Ozone-climate links in the Southern Hemisphere.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis details a number of the effects that changes in the abundance of stratospheric ozone over the period 1950-2100 are having on climate in the Southern Hemisphere. Beginning in the 1970s polar stratospheric ozone became increasingly depleted due to anthropogenic emissions of ozone depleting substances (ODSs), leading to the formation of what is known as the “ozone hole”. The cessation of ODS emissions as a result of the Montreal Protocol and its amendments and adjustments is projected to lead to the recovery of polar stratospheric ozone over the course of the 21st century. Ozone has a large effect on temperature in the stratosphere via its absorption of solar radiation and outgoing longwave radiation. Changes in ozone therefore will have an effect on stratospheric climate and also, as is demonstrated in this thesis, tropospheric climate.
This study utilizes the NIWA-UKCA coupled atmosphere-ocean-chemistry climate model (AOCCM). This model interactively simulates ozone chemistry, which is an advance on prior generations of general circulation models (GCMs) in which ozone is often prescribed. Comparison of model runs in which ODS concentrations vary according to historical and projected future values, to runs in which ODS concentrations are fixed at pre-ozone hole levels allows for the attribution of various changes in climate to changes in ozone.
Southern Hemisphere climate can be described, in large part, by the Southern Annular Mode (SAM). The SAM describes an oscillation of atmospheric mass between mid-latitudes and the pole. This results in a vacillation of the strength of the polar vortex in the stratosphere and a meridional meandering of the polar frontal jet in the troposphere. This thesis shows an increase in the frequency of extreme SAM events and an increase in persistence of the SAM in the stratosphere as a result of ozone depletion.
The SAM is also useful for examining coupling between the stratosphere and troposphere. Extreme SAM events in the stratosphere have been shown in previous studies to be followed by SAM anomalies in the troposphere that persist for around two months - much longer than the typical persistence in the troposphere. This thesis shows that the strength of this coupling increases as a result of ozone depletion, with the tropospheric SAM showing an increased anomaly lagging the stratospheric anomaly by 40-60 days.
Another feature of tropospheric climate is atmospheric blocking. Blocking describes large, quasi-stationary, persistent anticyclonic anomalies that impede the mid-latitude zonal flow. It is shown that ozone depletion leads to a increase in the frequency of blocking during summer in the South Atlantic, followed by a decrease in frequency as ozone recovers over the 21st century. In contrast, changes in ozone forcing show no influence on blocking in the South Pacific. The difference between the two regions is likely linked to the SAM as it is shown that blocking events in the Atlantic are preceded by positive SAM anomalies which is not the case for Pacific blocking events. The distribution of ozone is also examined. It is known that ozone distribution is somewhat asymmetric, with the ozone hole displaced from the pole toward South America. This asymmetry is an important factor in the determining the temperature of the polar stratosphere. An eastward trend in the location of the ozone minimum over 1960-1999 is attributed to ozone depletion. An increase in greenhouse gas (GHG) forcing opposing the effect of ozone recovery during the 21st century results in a relatively constant location of this minimum over this timespan.
The results presented in this thesis demonstrate the utility of accounting for changes in ozone and, in general, of accurately simulating the stratosphere when modelling the climate or producing long range weather forecasts.