Impacts of the atmospheric weather regimes and associated

Impacts of the atmospheric weather regimes and associated storminess on the ocean circula5on in the North Atlan5c Nicolas Barrier1,2 Anne-­‐Marie Treguier2 Christophe Cassou3 4,5 Julie Deshayes
1MIO, Aix-­‐Marseille Université-­‐Université de Toulon-­‐CNRS-­‐IRD, Marseille, France 2 LPO, CNRS-­‐Ifremer-­‐IRD-­‐UBO, Brest, France 3 CNRS-­‐Cerfacs, Toulouse, France 4 LOCEAN, CNRS-­‐IRD-­‐UPMC-­‐MNHN-­‐IPSL, Paris, France 5 Dept. of Oceanography, University of Cape Town, South Africa [email protected] Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Mo5va5ons (1/2) •  In 1995, the North AtlanTc subpolar gyre has shown a dramaTc warming. Curry and Mauritzen (2005) A?ributed to the ocean response to changes in the large-­‐scale atmospheric circula5on. HYDROBASE. Courtesy of Ruth Curry (WHOI). Updated from Curry and Mauritzen (2005) and Peterson et al. (2006) 2 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Mo5va5ons (2/2) •  However, strong uncertainTes remain. –  North AtlanTc OscillaTon (NAO) or East-­‐AtlanTc Pa_ern? •  Sarafanov et al. (2008) vs. Hakkinen et al. (2011) –  Interannual or decadal Tme scales? –  Wind-­‐driven or buoyancy-­‐driven response? •  Sarafanov et al. (2008) vs. Msadek et al. (2014) •  Need a be_er understanding of the impacts of the large-­‐scale atmospheric circulaTon on the ocean circulaTon. NAO+ Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Weather regimes: descrip5on •  Climate indices -­‐> Weather regimes. –  Large scale, recurrent and quasi-­‐staTonary atmospheric pa_erns •  PopulaTons of similar atmospheric situaTons –  Events of different strengths and duraTons •  Capture the interannual and decadal variability of surface forcings (Cassou et al., 2011). 4 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Weather regimes: descrip5on (1/2) Freq: 22.9% Freq: 26.0% Freq: 23.3% Freq: 27.7% Asymetry: NAO-­‐ ≠ -­‐ (NAO+) 5 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Weather regimes: descrip5on (2/2) Freq: 22.9% Freq: 26.0% Freq: 23.3% Freq: 27.7% Asymetry: NAO-­‐ ≠ -­‐ (NAO+) 6 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Model descrip5on •  Use of forced ocean only experiments –  NEMO model (Madec, 2006) –  Regional, North-­‐AtlanTc configuraTon •  Closed boundaries •  Horizontal resoluTon: 0.5∘ •  Hindcast experiment (REF) –  Interannual forcings (DFS4.3) •  SensiTvity experiments 7 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Overturning Depth (km) Gyre REF, mean state Barotropic streamfuncTon Overturning streamfuncTon 8 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 REF, fast response to WRs (1/2) •  Methodology: composite analysis –  ComputaTon of monthly circula5on anomalies –  Averaging over extreme WRs months •  Normalized number of occurrences > 1.5 std 9 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 REF, fast response to WRs (2/2) AR Projects on the mean posiTon of the gyres: weakening of the circulaTon Intergyre-­‐gyre (Marshall et al. 2001) Southward shiked intergyre-­‐gyre NAO-­‐ NAO+ Collocated with wind-­‐stress curl anomalies: fast adjustment to wind anomalies Topographic Sverdrup balance (Eden and Willebrand, 2001; Eden and Greatbach, 2003) 10 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Slow response to WRs: idealised experiments •  Idealised experiments –  Winter reference forcing anomalies -­‐> forcing anomalies of random events that belong to one single regime •  Two experiments per regime: –  Interannual winds and buoyancy fluxes (REF/WR) –  Interannual winds and climatological buoyancy fluxes (WREF/WWR) •  ComputaTon of circulaTon « anomalies » –  Mean(Idealised) – Mean(REF) 11 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Idealised experiments: gyres Top – Bo?om = Impacts of buoyancy forcings only (Biastoch et al., 2008) 12 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Idealised experiments: gyres Wind-­‐driven weakening