THE INTERANNUAL SPATIAL VARIABILITY OF THE SOUTHERN HEMISPHERE TOTAL OZONE COLUMN MIDLATITUDE MAXIMUM: AN INDICATOR OF TROPOSPHERIC-STRATOSPHERIC COUPLED DYNAMICS

Pablo O. Canziani 1,3, Eduardo Agosta1,3 and Elizabeth Castañeda

2,3

1. Equipo Interdisciplinario para el estudio de Procesos Atmosféricos en el Cambio Global, (PEPACG), Pontificia Universidad Católica Argentina

2. Departamento de Ciencias de la Atmósfera, Universidad de Buenos Aires

3. Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina

DATA:

Total Ozone Column (TOC) from the TOMS Nimbus v.8 in the period 1980-2002.

Reanalisys daily data for daily atmospheric fields at pressure levels from the ECMWF.

Reanalysis monthly data for divergent wind and relative vorticity at sigma levels from the NCEP-NCAR

Decadal snapshots of TOMS Total Ozone (TOC) two-year means:

So our motivation is to answer the question:

Why the midaltitude ozone deplation migrates to the east and to the west, back and forth,

interannually?; and, why it progresses eastwards for the last

decades?

We are showing the first insights following…

Such changes are also observed in the tropopause and the lower stratosphere

This may suggest that the overall horshoe structure is a lower stratosphere feature, that can correspond to the well-known stationary wave number 1.

However, why this structure interannualy moves zonally, back and forth, with a tendency towards the east, is unkown.

Transient eddies are an important source of stationary wave via direct interaction (mainly through mid-latitudes transient

eddy mechanical forcing activity) or latent heat release associated to the mid-

latitudes transient eddies or to tropical deep convection that modify stormtracks

location.

Therefore, let us see the transient activity associated with the interanually TOC variability for October ….

The variability map shows a distinct region with the highest variability in the SH over the high latitudes of the western Indian Ocean sector....

The interannual evolution of this variability is chosen as an index of the state of the high-latitude total ozone.

-180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 180-60

-50

-40

-30Standard deviation of October TOC in the period 1980-2001

The time series of TOC variability, compared with another mayor source of atmospheric variability – the southern hemisphere Pacific-South Atlantic (PSA) mode

Maximum Variance TOC

200

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400

450

1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

DU

-3,00E+03

-2,00E+03

-1,00E+03

0,00E+00

1,00E+03

2,00E+03

3,00E+03

4,00E+03

5,00E+03

Max Var TOC PSA Oct

We compound the TOC field and the zonal assymetry anomalies during the years of occurrence of high and low TOC variability in October, defined usign the first and third quartile criterion

Maximum Minimum

1980 1992

1981 1997

1984 1998

1988 1999

1991 2002

-180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 180-60

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-180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 180-60

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October Total Ozone composites for max and min values in TOC variability

The following plots show the composite fields for Octobers with high (max) and low (min) TOC variability. The statistical significance was calulated with unequal 2-tailed T test and the corresponing areas highlighted in gray.

Z comp – [Z]clim

maxmin

50 hPa

Z comp – [Z]clim Z comp – [Z]clim

MAXMIN

70 hPa

100 hPa

150 hPa

Z comp – [Z]clim Z comp – [Z]clim

MAX MIN

200 hPa

300 hPa

Hence the anomaly patterns in the stationary eddy waves have structures that appear to be coherent from the stratosphere into the upper troposphere (400 hPa not shown), which are statistically significant. The differences bewteen max and min composites is also significant. These significant anomalies in the stationary eddy waves reveal relevant changes in the planetary Rossby waves at tropical latitudes as well as in the midlatitudes transient eddies.

- To identify source regions for the lower stratosphere stationary waves the wave activity flux is estimated. The stationary wave activitiy Fs can be calculate following Plumb (1985):

Fs = p·cosφ[v´2 - A ·v´Ф´|´λ , -u´v´+ A · u´ Ф´|´λ ]

Where a = 1/a2Ωsin(2φ)

CONV Fs is the provision of energy to transient eddies by the assymetric flow (shaded in light grey in the following figures)

DIV Fs is the absorption of energy from transient eddies by the assymetric flow (shaded in dark grey).

cos.pFs

Storm tracks in the SH can be identified by the kinetic energy component:

Ke = 0.5 [u´2 + v´2 ]

The local eddy wave activity can be estimated following Trenberth (1986):

Eu α [(v´2-u´2), -u´v´, T´v´] α dU/dt

Divergence (convergence) of E indicates local rotational forcing consistent with westerly (easterly) wind acceleration by synoptic-scale eddies, wich in turns can be associated with locally reduced (enhanced) meridional secondary circulation and thus it is a proxy of local TOC variation.

