Department of GeophysicsUniversity of Bergen
Spin-up; monthly forcing; daily forcing; relaxation T and/or S
Topics;• Variability guided by observed changes (mainly) in the Atlantic; particularly the gyre variability• Inflow to the Nordic Seas/Arctic• Water mass transformation in the Nordic Seas• AMOC variability• Physical forcing of the marine biota• Idealised experiments; role of the ocean preconditioning and atmospheric forcing
Have used NCAR/NCEP and ERA40, error in implementation of CORE; will be rerun
Department of GeophysicsUniversity of Bergen
Spin-up … min full 4 cycles with daily forcing; usually 6 full cycles (ca. 300 yr)
Always start with (strong) SSS-relaxation; 30 days for 50 m thick ML; scales with ML depth; limited to |ΔSSS<0.5| everywhere; no relaxation under max sea ice extent
Diagnose SSS-nudging when model is steady (5th or 6th cycle); applying diagnosed SSS-fluxes for the production runs with very weak Newtonian relaxation (360 days time scale for 50 m ML and |ΔSSS<0.5| )
Temperature relaxation is not critical (in our system)
Department of GeophysicsUniversity of Bergen
Department of GeophysicsUniversity of Bergen
Department of GeophysicsUniversity of Bergen
AMOC, last 4 cycles
Department of GeophysicsUniversity of Bergen
NA Sub-Polar Gyre SSH, last 4 cycles
Department of GeophysicsUniversity of Bergen
Department of GeophysicsUniversity of Bergen
Department of GeophysicsUniversity of Bergen
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3) Increased salinity inthe Subpolar gyre (SPG)
2) Increased salinity in thesubtropical gyre (STG)
1) Relative contributionfrom the two gyres
(Dynamics)
4) Enhanced Evaporation minus Precipitation (E-P)
Possible mechanisms causing the rapid increase in salinity (and temperature)
Hatun et al., Science (2005)
Department of GeophysicsUniversity of Bergen
Altimetry
MICOM
SSH EOF (Häkkinen and Rhines, Science, 2004)
I
I
F
F
R
R
Dynamical SPG influence
Department of GeophysicsUniversity of Bergen
RI
A longer term perspective,using the simulated Gyre Index
as a proxy for the circulation
Irminger Current (obs)
Rockall Through (obs)
Gyre index (model)
Department of GeophysicsUniversity of Bergen
NB: Relationship also valid for temperature
Department of GeophysicsUniversity of Bergen
Forcing mechanisms (I)
Hatun et al., in prep.
Department of GeophysicsUniversity of Bergen Hatun et al., in prep
Forcing mechanisms (II)
Department of GeophysicsUniversity of Bergen
Wind stress and NAO consistent with the Gyre Index before - but not after - 1995
Department of GeophysicsUniversity of Bergen
– SPG is of key importance for the decadal-scale variations in the Atlantic inflow to the Arctic Mediterranean and along the cost off South Greenland
– The strength of the SPG is governed by the Subpolar (winter) buoyancy forcing
– The latter follows, in general, the North Atlantic wind stress (NAO) forcing, but not after 1995/96
– The Subpolar Gyre Index, rather than NAO, should be used as a proxy for the variability of the marine climate in the North Atlantic region (can be deduced from observations or hind-cast model simulations)
Conclusions (1)
Department of GeophysicsUniversity of Bergen
Recent North Atlantic Warming and Some Consequences Thereof
Helge Drange, Katja Lohmann, Mats Bentsen, Hjalmar Hatun, Anne Britt Sandø and colleagues
Department of GeophysicsUniversity of Bergen
0 1 2 3 4 5 6 7 8 9 10 °C
Orvik and Niiler, GRL, 2002
Observed sea surface temperature
Greenland
Norway
UK
Iceland
This presentation: Focus on the rapid (northern) North Atlantic warming after 1995/96
Not addressed here: The superimposed global warming signal
Department of GeophysicsUniversity of Bergen
0 1 2 3 4 5 6 7 8 9 10 °C
Barents
Faroes
Observed hydrography, N Atl/Nordic Seas (NB: post 95-changes!)
Fram Strait
Svinøy section
Department of GeophysicsUniversity of Bergen
(Hátún, AGU Monogr 158, 2005)
SST variability (Atl inflow), Faroe Islands
Record high temperature(and salinity)
Simulated temperature
Department of GeophysicsUniversity of Bergen
1 °C increase in T of Atlantic Water entering the Nordic Seas since 1997
(ca 0-600 m)
Orv
ik, K
jell
Aril
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kags
eth,
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tein
(20
05),
Hea
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ns in
th
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ster
n N
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n A
tlant
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urre
nt to
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d th
e A
rctic
from
m
oore
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. Geo
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No.
14,
L1
4610
10.
