ABSTRACT: The assessment model for ultraviolet radiation and risk "AMOUR" is applied to output from two chemistry-climate models (CCMs). Results from the UK Chemistry and Aerosols CCM are used to quantify the worldwide skin cancer risk avoided by the Montreal Protocol and its amendments: by the year 2030, two million cases of skin cancer have been prevented yearly, which is 14% fewer skin cancer cases per year. In the "World Avoided," excess skin cancer incidence will continue to grow dramatically after 2030. Results from the CCM E39C-A are used to estimate skin cancer risk that had already been inevitably committed once ozone depletion was recognized: excess incidence will peak mid 21st century and then recover or even super-recover at the end of the century. When compared with a "No Depletion" scenario, with ozone undepleted and cloud characteristics as in the 1960s throughout, excess incidence (extra yearly cases skin cancer per million people) of the "Full Compliance with Montreal Protocol" scenario is in the ranges: New Zealand: 100-150, Congo: -10-0, Patagonia: 20-50, Western Europe: 30-40, China: 90-120, South-West USA: 80-110, Mediterranean: 90-100 and North-East Australia: 170-200. This is up to 4% of total local incidence in the Full Compliance scenario in the peak year.
Photochemistry and Photobiology 08/2012; · 2.41 Impact Factor
Atmospheric Chemistry and Physics.
Atmospheric Chemistry and Physics. 9(2009):6017-6031.
Atmospheric Chemistry and Physics. 9(2009):8935-8948.
ABSTRACT: The stratospheric climate and variability from simulations of sixteen chemistryclimate models is evaluated. On average the polar night jet is well reproduced though its variability is less well reproduced with a large spread between models. Polar
temperature biases are less than 5 K except in the Southern Hemisphere (SH) lower stratosphere in spring. The accumulated area of low temperatures responsible for polar stratospheric cloud formation is accurately reproduced for the Antarctic but underestimated
for the Arctic. The shape and position of the polar vortex is well simulated, as is the tropical upwelling in the lower stratosphere. There is a wide model spread in the frequency of major sudden stratospheric warnings (SSWs), late biases in the breakup of the SH
vortex, and a weak annual cycle in the zonal wind in the tropical upper stratosphere.
Quantitatively, “metrics” indicate a wide spread in model performance for most diagnostics with systematic biases in many, and poorer performance in the SH than in the Northern Hemisphere (NH). Correlations were found in the SH between errors in the final warming, polar temperatures, the leading mode of variability, and jet strength, and in the NH between errors in polar temperatures, frequency of major SSWs, and jet strength. Models with a stronger QBO have stronger tropical upwelling and a colder NH vortex. Both the qualitative and quantitative analysis indicate a number of common and long‐standing model problems, particularly related to the simulation of the SH
and stratospheric variability.
Journal of Geophysical Research. 116(2011-D05102):1-21.
EGU General Assembly 2010;
Atmospheric Chemistry and Physics. 11(2011):7533-7545.
ABSTRACT: Chemistry-climate models (CCMs) are commonly
used to simulate the past and future development of Earth’s ozone layer. The fully coupled chemistry schemes calculate the chemical production and destruction of ozone interactively and ozone is transported by the simulated atmospheric
flow. Due to the complexity of the processes acting on ozone it is not straightforward to disentangle the influence of individual processes on the temporal development of ozone concentrations. A method is introduced here that quantifies the influence of chemistry and transport on ozone concentration changes and that is easily implemented in CCMs and chemistry-transport models (CTMs). In this method, ozone tendencies (i.e. the time rate of change of ozone) are partitioned into a contribution from ozone production and destruction (chemistry) and a contribution from transport of ozone (dynamics). The influence of transport on ozone in a specific region is further divided into export of ozone out of that region and import of ozone from elsewhere into that region. For this purpose, a diagnostic is used that disaggregates the ozone mixing ratio field into 9 separate fields according to in which of 9 predefined regions of the atmosphere the ozone originated. With this diagnostic the ozone mass fluxes between these regions are obtained. Furthermore, this method is used here to attribute long-term changes
in ozone to chemistry and transport. The relative change in ozone from one period to another that is due to changes in production or destruction rates, or due to changes in import or export of ozone, are quantified. As such, the diagnostics introduced here can be used to attribute changes in ozone on monthly, interannual and long-term time-scales to the responsible
mechanisms. Results from a CCM simulation are
shown here as examples, with the main focus of the paper being on introducing the method.
Geoscientific Model Development. 4(2011):271-286.
