Euro-Atlantic winter atmospheric response to the Tropical Atlantic Variability

Euro-Atlantic winter atmospheric response to the Tropical Atlantic Variability

T. Losada (1), B. Rodríguez-Fonseca (1), J. García-Serrano (1) C.R. Mechoso (2), F. Kucharski (3)

(1) Dpto. de Geofísica y Meteorología, UCM, Madrid, Spain(2) Departmet of Atmospheric and Oceanic Sciences, UCLA, USA

(3) International Centre for Theoretical Physics, Trieste, Italy

Tropical Atlantic principal modes of variability

EQUATORIAL MODE: during SUMMER the #1 EOF of the SST variability over the Equatorial Atlantic shows a NIÑO-LIKE PATTERN, a highly ocean-atmosphere coupled mode in which the Bjerknes feedback operates(Zebiak 1993).

SUBTROPICAL NORTH ATLANTIC MODE: Shares its variability part with the TRIPOLE PATTERN, the WINTER #1 EOF of the SST variability over the North Atlantic , in which poles centred towards the west of the basin alternate in the meridional direction (Sutton and Allen 1997).

SUBTROPICAL NORTH ATLANTIC INFLUENCE ON THE EURO-ATLANTIC WINTER

What are the dynamical mechanisms at work in the relation SST – Atmospheric Circulation ?

Are the subtropical SST anomalies the responsible of the anomalous atmospheric circulation in the North Atlantic

sector?

or

have the extratropical SST anomalies a key role in the generation of the atmospheric anomalies observed in high

latitudes?

UCLA AGCM SIMULATIONS

Resolution: 2.5ºlongitude x 2ºlatitude x 29 sigma levels.

Initial conditions: April, 16, 1995.

Simulation: April 16 1995 to March 1, 1996.

Ensemble simulation: 10 members.

Experiment design

ATL

Atlantic observed SST. 60S-60N. Climatology elsewhere.

TROP

Tropical Atlantic observed SST. Climatology elsewhere.


ATL geopotengial height and vector wind

200 hPa

925 hPa

TROP geopotengial height and vector wind

200 hPa

925 hPa

DJF anomalies

u200 TROP u200 ATL


zonal wind anomalies (contours)

omega-meridional wind anomalies (arrows)

averaged between 75W-50W, 30W-0

vertical circulationDJF anomalies

TROP 75w-50w

ATL 75w-50w ATL 30w-0

TROP 30w-0


Extratropical SST impact: Eddy activity and eddy – mean flow interaction

horizontal projection of EEP 2 2E= ( v' u' , u'v' )

energy transfer between the eddies and the mean flow

E·(grad(ū)) gives approximately the barotropic conversion term in

the eddy kinetic energy equation

grad(ū) > 0

ū

u’v’< 0 grad(ū) < 0

u’v’> 0

E·(grad(ū))< 0

the eddies give energy to the mean

zonal flow

ATL - TROP

Difference of barotropic energy conversion

DJF mean


extended EP-flux vector in the meridional plane

averaged between 75W-50W

ATL - TROP

vertical component of EEP

Is proportional to the northward heat flux by eddies

gives a qualitative idea of the eddy growth through baroclinic processes

f 0 v'θ' /Θ p


Explaining the winter circulation anomalies in the North Atlantic-European sector in 1995/96 requires consideration of both subtropical and extratropical anomalies in the Atlantic.

The tropical anomalies enhance the direct circulation.

The extratropical anomalies contribute to the enhancement of baroclinic eddy activity.

The enhanced kinetic energy of the transient eddies is then transferred to the mean zonal flow in the upper troposphere through barotropic processes.

SUMMARY AND CONCLUSIONS

Losada T, Rodriguez-Fonseca B, Mechoso CR, Ma H-Y (2007) Impacts of sst anomalies on the North Atlantic atmospheric circulation: A case study for the northern winter 1995/96. Cim Dyn. doi: 10.1007/s00382-007-0261-x


EQUATORIAL MODE INFLUENCE ON THE EURO-ATLANTIC WINTER CIRCULATION

Polo et al. (2008) show the Equatorial Mode evolution and damping, while García-Serrano et al. (2008) focus their study in the evolution of the convection associated with the summer Equatorial Mode anomalies.

Although no forecast capacity was described in García-Serrano et al. (2008) for winter rainfall, regressions of the precipitation on the mode described in Polo et al. (2008) show an anomalous dipole of precipitation in Europe.

Also, Polo et al. (2008) point to an statistical link link between Atlantic Niños and following winter Pacific Niñas that operates from the 70's. This feature has been confirmed by Rodríguez-Fonseca et al. (2009).

Is the Equatorial Mode linked to a pattern of winter European precipitation anomalies?

What is the nature of the physical mechanisms behind this link?

Is the Atlantic-Pacific connetion having any role on the generation of the winter atmospheric anomalies over the

North Atlantic sector?

Experiment design

1) JJAS to NDJF regression of precipitation, sst, 200hPa velocity potential and streamfunction onto the #1 EC of the 1979-1998 EMCA between JJAS precipitation over West Africa and FMAM to SOND tropical Atlantic SSTs for:

1a) Observations.

1b) UCLA AGCM simulation with observed SSTs: OBS simulation

2) Response of the UCLA AGCM to an anomalous Equatorial Mode added to the 1979-1998 climatology: EM simulation


OBSERVATIONS


UCLA AGCM OBS results


EM SIMULATION results


During summer, atmospheric conditions related to the Equatorial Mode forcing would favor the development of opposite anomalies of SST over the Pacific ocean.

From autumn to winter, the well developed SST anomalies over the Pacific affect the convection over the Caribbean region.

The anomalous upward motion over the Amazon region produces an increase of precipitation in this area.

An anomalous wave-like pattern arises from that region and reaches the Euro-Atlantic sector, producing a dipole of anomalous precipitation.

The Equatorial Mode simulation doesn’t show any significant response from autumn, pointing to the key role of the Pacific anomalies in the winter response.


SUMMARY AND CONCLUSIONS

GRACIAS

Euro-Atlantic winter atmospheric response to the Tropical Atlantic Variability

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