department of geophysics university of bergen spin-up; monthly forcing; daily forcing; relaxation t...

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


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)


AMOC, last 4 cycles


NA Sub-Polar Gyre SSH, last 4 cycles


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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)


Altimetry

MICOM

SSH EOF (Häkkinen and Rhines, Science, 2004)

I

I

F

F

R

R

Dynamical SPG influence


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)


NB: Relationship also valid for temperature


Forcing mechanisms (I)

Hatun et al., in prep.

Department of GeophysicsUniversity of Bergen Hatun et al., in prep

Forcing mechanisms (II)


Wind stress and NAO consistent with the Gyre Index before - but not after - 1995


– 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)


Recent North Atlantic Warming and Some Consequences Thereof

Helge Drange, Katja Lohmann, Mats Bentsen, Hjalmar Hatun, Anne Britt Sandø and colleagues

[email protected]


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


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


(Hátún, AGU Monogr 158, 2005)

SST variability (Atl inflow), Faroe Islands

Record high temperature(and salinity)

Simulated temperature


1 °C increase in T of Atlantic Water entering the Nordic Seas since 1997

(ca 0-600 m)

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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


Observed air temperature, Nuuk (W Greenland)

Stein (2007)


http://svs.gsfc.nasa.gov

Jakobshavn glacier, West Greenland


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)


The model system used


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


Subpolar gyresource water

Western Subtropical gyreSource water

Western North Atlantic Water (WNAW)

Eastern NorthAtlantic Water (ENAW)

Source Watersfor the Atlantic Inflows

Simulatedsalinity


Irminger (I) Faroe (F) Rockall (R)

Observed and simulated salinity anomalies at three locations in the northern North Atlantic

Hatun et al., Science (2005)


RIGyre index

Irminger Current (obs)

Rockall Trough (obs)

Gyre index


Atmospheric forcing

Hatun et al., 2007

Department of GeophysicsUniversity of BergenHatun et al., 2007

Atmospheric forcing


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?


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


Sea surface height, NAO+ minus NAOn


Steric height, NAO+ minus NAOn

T

S


SPG index, NAO+ minus NAOn

Strong

Weak


Sea surface height, NAO- minus NAOn


Sea surface height, NAO+ minus NAO-

Looks like a gradual (linear) change, but is a composite of two different temporal and

spatial responses


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)


NAO index

Q3 How important was the ocean initial state in 1995 for the strong and rapid weakening of the SPG?


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


Post 1995 forcing (I)

Control integration (“reality”)



Control integration (“reality”)

Post 1995 forcing (II)



Additional sensitivity experiments

Post 1982 forcing (red arrow) applied to ocean initial state from ✓ 1975, 1985, 1990, 1995, 2000, 2005



Post 1982 forcing

Control integration 1995



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


Hatun et al, Prog. Oceanogr. (2009)


Response of phytoplankton (PCI) and zooplankton (Cal. Fin.)

department of geophysics university of bergen spin-up; monthly forcing; daily forcing; relaxation t...

Documents

atmospheric forcing

gyre variability inflow

strong sssrelaxation

ocean preconditioning

implementation of core