ABSTRACT: Drivers of upwelling in the tropical lower stratosphere are investigated using the E39C-A chemistry–
climate model. The climatological annual cycle in upwelling and its wave forcing are compared to the interim ECMWF Re-Analysis (ERA-Interim). The strength in tropical upwelling and its annual cycle can be largely
explained by local resolved wave forcing. The climatological mean forcing is due to both stationary planetaryscale waves that originate in the tropics and extratropical transient synoptic-scale waves that are refracted equatorward.
Increases in atmospheric greenhouse gas (GHG) concentrations to 2050 force a year-round positive trend
in tropical upwelling, which maximizes in the lowermost stratosphere. Tropical ascent is balanced by
downwelling between 208 and 408. Strengthening of tropical upwelling can be explained by stronger local
forcing by resolved wave flux convergence, which is driven in turn by processes initiated by increases in
tropical sea surface temperatures (SSTs). Higher tropical SSTs cause a strengthening of the subtropical jets
and modification of deep convection affecting latent heat release. While the former can modify wave propagation and dissipation, the latter affects tropical wave generation. The dominant mechanism leading to
enhanced vertical wave propagation into the lower stratosphere is an upward shift of the easterly shear zone due to the strengthening and upward shift of the subtropical jets.
Journal of Atmospheric Science. 68(2011):1214-1233.
ABSTRACT: The effect of the winter Brewer-Dobson circulation (BDC) on the seasonal and decadal evolution of total ozone in both hemispheres is investigated using satellite total ozone data from the merged GOME/SCIAMACHY/GOME-2 (GSG) data set (1995–
2010) and outputs from two chemistry-climate models
(CCM), the FUB-EMAC and DLR-E39C-A models. Combining data from both hemispheres a linear relationship between the winter average extratropical 100 hPa eddy heat flux and the ozone ratio with respect to fall ozone levels exists and is statistically significant for tropical as well as polar ozone. The high correlation at high latitudes persists well into the summer months until the onset of the next winter season. The anti-correlation of the cumulative eddy heat flux with tropical ozone ratios, however, breaks down in spring as the polar vortex erodes and changes to a weak positive correlation similar to that observed at high latitudes. The inter-annual variability and decadal evolution of ozone in each hemisphere in winter, spring, and summer are therefore driven by the cumulative effect of the previous winter’s meridional circulation.
This compact linear relationship is also found in both
CCMs used in this study indicating that current models realistically describe the variability in stratospheric circulation and its effect on total ozone. Both models show a positive trend in the winter mean eddy heat flux (and winter BDC (strength) in both hemispheres unti l year 2050, however the inter-annual variability (peak-to-peak) is two to three times larger than the mean change between 1960 and 2050. It is, nevertheless, possible to detect a shift in this compact linear
relationship related to past and future changes in the stratospheric halogen load. Using the SBUV/TOMS/OMI (MOD V8) merged data set (1980–2010), it can be shown that from the decade 1990–1999 to 2000–2010 this linear relationship remained unchanged (before and after the turnaround in the stratospheric halogen load), while a shift is evident between 1980–1989 (upward trend in stratospheric halogen) and the 1990s, which is a clear sign that an onset of recovery is detectable despite the large variability in polar ozone. Because of the large variability from year to year in the BDC circulation substantial polar ozone depletion may still occur in coming decades in selected winters with weak BDC and very low polar stratospheric temperatures.
Atmospheric Chemistry and Physics. 11(2011):11221-11235.
ABSTRACT: Current measurements signify a cooling of the tropical lower stratosphere (TLS) and most chemistry-climate models (CCMs) demonstrate a link to an intensifying tropical upwelling, caused by the anthropogenic increase in well-mixed greenhouse gas (GHG) concentrations. In particular, the associated rise in tropical sea surface temperatures (SSTs) appears to stimulate the dissipation of resolved waves in the TLS, and hence the local upwelling. Yet, the relevant studies are ambiguous about the mechanisms behind the dissipation enhancement. We present findings from our studies with the CCMs E39C and E39C-A. For E39C, we compare two transient scenarios that share the same boundary conditions, but differ via prescribed SSTs and GHG concentrations. In the summer hemisphere tropics, the higher SSTs amplify the deep-convective excitation of quasi-stationary waves. These waves propagate upward, intensifying the wave dissipation in the TLS. Our findings for E39C-A are based both on a multi-decadal transient simulation and on a set of sensitivity simulations in time slice mode. The former includes a steady increase in GHG concentrations and SSTs. The latter serve to isolate the sensitivity to differences in SSTs and GHG concentrations. Again, the dissipation of resolved waves intensifies in the TLS mainly in response to higher tropical SSTs. The intensification, however, occurs year-round and is associated with quasi-stationary and transient waves which propagate into the TLS more efficiently due to altered zonal winds. Resolving the discrepancy is an important task and, given the entanglement of zonal winds and waves, should not only involve CCMs, but also simpler mechanistic models.