of the gyre circulaTon. Significant contribuTon of heat fluxes in the subpolar gyre 13 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Idealised experiments: gyres Buoyancy driven intensificaTon of the subpolar gyre. No influence of BLK winds 14 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Idealised experiments: gyres Southward shik of the gyres. Buoyancy driven intensificaTon in the Labrador Sea 15 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Idealised experiments: gyres Wind driven intensificaTon of the subtropical gyre. Buoyancy driven intensificaTon of the subpolar gyre. 16 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Idealised experiments: MOC 17 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Idealised experiments: MOC Large-­‐scale weakening of the MOC due to reduced heat loss in the Labrador Sea. 18 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Idealised experiments: MOC Large-­‐scale intensificaTon of the MOC due to increased heat loss in the Labrador Sea. 19 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Conclusions AR BLK Wind-­‐driven slackening of the STG and SPG Buoyancy-­‐driven strengthening of the SPG and MOC NAO-­‐ Buoyancy-­‐driven weakening of the northern SPG and MOC; wind-­‐driven intergyre-­‐gyre NAO+ Buoyancy-­‐driven strengthening of the SPG and MOC; wind-­‐driven strengthening of the STG Barrier, N., Treguier, A.M., Cassou, C. and Deshayes, J., 2014: Influence des régimes de temps atmosphériques sur la circulaTon océanique de l’AtlanTque Nord. La Météorologie, 87, 38-­‐44 Barrier, N., Cassou, C., Deshayes, J., and Treguier, A.M., 2014: Response of North AtlanTc Ocean CirculaTon to Atmospheric Weather Regimes. Journal of Physical Oceanography, 44, 179 201. 20 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 EXTRA SLIDES 21 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Methodology: heat budgets (1/2) •  Heat budgets are computed using four ocean hindcast simulaTons. –  Drakkar reference simulaTons •  Western and eastern subpolar gyre are considered separately. West East 22 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Methodology: heat budgets (1/2) "
!
∂hc
Qnet dx dy + ρ0 Cp [U Tint ] dl dz + ε
=
∂t
So
Sa
Surface heat fluxes Ocean heat convergence (determined using the Physical Analysis of Gridded Ocean, Deshayes et al. 2014) West East 23 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Results Lagged response to the strong NAO+ of 1988-­‐1995 Overturning In-­‐phase response to the NAO shik of 1995 Gyre circula5on Barrier, N., Deshayes, J., Treguier, A.M. and Cassou, C., 2014: Heat budget in the North AtlanTc subpolar gyre: Impacts of atmospheric weather regimes on the 1995 warming event. Progress in Oceanography, 130, 75-­‐90. Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Weather regimes: added values •  The tradiTonal NAO index can be polluted by synopTc storms or by circulaTon anomalies that are not representaTve of the NAO dipolar pa_ern •  Winters of posiTve NAO index may include a significant number of days that belong to the three other weather regimes Strongly posi5ve NAO (index>1) 3
50
2
40
1
30
0
20
−1
10
−2
0
1960
1970
1980
1990
2000
Percentage of NAO+ days NAO index
% of NAO+ days
NAO index 60
Strong NAO index but few NAO+ days (<35%) −3
2010
25 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Methodology: Idealised configura5ons •  « Regime-­‐only » experiments •  Winter observed forcing anomalies -­‐> forcing anomalies of random events that belong to one single regime –  The staTsTcs of the regimes (distribuTon and persistence) are respected –  The staTsTcs of the forcings are respected (Barrier et al. 2013) –  Integrated from 1958 to 2002 December January February March December January February March 26 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Extra experiments(! !"#$%##"&'#()*+, •  "#$!%&#'($!)*!+,-!./!0$1-%2!%'./01"#2$3(
•  3$4!$56$-&7$#8%2!4$!-$7,9$!,#$!%&#'($!)*!
