Low-frequency variability in the mid-latitude atmosphere
induced by an oceanic thermal front:
Application to the North Atlantic Ocean
Yizhak Feliks1,2 Michael Ghil2,3 and Andrew W. Robertson4
1
Mathematics Dept., Israel Institute of Biological Research, Ness Ziona, Israel.
2 Dept. of Atmospheric & Oceanic Sciences and Institute of Geophysics & Planetary
Physics, UCLA, Los Angeles, CA, USA.
3 Geosciences Department and Laboratoire de Météorologie Dynamique (CNRS and
IPSL), Ecole Normale Supérieure, Paris, France.
4 International Research Institute for Climate and Society, Columbia University,
Palisades, NY, USA.
Outline
• A model of atmospheric response to SST fronts
‣ Marine atmospheric boundary layer (MABL) + QG free atmosphere
• SST front specification
‣ Steady SST front 6 oC/100 km
‣ Adding interannual oscillations of 1 oC /100 km to the SST front
• Gulf Stream SST front
‣ spectral analyses of the SST field (SODA reanalysis, 1958–2007) in two regions
along the Gulf Stream front, in which the interannual oscillations are prominent
‣ atmospheric model response to SODA monthly history
Evolution of the barotropic mode in a
domain 5000 km x 5000 km
Three kinds of unstable oscillatory modes
First, antisymmetric instabilities are baroclinic;
they have a standing dipole structure.
The dominant mode has a period of 270 days.
Second, symmetric instabilities are barotropic;
they develop at the eastern edge of the eastward jet.
This mode was also obtained in an equivalent-barotropic model.
The dominant mode has a period of 30 days, cf. Feliks et al. (JAS, 2004).
Third, northward propagating instabilities can be decomposed
into two standing parts, an antisymmetric and a symmetric part.
The dominant mode has a period of 103 days.
The spatio-temporal evolution of this mode resembles
the observed 70-day mode of Plaut and Vautard (1994).
Conclusion:
• The SST front spins up an eastward jet in the free atmosphere.
• Three kinds of unstable oscillatory modes are obtained:
(1) antisymmetric due to baroclinic instability, with a period of 6–8 months.
(2) Symmetric due to barotropic instability, with a period of 30 days.
(3) Northward propagating, with an antisymmetric and a symmetric part,
and a period of 2-3 months.
• These effects depend of the atmospheric model’s high resolution of
50 km x 50 km (not shown)!
• The role of interannual oscillations of the SST front in the atmospheric
evolution was studied next.
Evolutive spectral analysis
30-day oscillation
70-day oscillation
The atmospheric response to the observed North Atlantic SST field
10 km
Next we examined the atmospheric effects of SST anomalies over and near
the Gulf Stream with the general circulation model (GCM) of the Laboratoire
de Météorologie Dynamique (LMD-Z) that has a zooming capability over the
Gulf Stream. Francis Codron’s talk will summarize this study.
Additional slides
In the barotropic model, the
instability is symmetric.
A bifurcation point appears at
ΔT=6 0C:
Δ t < 6 0C, the eddies are weak
and the dominant mode has a
period of 30 days;
Δ t > 6 0C, the eddies are
strong and the dominant mode
has a period of 70 days.
Schematic illustration of the FGS mechanism [FGS(a,b)]
of SST front impacts. The sharp SST gradient forces a mesoscale
cross-front circulation. The resulting vertical velocity at the top of
the MABL induces vorticity anomalies in the free troposphere and
a jet parallel to the surface isotherms. The vertical velocity at the
top of the MABL has a thermal component, similar to that of
Lindzen and Nigam (1987) in the tropics, and a mechanical one,
which is substantial in mid-latitudes.