IUGG Genral Assembly;
Photochemistry and Photobiology.
ABSTRACT: One of the most significant events in the evolution
of the ozone layer over southern mid-latitudes since the
late 1970s was the large decrease observed in 1985. This
event remains unexplained and a detailed investigation of
the mechanisms responsible for the event has not previously
been undertaken. In this study, the 1985 Southern Hemisphere
mid-latitude total column ozone anomaly is analyzed
in detail based on observed daily total column ozone fields,
stratospheric dynamical fields, and calculated diagnostics of
stratospheric mixing. The 1985 anomaly appears to result
from a combination of (i) an anomaly in the meridional circulation
resulting from the westerly phase of the equatorial
quasi-biennial oscillation (QBO), (ii) weaker transport
of ozone from its tropical mid-stratosphere source across the
sub-tropical barrier to mid-latitudes related to the particular
phasing of the QBO with respect to the annual cycle,
and (iii) a solar cycle induced reduction in ozone. Similar
QBO and solar cycle influences prevailed in 1997 and 2006
when again total column ozone was found to be suppressed
over southern mid-latitudes. The results based on observations
are compared and contrasted with analyses of ozone
and dynamical fields from the ECHAM4.L39(DLR)/CHEM
coupled chemistry-climate model (hereafter referred to as
E39C). Equatorial winds in the E39C model are nudged towards
observed winds between 10<sup>o</sup> S and 10<sup>o</sup> N and the ability
of this model to produce an ozone anomaly in 1985, similar
to that observed, confirms the role of the QBO in effecting
Atmospheric Chemistry and Physics. 7(2007-11-14):5625-5637.
ABSTRACT: Changes in climate are likely to drive changes
in stratospheric mixing with associated implications for
changes in transport of ozone from tropical source regions
to higher latitudes, transport of water vapour and source
gas degradation products from the tropical tropopause layer
into the mid-latitude lower stratosphere, and changes in the
meridional distribution of long-lived trace gases. To diagnose
long-term changes in stratospheric mixing, global
monthly fields of Lyapunov exponents were calculated on
the 450 K, 550 K, and 650K isentropic surfaces by applying
a trajectory model to wind fields from NCEP/NCAR reanalyses
over the period 1979 to 2005. Potential underlying
geophysical drivers of trends and variability in these mixing
fields were investigated by applying a least squares regression
model, which included basis functions for a mean
annual cycle, seasonally dependent linear trends, the quasibiennial
oscillation (QBO), the solar cycle, and the El Ni˜no
Southern Oscillation (ENSO), to zonal mean time series of
the Lyapunov exponents.
Long-term positive trends in mixing are apparent over
southern middle to high latitudes at 450K through most of
the year, while negative trends over southern high latitudes
are apparent at 650K from May to August. Wintertime negative
trends in mixing over northern mid-latitudes are apparent
at 550K and 650 K. Over low latitudes, within 40<sup>o</sup> of the
equator, the QBO exerts a strong influence on mixing at all
three analysis levels. This QBO influence is strongly modulated
by the annual cycle and shows a phase shift across
the subtropical mixing barrier. Solar cycle and ENSO influences
on mixing are generally not significant. The diagnosed
long-term changes in mixing should aid the interpretation of
trends in stratospheric trace gases.
Atmospheric Chemistry and Physics. 7(2007-11-14):5611-5624.
4th SPARC General Assembly;
ABSTRACT: The internal variability and coupling between the stratosphere and troposphere in CCMVal‐2 chemistry‐climate models are evaluated through analysis of the annular mode
patterns of variability. Computation of the annular modes in long data sets with secular trends requires refinement of the standard definition of the annular mode, and a more
robust procedure that allows for slowly varying trends is established and verified. The spatial and temporal structure of the models’ annular modes is then compared with that of
reanalyses. As a whole, the models capture the key features of observed intraseasonal variability, including the sharp vertical gradients in structure between stratosphere
and troposphere, the asymmetries in the seasonal cycle between the Northern and Southern hemispheres, and the coupling between the polar stratospheric vortices and tropospheric midlatitude jets. It is also found that the annular mode variability changes little in time throughout simulations of the 21st century. There are, however, both common biases and significant differences in performance in the models. In the troposphere, the annular mode in models is generally too persistent, particularly in the Southern Hemisphere summer, a bias similar to that found in CMIP3 coupled climate models. In the stratosphere, the periods of peak variance and coupling with the troposphere are delayed by about a month in both hemispheres. The relationship between increased variability of the stratosphere and increased persistence in the troposphere suggests that some tropospheric biases may be related to stratospheric biases and that a well‐simulated stratosphere can improve simulation of tropospheric intraseasonal variability.
Journal of Geophysical Research. 115(2010-D00M06):1-15.