Impact of Antarctic Ozone Depletion and Recovery on Southern Hemisphere Precipitation, Evaporation, and Extreme Changes
Journal of Climate
4 The possible impact of Antarctic ozone depletion and recovery on Southern Hemisphere 5 (SH) mean and extreme precipitation and evaporation is examined using multimodel output 6 from the Climate Model Intercomparison Project 3 (CMIP3). By grouping models into four 7 sets, those with and without ozone depletion in 20 th century climate simulations and those 8 with and without ozone recovery in 21 st century climate simulations, and comparing their 9 multimodel-mean trends, it is shown that
... is shown that Antarctic ozone forcings significantly modulate 10 extratropical precipitation changes in austral summer. The impact on evaporation trends is 11 however, minimal especially in 20 th century climate simulations. In general, ozone depletion 12 has increased precipitation in high-latitudes and decreased it in mid-latitudes, in agreement 13 with the poleward displacement of the westerly jet and associated storm tracks by Antarc-14 tic ozone depletion. Although weaker, the opposite is also true for ozone recovery. These 15 precipitation changes are primarily associated with changes in light precipitation (1-10 mm 16 day −1 ). Contributions by very-light precipitation (0.1-1 mm day −1 ) and moderate-to-heavy 17 precipitation (>10 mm day −1 ) are minor. Likewise, no systematic changes are found in 18 extreme precipitation events, although extreme surface wind events are highly sensitive to 19 ozone forcings. This result indicates that, while extratropical mean precipitation trends are 20 significantly modulated by ozone-induced large-scale circulation changes, extreme precipita-21 tion changes are likely more sensitive to thermodynamic processes near the surface than to 22 dynamical processes in the free atmosphere. 23 1 41 mer, have driven SH extratropical circulation changes in a similar way, the cumulative effect 42 resulting in more significant tropospheric climate change in austral summer than in other 43 seasons. In the future, the effects of these two forcings are however predicted to oppose each 44 other (Shindell and Schmidt 2004; Perlwitz et al. 2008; McLandress et al. 2011), as Antarctic 45 ozone concentrations are anticipated to increase due to the implementation of the Montréal 46 Protocol (Austin et al. 2010). 47 While the surface climate impact of increasing greenhouse gases is relatively well un-48 derstood, our understanding of stratospheric ozone-related climate change at the surface, 49 especially its mechanisms, is somewhat limited. In particular the impact of stratospheric 50 2 ozone changes on the hydrological cycle in the SH is not well understood. A series of recent 51 studies have shown that stratospheric ozone depletion has likely enhanced austral-summer 52 precipitation changes in the subtropics and high-latitudes but reduced them in mid-latitudes, 53 consistent with the poleward displacement of the westerly jet, or equivalently the positive 54 trend in the Southern Annular Mode (SAM) index (Son et al. 2009; McLandress et al. 2011; 55 Polvani et al. 2011; Kang et al. 2011). Although they are crucial for understanding salinity 56 changes in the Southern Ocean, net hydrological changes, including evaporation, are not yet 57 well understood. In addition and arguably more importantly, potential changes in extreme 58 precipitation events are yet to be investigated. It is known that individual precipitation 59 events are likely to get more intense as the climate warms (Emori and Brown 2005; Sun 60 et al. 2007; O'Gorman and Schneider 2009). Previous studies suggest that it is predomi-61 nantly thermodynamics that control changes in extratropical extreme precipitation (Emori 62 and Brown 2005; O'Gorman and Schneider 2009). Thus, it is questionable whether extreme 63 precipitation events will respond to dynamical changes driven by the Antarctic ozone hole. 64 The purpose of this study is to bridge the existing gap in understanding the relative 65 contributions of anthropogenic greenhouse gas emissions and stratospheric ozone changes 66 in forcing changes in the hydrological cycle. Multimodel output from the Climate Model 67 Intercomparison Project 3 (CMIP3; Meehl et al. (2007)) are analysed. By grouping models 68 into those with prescribed ozone depletion and recovery, and those without it, we show 69 that Antarctic ozone forcings significantly affect seasonal-mean precipitation trends in the 70 extratropics during austral summer, but play a minimal role in evaporation and extreme 71 precipitation trends. 72 2. Data and methods 73 CMIP3 data from the 20 th century climate simulations (20C3m) and 21 st century climate 74 simulations with the special report on emissions scenarios A1B forcing (A1B) are analysed. 75 those associated with model-dependent internal variabilities (Son et al. 2009). 101 Since daily data are archived only for selected decades in the A1B runs, long-term trends 102 are estimated in this study using decadal differences. The 20 th century change, reflecting the 103 impact of ozone depletion, is defined by the difference between 1990-1999 and 1961-1970 104 means. Likewise the 21 st century change, reflecting the impact of ozone recovery, is defined 105 by the difference between 2056-2065 and 1990-1999 means. Since decadal differences are 106 qualitatively similar to the linear trends computed from monthly-mean data over 1960-1999 107 and 2000-2079 (not shown), they are simply referred to as "trends" in this study. The 108 possible changes in extreme precipitation events are examined by decomposing seasonal-109 mean precipitation trends into three regimes (Sun et al. 2007): very-light (0.1-1 mm day −1 ), 110 light (1-10 mm day −1 ) and moderate-to-heavy (>10 mm day −1 ) precipitation changes. Five 111 extreme precipitation indices are also examined. They are the sum of precipitation on all 112 wet days divided by the number of wet days, the sum of rainfall on days exceeding the 113 95 th percentile threshold as determined for the base period of 1961-1990 (hereafter 95 th 114 percentile precipitation), the sum of rainfall on days exceeding the 99 th percentile threshold 115 as determined for the base period of 1961-1990, seasonal maximum one-day precipitation, 116 and seasonal maximum five-day consecutive precipitation (ETCCDI/CRD 2009). 117 3. Results 118 Multimodel-mean trends of austral-summer (DJF) precipitation and evaporation are pre-119 sented in Fig. 1. Only the extratropics, poleward of 30 • S, are shown, as tropical and subtrop-120 ical trends are noisy and largely insignificant. In the 20 th century the poleward displacement 121 of storm tracks by increasing greenhouse gases causes a dipolar trend in precipitation (Yin 264 Adler, R., et al., 2003: The Version-2 Global Precipitation Climatology Project (GPCP) 265 monthly precipitation analysis (1979-present). Austin, J., et al., 2010: Decline and recovery of total column ozone using a multi-269 model time series analysis.