1029
/200
5GL0
2348
7
Department of GeophysicsUniversity of Bergen
Ingvaldsen, Loeng and Ådlandsvik, 2007
Temperature anomaly (ºC), 50-200 m Barents Sea
Department of GeophysicsUniversity of Bergen Ursula Schauer, AWI
Observed vertically avg temperature, Fram Strait
Department of GeophysicsUniversity of Bergen
Observed air temperature, Nuuk (W Greenland)
Stein (2007)
Department of GeophysicsUniversity of Bergen
http://svs.gsfc.nasa.gov
Jakobshavn glacier, West Greenland
Department of GeophysicsUniversity of Bergen
Department of GeophysicsUniversity of Bergen
Rapid warming is observed throughout the North Atlantic since 1995/96
Followed by rapid changes in the cryosphere and the marine ecosystems, possibly/likely linked to the North Atlantic warming
North Atlantic salinity is increasing in concert with temperature (post 1995)
Department of GeophysicsUniversity of Bergen
The model system used
Department of GeophysicsUniversity of Bergen
MICOMThe NERSC version
Simulatedsalinity
Global model:(40 km in the Nordic Seas) run forthe 1948-2003 period; forced with daily atmosphericNCAR/NCEP re-analyses fields
Nordic Seas
Regional model: (20 km in the North Atlantic) run for
the same period (1948-2003); forcing fields as for the global model;
boundary fields from the global model
Department of GeophysicsUniversity of Bergen
Subpolar gyresource water
Western Subtropical gyreSource water
Western North Atlantic Water (WNAW)
Eastern NorthAtlantic Water (ENAW)
Source Watersfor the Atlantic Inflows
Simulatedsalinity
Department of GeophysicsUniversity of Bergen
Irminger (I) Faroe (F) Rockall (R)
Observed and simulated salinity anomalies at three locations in the northern North Atlantic
Hatun et al., Science (2005)
Department of GeophysicsUniversity of Bergen
RIGyre index
Irminger Current (obs)
Rockall Trough (obs)
Gyre index
Department of GeophysicsUniversity of Bergen
Atmospheric forcing
Hatun et al., 2007
Department of GeophysicsUniversity of BergenHatun et al., 2007
Atmospheric forcing
Department of GeophysicsUniversity of Bergen
Q1 How does the subpolar gyre (SPG) respond to a persistent, decadal time scale positive – or negative – NAO forcing?
Q2 How linear is the response of the SPG forced with positive – or negative – phases of the NAO?
Q3 How important was the ocean initial state in 1995 for the strong and rapid weakening of the SPG?
Department of GeophysicsUniversity of Bergen
Q1 How does the subpolar gyre (SPG) respond to a persistent, decadal time scale positive – or negative – NAO forcing?
Q2 How linear is the response of the SPG forced with positive – or negative – phases of the NAO?
Q3 How important was the ocean initial state in 1995 for the strong and rapid weakening of the SPG?
Department of GeophysicsUniversity of Bergen
Idealized experiments (Lohmann et al., Clim Dyn, 2008)
40 year sensitivity experiments, forced with persistent NAO+, NAO- and NAOn fields (cycling through the marked years)
NAO indexHigh NAO-years
Low NAO-years
Department of GeophysicsUniversity of Bergen
Sea surface height, NAO+ minus NAOn
Department of GeophysicsUniversity of Bergen
Steric height, NAO+ minus NAOn
T
S
Department of GeophysicsUniversity of Bergen
SPG index, NAO+ minus NAOn
Strong
Weak
Department of GeophysicsUniversity of Bergen
Sea surface height, NAO- minus NAOn
Department of GeophysicsUniversity of Bergen
Sea surface height, NAO+ minus NAO-
Looks like a gradual (linear) change, but is a composite of two different temporal and
spatial responses
Department of GeophysicsUniversity of Bergen
Conclusions (2)
• NAO+ Initial strengthening of SPG After ~10 years replaced by weakening Advective warming overruns local buoyancy forcing
• NAO- Steady weakening of SPG No ocean advective feedback
• Nonlinearity: SPG response to NAO+ and NAO- forcing is nonlinear, so the difference NAO+ minus NAO- is misleading
• NAOn forcing can reveal temporal / spatial nonlinearities in ocean’s response to NAO (for analysis, use e.g. NAO+ minus NAOn and NAO- minus NAOn)
Department of GeophysicsUniversity of Bergen
NAO index
Q3 How important was the ocean initial state in 1995 for the strong and rapid weakening of the SPG?
Department of GeophysicsUniversity of Bergen
Sensitivity experiments
Post 1995 forcing (red arrow) applied to ocean initial state from ✓ 1975, 1980, 1985, 1990, 2000, 2005, and✓ every year between 1991 and 1997
Simulated strength of the SPG
Department of GeophysicsUniversity of Bergen
Post 1995 forcing (I)
Control integration (“reality”)
Simulated strength of the SPG
Department of GeophysicsUniversity of Bergen
Control integration (“reality”)
Post 1995 forcing (II)
Simulated strength of the SPG
Department of GeophysicsUniversity of Bergen
Additional sensitivity experiments
Post 1982 forcing (red arrow) applied to ocean initial state from ✓ 1975, 1985, 1990, 1995, 2000, 2005
Simulated strength of the SPG
Department of GeophysicsUniversity of Bergen
Post 1982 forcing
Control integration 1995
Simulated strength of the SPG
Department of GeophysicsUniversity of Bergen
Conclusions (3)
What happened after 1995? ✓ SPG close to maximum strength and approaching
weakening even with unchanged NAO+ forcing (preconditioning)
NAO forcing dropped from high to low value the winter 1995/96 (right forcing)
Important implications for decadal-scale predictability of the climate in the North Atlantic – the ocean initial state is of key imortance
Department of GeophysicsUniversity of Bergen Large increase in the blue whiting SSB
Large changes in the distribution of redfish
Changessince 1995
Cod is heading back toGreenlandic waters
Warming
Warming
Warming
Warming
Herring is heading backto Icelandic and Faroese waters
Department of GeophysicsUniversity of Bergen
Hatun et al, Prog. Oceanogr. (2009)
Department of GeophysicsUniversity of Bergen
Response of phytoplankton (PCI) and zooplankton (Cal. Fin.)