Low-frequency variability in the mid-latitude atmosphere
Download
1 / 28

Low-frequency variability in the mid-latitude atmosphere induced by an oceanic thermal front: - PowerPoint PPT Presentation


  • 124 Views
  • Uploaded on

Low-frequency variability in the mid-latitude atmosphere induced by an oceanic thermal front: Application to the North Atlantic Ocean. Yizhak Feliks 1,2 Michael Ghil 2,3 and Andrew W. Robertson 4. 1 Mathematics Dept., Israel Institute of Biological Research, Ness Ziona, Israel.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' Low-frequency variability in the mid-latitude atmosphere induced by an oceanic thermal front:' - cato


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

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



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
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.


30-day oscillation

70-day oscillation



10 km field


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
Additional slides 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


In the barotropic model, the instability is symmetric. 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

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)] 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

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.


ad