to surface heat fluxes
tendency due to advection
Winter 2015
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