October Mean Conditions

October Mean Conditions

50 hPa

Fs

Max Composites

Fs

Max Composites

Second term associated with the divergent wind and the absolute vorticity gradient.

Fs

Max Composites

Div

Con

EP flux

50 hPa

Fs Min Composites

50 hPa

Fs

Min Composites

Second term associated with the divergent wind and the absolute vorticity gradient.

50 hPa

Fs

Min Composites

Conclusion:

1)West-to-east movement of the high-latidues ozone deplation during October is not related to SAM variability nor to the Pacific South Atlantic Oscillation2)This variability couples the lower stratosphere/upper troposphere, showing possible feedbacks.3)This behaviour is related to the Rossby-Wave Sources at subtropical latitudes associated with the divergent winds (due to equatorial-tropical deep convection and latent heat release) and with the strong subtropical vorticity gradient together with the transient eddy activity in troposphere wich interacts with the stationary wave 1 and possibly wave 2 in the stratosphere.4)Locally the transient eddy activity can influence the TOC variation via the Brewer-Dobson induced circulation.

During Maximum TOC variability, the midlatitude transient eddy activity is significantly ehanced in the troposphere over the Indean, the western South Pacific and the western South Atlantic oceans. The midlatitude transient eddy activity over the southwestern South Atlantic, enhanced by South American Rossby Wave Source, can account for the stratospheric stationary wave location.

During Minimum TOC variability, the midlatitude transient eddy activity is significantly enhanced over the Pacific and Altantic (and significantly decreased in the Indean ocean) associated with transient activity in lower stratosphere. The Rossby Wave Source may be playing a relevant role in the storm track location and the local stratospheric transient activity over the Indean ocean.

Therefore reason for the eastward migration of the highlatitude ozone deplation could be probably atributed to positive trends in the Indean Rossby Wave Source due to the increase in the zonally assymetric gradient of tropical SSTs (and thus enhanced tropical/equatorial convection over the Indean ocean).

This needs further investigation…

That´s all for now, thank you!

For further questions, I invite you to discuss personally so I will be

able to understand you…

A first Hypothesis:

Could there be some sort of spatial variability in SAM that affects TOC?

For example, has this EOF pattern slowly rotated over the years?

From JISAO website

Following Marshall´s SAM index calculation using surface station SLP observations, we estimated the spatial variability of the SAM index at 400hPa and 70 hPa using ERA-40 geopotential data products. The index was calculated at 10°longitude intervals and plotted as a Hovmoller diagram to detect such a possibility, for months between June and December

1980

1982

1984

1986

1988

1990

1992

1994

1996

1998

2000

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25

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October

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-1

0

1

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6

1980

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1982

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2001

In case you do not believe us, this is the zonal mean time series of our longitudinal SAM indices at 400 and 70 hPa (dashed and dotted lines respectively) compared with Marshall’s SLP SAM index (solid line) for the months of October in the period 1980-2001....

Not too bad considering the differences between reanalysis and observations over the SH as well as the height differences

Marshall vs 400 hPa

Marshall vs 70 hPa

Jan 0.886 0.857

Jun 0.732 0.452

Sep 0.896 0.402

Oct 0.828 0.536

Nov 0.888 0.441

Dec 0.932 0.646

Correlations significant at the 95% significance level are underlined.

Correlations between Marshall´s SAM INDEX and our zonal mean proxies were calculated

We do not see any well defined rotation pattern in the longitudinal SAM index.

Hence the SAM Variability Hypothesis....

According to Inatsu and Hoskins (2004, and references herein) the SH storm tracks location is determined by:i)The assymetry of tropical SSTs that induced convective heat release as a source of Rossby wave propagation into extratropics over the Indian Ocean mainly. Normally the quasistationary propagating waves modify the subtropical jet position, which acts as a waveguide for storm tracks.ii)Meridional SSTs gradient at midlatitudes that influence the mean baroclinic instability of the basic flow in the lower troposphere. Eddy development takes place downstream of the areas of high mean baroclinic instability of the basic flow.iii)Orography as the Andes and the African plateau can lead to cyclogenesis (cyclolisis) downstream (upstream) contributing to the overall eddy activity.