–  *$(1:9$!6$-;$#81'$!,+!8<$!,8<$-!-$'&7$%!&%!
;,#%$-9$=!
4
Subtropical Gyre anomalies (Sv)
No AR
No BLK
No NAONo NAO+
Subpolar Gyre anomalies (Sv)
10
2.0
3
1.5
2
5
1.0
1
0.5
0
0
0.0
−1
−0.5
−2
−3
−4
Maximum Overturning anomalies (Sv)
−5
−1.0
>?@!1#,71(&$%!A>9B!
1960 1965 1970 1975 1980 1985 1990 1995 2000
Years
−10
1960
>C@!1#,71(&$%!A>9B!
1965 1970 1975 1980 1985 1990 1995
!
Years
−1.5
2000
−2.0
D"E!1#,71(&$%!A>9B!
1960 1965 1970 1975 1980 1985 1990 1995 2000
Years
27 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Forcing reconstruc5on (1/2) 28 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Forcing reconstruc5on (1/2) no
Random
draft of
distance
vector R
Selection of
the closest
day within
the observed distances O
Does the
day belong
to the right
regime?
yes
Does the
event last
at least
3 days?
yes
Selection of
the whole
event
no
29 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Weather regimes •  Determined from daily mean-­‐sea level pressure anomalies (NCEP/NCAR reanalysis) –  North AtlanTc domain: (20◦N-­‐80◦N,80◦W-­‐30◦E) •  Clustering method of Michelangi et al. (1995); Cassou (2008) –  Aggregate days that share some resemblance (recurrent pa_erns) •  Only winter regimes (DJFM) are considered –  Higher mean and variance of surface forcings 30 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Western subpolar gyre 100
LR1
hflux
hconv
LR2
6
input
hc
4
50
0
0
ZJ
TW
2
−2
−50
−4
−6
−100
LR3
100
HR
6
4
50
0
0
−2
−50
−100
8
198
ZJ
TW
2
−4
−6
0
199
2
199
4
199
199
6
199
8
0 8
200 198
0
199
199
2
199
4
6
199
8
199
200
0
31 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Eastern subpolar gyre LR1
LR2
60
hflux
hconv
3
2
20
1
0
0
−20
−1
−40
−2
ZJ
TW
40
4
input
hc
−3
−60
−4
LR3
HR
4
3
40
2
20
1
0
0
−20
−1
−40
−2
ZJ
TW
60
−3
−60
8
198
0
199
2
199
4
199
1
996
1
998
0 8
200 198
1
990
1
992
1
994
1
996
1
998
2
000
−4
32 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 REF, fast response to WRs Reference bathymetry AR Constant bathymetry Analy5cal Sverdrup AR AR NAO-­‐ NAO-­‐ NAO-­‐ NAO+ NAO+ NAO+ Barotropic set-­‐up 33 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 Choice on regime numbers •  We chose 4 clusters but: –  Regimes are robust •  AddiTonal one: AL (opposed to AR) •  Weakening of the gyre circulaTon. •  Only impact on the wind 34 Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 REF, fast response to WRs (3/3) AR NAO-­‐ NAO+ Top to bo_om anomalies. Adjustement to Ekman transport anomalies (Eden & Willebrand, 2001) 35 Sv
0
Nicolas Barrier, Ateliers de ModélisaTon de l’Atmosphère 2015 −5
Closed boundaries −10
−15
3
Maximum Overturning at 46◦ N (corr = 0.94)
2
1
0
Subpolar Gyre (corr
15
REF
G70
−1
10
−2
−3
1965
5
1970
1975
Sv
Sv
Reminder: close boundaries at 20°S/80°N MOC consistent with global ORCA05-­‐G70 (Barnier et al., 2006) 1980 1985
Years
1990
1995
2000
0
−5
−10
−15
3
Maximum Overturning at 46
2
1
36