Upload
dorthy-blake
View
236
Download
1
Tags:
Embed Size (px)
Citation preview
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
250
300
350
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
-50
-40
-30
-180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 180-60
-50
-40
-30
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
-175
-125
-75
-25
25
75
125
175
October
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
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.